bio exam 4

If a heterozygous plant is allowed to self-pollinate, what proportion of the offspring will also be heterozygous?

1/2 -The law of segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. For the flower-color character in Mendel's peas, the law of segregation predicts that the two different alleles present in an F1 individual will segregate into gametes such that half the gametes will have the purple-flower allele and half will have the white-flower allele. During self-pollination, gametes of each class unite randomly. An egg with a purple-flower allele has an equal chance of being fertilized by a sperm with a purple-flower allele or one with a white-flower allele. Because the same is true for an egg with a white-flower allele, there are four equally likely combinations of sperm and egg. In the case listed here, one-quarter of the offspring will be homozygous for the dominant allele, one-quarter will be homozygous for the recessive allele, and the remaining half will be heterozygous.

Ignoring crossover, how many kinds of gametes can be produced by an organism with a diploid number of 8?

16 -One aspect of sexual reproduction that generates genetic variation is the random orientation of pairs of homologous chromosomes at metaphase of meiosis I. Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin. Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called independent assortment. The number of possible combinations when chromosomes sort independently during meiosis is 2n, where n is the haploid number of the organism. A cell with a diploid number of 8 would have a haploid, or n number, of 4. The possible number of combinations of chromosomes would therefore be 16.

The egg (ovum) of a rabbit contains 22 chromosomes. How many chromosomes are in the somatic (body) cells of a rabbit?

44 -Somatic cells are all the cells in the body of an animal except the gametes and their precursors. Somatic cells contain two sets of each chromosome. Any cell with two chromosome sets is called a diploid cell and has a diploid number of chromosomes, abbreviated 2n. In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). Because the egg of a rabbit contains 22 chromosomes, a somatic cell of a rabbit would contain twice as many chromosomes, for a total of 44.

In werewolves (hypothetically), pointy ears (P) are dominant over round ears (p). The gene is on the X chromosome. (Sex determination in werewolves is the same as for "other" humans.) A certain female werewolf has pointy ears even though her father had round ears. What percentage of her sons will have round ears if she mates with a werewolf with round ears?

50% -The human X chromosome contains approximately 1,100 genes, which are called X-linked genes. The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes. Fathers pass X-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass X-linked alleles to both sons and daughters. The female werewolf must be heterozygous, because her father has round ears. Each of her sons has a 50 percent chance of inheriting the round allele.

Why did Morgan choose Drosophila for his genetics experiments?

A single mating can produce many offspring and Drosophila chromosomes can be easily distinguishable under a light microscope. -The first solid evidence associating a specific gene with a specific chromosome came early in the 20th century from the work of Thomas Hunt Morgan, an experimental embryologist at Columbia University. Although Morgan was initially skeptical about both Mendelism and the chromosome theory, his early experiments provided convincing evidence that chromosomes are indeed the location of Mendel's heritable factors. For his work, Morgan selected a species of fruit fly, Drosophila melanogaster, a common insect that feeds on the fungi growing on fruit. Fruit flies are prolific breeders; a single mating will produce hundreds of offspring, and a new generation can be bred every two weeks. Another advantage of the fruit fly is that it has only four pairs of chromosomes, which are easily distinguishable with a light microscope.

Human blood groups are governed by three alleles, IA, IB, and i. IA and IB are codominant and i is recessive to both. A man who has type B blood and a woman who has type A blood could have children of which of the following phenotypes?

A, B, AB, or O -Only two alleles exist for the pea characters that Mendel studied, but most genes exist in more than two allelic forms. The ABO blood groups in humans, for instance, are determined by three alleles of a single gene: IA, IB, and i. A person's blood group (phenotype) may be one of four types: A, B, AB, or O. These letters refer to two carbohydrates—A and B—that may be found on the surface of red blood cells. A person's blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O). The genotype of the man with type B blood could be IBIB or IBi, so his gametes could be IB or i. The genotype of the woman with type A blood could be IAIA or IAi, so her gametes could be IA or i.

What appears to be the mechanism for genomic imprinting?

DNA methylation that silences particular genes and DNA methylation that activates particular genes -In recent years, geneticists have identified two to three dozen traits in mammals that depend on which parent passed along the alleles for those traits. Such variation in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting. Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes. Because these genes are imprinted differently in sperm and eggs, a zygote expresses only one allele of an imprinted gene, which it inherited from either the female or the male parent. What exactly is a genomic imprint? In many cases, it seems to consist of methyl (-CH3) groups that are added to cytosine nucleotides of one of the alleles. Such methylation may silence the allele, an effect consistent with evidence that heavily methylated genes are usually inactive. However, for a few genes, methylation has been shown to activate expression of the allele. Crossing over does not appear to play a part in genomic imprinting. Nondisjunction does not appear to play a role in genomic imprinting.

Regarding the role of cohesin protein in maintaining cohesion between sister chromatids, which of the following statements is false?

During meiosis II, cohesion holds sister chromatids together along their lengths as the second meiotic spindle forms. -Sister chromatids are attached along their lengths by protein complexes called cohesins. In mitosis, this attachment lasts until the end of metaphase, when enzymes cleave the cohesins, freeing the sister chromatids to move to opposite poles of the cell. In meiosis, sister chromatid cohesion is released in two steps, one at the start of anaphase I and one at anaphase II. In metaphase I, homologs are held together by cohesion between sister chromatid arms in regions beyond points of crossing over, where stretches of sister chromatids now belong to different chromosomes. The combination of crossing over and sister chromatid cohesion along the arms results in the formation of a chiasma. Chiasmata hold homologs together as the spindle forms for the first meiotic division. At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to separate. At anaphase II, the release of sister chromatid cohesion at the centromeres allows the sister chromatids to separate. Thus, sister chromatid cohesion and crossing over, acting together, play an essential role in the lining up of chromosomes by homologous pairs at metaphase I.

Queen Victoria was a carrier of a recessive sex-linked allele for hemophilia. Which of the following possibilities could explain the presence of the hemophilia allele in her genotype?

Either her mother was a carrier or her father had hemophilia. -Hemophilia is an X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting. When a person with hemophilia is injured, bleeding is prolonged because a firm clot is slow to form. Small cuts in the skin are usually not a problem, but bleeding in the muscles or joints can be painful and can lead to serious damage. Queen Victoria must have had the hemophilia allele on one of her X chromosomes. Either her mother was a carrier or her father was affected. A homozygous dominant mother could not be responsible for a carrier daughter.

In a certain fish, fin rays (supporting structures for the fins) can be either bony or soft in adult fish. Sex linkage in a fish is similar to that in humans. What evidence would most strongly support the idea that the ray locus is on the X chromosome?

Matings of soft ray males and bony ray females give different results from the matings of bony ray males and soft ray females. -X-linked genes in many animals, including fish, follow the same pattern of inheritance that Morgan observed for the eye-color locus he studied in Drosophila. Fathers pass X-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass X-linked alleles to both sons and daughters. Genes affecting sex-linked traits will give particular sex ratios in the offspring, and reciprocal crosses (males with the dominant trait crossed with females with the recessive trait, and males with the recessive trait crossed with females with the dominant trait) would not have the same results. Bony ray males passing on this trait only to their male offspring would support Y-linkage. An ability or inability to reproduce is not indicative of sex linkage. Females can pass sex-linked traits to their offspring.

In a typical pea experiment, two true-breeding plants with distinct traits of a single character are called the __________, and the offspring are called the __________, which will always be __________.

P (or parental) generation; F1 (or first filial) generation; hybrid -In a typical breeding experiment, Mendel cross-pollinated two contrasting, true-breeding pea varieties—for example, purple-flowered plants and white-flowered plants. This mating, or crossing, of two true-breeding varieties is called hybridization. The true-breeding parents are referred to as the P generation (parental generation), and their hybrid offspring are the F1 generation (first filial generation; the word filial is from the Latin word for "son").

Which choice below is a basic difference between Mendel's particulate hypothesis and the hypothesis of blending inheritance?

The blending inheritance hypothesis, but not the particulate hypothesis, maintained that after a mating, the genetic material provided by each of the two parents is mixed in the offspring, losing its individual identity. -The explanation of heredity most widely in favor during the 1800s was the "blending" hypothesis, the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. This hypothesis predicts that over many generations, a freely mating population will give rise to a uniform population of individuals. However, our everyday observations and the results of breeding experiments with animals and plants contradict that prediction. The blending hypothesis also fails to explain other phenomena of inheritance, such as traits reappearing after skipping a generation. An alternative to the blending model is a "particulate" hypothesis of inheritance: the gene idea. According to this model, parents pass on discrete heritable units—genes—that retain their separate identities in offspring. An organism's collection of genes is more like a deck of cards than a pail of paint. Like playing cards, genes can be shuffled and passed along, generation after generation, in undiluted form. Modern genetics had its genesis in an abbey garden, where a monk named Gregor Mendel documented a particulate mechanism for inheritance. Neither hypothesis speculated on the origin of different alleles. However, mutation is the ultimate source of all genetic variation. The blending inheritance hypothesis did not specifically address alleles and loci, or whether alleles were the same or different; rather, it generally dealt with what happens to the genetic material upon mating. With the exception of sex-linked genes, males and females both contribute genes that affect the same characters.

What kind of protection does the Genetic Information Nondiscrimination Act of 2008 provide the public with regard to the use of genetic information?

The law prohibits the use of genetic test information to deny insurance coverage or employment to individuals. -Tests for identifying carriers enable people with family histories of genetic disorders to make informed decisions about having children, but raise other issues. Could carriers be denied health or life insurance or lose the jobs providing those benefits, even though they themselves are healthy? The Genetic Information Nondiscrimination Act, signed into law in the United States in 2008, allays these concerns by prohibiting discrimination in employment or insurance coverage based on genetic test results. The law provides protection from discrimination but not based on their race, sex, age, religious beliefs, or sexual orientation. The law does not mandate people to be tested but provides protection for those individuals who voluntarily undergo genetic testing. The law does not compel individuals to undergo any type of testing, fetal or not, but does provide protection for how the results of genetic testing can be used.

What is a locus?

The precise location of a gene on a chromosome. -Parents endow their offspring with coded information in the form of hereditary units called genes. The genes we inherit from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as shared eye color or freckles. Our genes program the specific traits that emerge as we develop from fertilized eggs into adults. The location of a gene within the genome, including the identity of the particular chromosome that it maps to and its specific position on that chromosome, is called its locus.

Which of the following statements about homologous chromosomes is correct?

They have genes for the same traits at the same loci. -In humans, each somatic cell has 46 chromosomes. Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. Homologous chromosomes are pairs of chromosomes that have the same length, among other characteristics. In addition, both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye color is situated at a particular locus on a certain chromosome, then the homolog of that chromosome will also have a version of the same gene specifying eye color at the equivalent locus. Both plant and animal cells contain homologous chromosomes. Cells in prophase II are haploid; homologous chromosomes pair up in prophase I. Haploid cells have only one of each homologous pair. The cells of both males and females contain homologous chromosomes.

Duchenne muscular dystrophy is caused by a sex-linked recessive allele. Its victims are almost invariably boys, who usually die before the age of 20. Why is this disorder almost never seen in girls?

To express an X-linked recessive allele, a female must have two copies of the allele. -The human X chromosome contains approximately 1,100 genes, which are called X-linked genes. The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes. Fathers pass X-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass X-linked alleles to both sons and daughters. An example is Duchenne muscular dystrophy, which affects about one out of every 3,500 males born in the United States. The disease is characterized by a progressive weakening of the muscles and loss of coordination. In order to express an X-linked recessive allele, a female must have two copies of the allele. In addition, males carrying the allele have less of an opportunity to have offspring.

Huntington's disease is an example of a genetic disorder caused by __________.

a lethal dominant allele that afflicts an individual later in life -The timing of the onset of a disease significantly affects its inheritance. A lethal dominant allele is able to be passed on if it causes death at a relatively advanced age. By the time symptoms are evident, the individual with the allele may have already transmitted it to his or her children. For example, Huntington's disease, a degenerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal.

Female mammals have two X chromosomes, one from their mother and one from their father. During X inactivation, one X chromosome in each embryonic cell is randomly inactivated; therefore, all of the tissues that arise from that cell have the same inactivated X chromosome. Female mammals are therefore considered to be __________ of the maternal and paternal cells.

a mosaic -British geneticist Mary Lyon demonstrated that the selection of which X chromosome forms the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation. As a consequence, females consist of a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother. After an X chromosome is inactivated in a particular cell, all mitotic descendants of that cell have the same inactive X. Thus, if a female is heterozygous for a sex-linked trait, about half her cells express one allele, and the others express the alternate allele. The illustration above shows how this mosaicism results in the mottled coloration of a tortoiseshell cat. In humans, mosaicism can be observed in a recessive X-linked mutation that prevents the development of sweat glands. A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands.

__________ occurs when a single gene affects the phenotype of many characters in an individual.

a pleiotropic effect -In Mendelian inheritance, each gene affects only one phenotypic character. Most genes, however, have multiple phenotypic effects, a property called pleiotropy (from the Greek pleion, more). In humans, for example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.

The F1 generation differed from the F2 in Mendel's experiments in that __________.

all of the F1 showed the dominant phenotype, but only three-fourths of the F2 did -If the blending model of inheritance were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers, a trait intermediate between those of the P generation. However, when Mendel did his crosses, all the F1 offspring had flowers just as purple as the purple-flowered parents. But when Mendel allowed the F1 plants to self-pollinate and planted their seeds, the white-flower trait reappeared in the F2 generation. Mendel reasoned that the heritable factor for white flowers did not disappear in the F1 plants, but was somehow hidden, or masked, when the purple-flower factor was present. In Mendel's terminology, purple flower color is a dominant trait, and white flower color is a recessive trait. The reappearance of white-flowered plants in the F2 generation was evidence that the heritable factor causing white flowers had not been diluted or destroyed by coexisting with the purple-flower factor in the F1 hybrids. All of the F1 were heterozygous and therefore showed the dominant phenotype. In the F2, one-half were heterozygous, and another quarter were homozygous for the dominant allele.

If an organism that is homozygous dominant is crossed with a heterozygote for that trait, the offspring will be __________.

all of the dominant phenotype -If the two alleles at a locus for a particular character differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. The offspring of an AA x Aa cross are either AA or Aa. All will express the dominant phenotype. The offspring of a homozygous dominant parent cannot have a recessive phenotype. Because one of the parents has a recessive allele, all of the offspring cannot be homozygous dominant. The offspring of a homozygous dominant parent cannot have a recessive phenotype. A 9:3:3:1 ratio is true of the F2 generation of a dihybrid cross with two true-breeding individuals, not those of a monohybrid cross.

An alternative version of a gene is called a(n) __________.

allele -Alternative versions of genes account for variations in inherited characters. The gene for flower color in pea plants, for example, exists in two versions, one for purple flowers and the other for white flowers. These alternative versions are alleles. Today, we can also relate this concept to chromosomes and DNA. Each gene is a sequence of nucleotides at a specific place, or locus, along a particular chromosome. The DNA at that locus, however, can vary slightly in its nucleotide sequence. This variation in information content can affect the function of the encoded protein and thus the phenotype of the organism. The purple-flower allele and the white-flower allele are two possible DNA sequence variations at the flower-color locus on one of a pea plant's chromosomes; one variation allows synthesis of purple pigment and the other variation does not.

The life cycle called __________ in plants has two multicellular stages: the __________ and the __________.

alternation of generations; haploid gametophyte; diploid sporophyte -Plants and some species of algae exhibit a second type of life cycle called alternation of generations. This type includes both diploid and haploid stages that are multicellular. The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides mitotically, generating a multicellular haploid stage called the gametophyte. Cells of the gametophyte give rise to gametes by mitosis. Fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next sporophyte generation. Therefore, in this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation. The term alternation of generations fits well as a name for this type of life cycle.

Fetal cells may be removed along with fluid from the womb by a process known as __________.

amniocentesis -Suppose a couple expecting a child learns that they are both carriers of the Tay-Sachs allele. In the 14th-16th week of pregnancy, tests performed along with a technique called amniocentesis can determine whether the developing fetus has Tay-Sachs disease. In this procedure, a physician inserts a needle into the uterus and extracts about 10 mL of amniotic fluid, the liquid that bathes the fetus. Some genetic disorders can be detected from the presence of certain molecules in the amniotic fluid itself. Tests for other disorders, including Tay-Sachs disease, are performed on the DNA of cells cultured in the laboratory, descendants of fetal cells sloughed off into the amniotic fluid.

Examples of alterations of chromosomal structures include all of the following except __________.

aneuploidy -Aneuploidy results from nondisjunction, or the failure of homologous chromosomes or sister chromatids to separate in meiosis. Alterations in chromosomal structures are due to errors in meiosis, or damaging agents such as radiation can cause breakage of a chromosome, which can lead to four types of changes in chromosome structure. A deletion occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes. The "deleted" fragment may become attached as an extra segment on a sister chromatid, producing a duplication. Alternatively, a detached fragment could attach to a nonsister chromatid of a homologous chromosome. In that case, though, the "duplicated" segments might not be identical because the homologs could carry different alleles of certain genes. A chromosomal fragment may also reattach to the original chromosome but in the reverse orientation, producing an inversion. A fourth possible result of chromosomal breakage is that the fragment joins a nonhomologous chromosome, a rearrangement called a translocation.

In Klinefelter syndrome, individuals are phenotypically male, but have reduced sperm production and may have some breast development in adolescence. The cells of Klinefelter individuals have two X chromosomes and one Y (they are XXY instead of XY). This occurs because of what meiotic error?

aneuploidy -Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally. If either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome, a condition known as aneuploidy. Nondisjunction of sex chromosomes produces a variety of aneuploid conditions. Having an extra X chromosome affects males much more than females. In males, the extra X chromosome causes Klinefelter syndrome: abnormally small testes and sterility; some breast enlargement and other female body characteristics; and intelligence may be subnormal. A translocation occurs when fragments produced by chromosomal breakage join nonhomologous chromosomes. Polyploidy refers to a condition in which a cell has more than two complete chromosomal sets. Duplication refers to the presence of additional segments within a single chromosome, not to the presence of additional chromosomes. Monosomy means one chromosome too few.

Down syndrome is an example of __________, and the child is __________ for chromosome 21.

aneuploidy; trisomic -Down syndrome affects approximately one out of every 830 children born in the United States and is usually the result of an extra chromosome 21; thus each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21.

Somatic cells in animals differ from gametes in that somatic cells __________.

are all of the cells of the body except for the gametes and their precursors -The transmission of hereditary traits has its molecular basis in the replication of DNA, which produces copies of genes that can be passed from parents to offspring. In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During fertilization, male and female gametes (sperm and eggs) unite, passing on genes of both parents to their offspring. Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a eukaryotic cell is packaged into chromosomes within the nucleus. Every species has a characteristic number of chromosomes. For example, humans have 46 chromosomes in their somatic cells—which include all cells of the body except for the gametes and their precursors. Each chromosome consists of a single long DNA molecule elaborately coiled in association with various proteins. One chromosome includes several hundred to a few thousand genes, each of which is a specific sequence of nucleotides within the DNA molecule. A gene's specific location along the length of a chromosome is called the gene's locus (plural, loci; from the Latin, meaning "place"). Our genetic endowment (our genome) consists of the genes and other DNA that make up the chromosomes we inherited from our parents.

Linked genes __________.

are located near each other on the same chromosome -The number of genes in a cell is far greater than the number of chromosomes; in fact, each chromosome (except the Y) has hundreds or thousands of genes. Genes located near each other on the same chromosome tend to be inherited together in genetic crosses; such genes are said to be genetically linked and are called linked genes. When geneticists follow linked genes in breeding experiments, the results deviate from those that would be expected from Mendel's law of independent assortment. To see how linkage between genes affects the inheritance of two different characters, let's examine another of Morgan's Drosophila experiments. In this case, the characters are body color and wing size, each of which has two different phenotypes. Wild-type flies have gray bodies and normal-sized wings. In addition to these flies, Morgan managed to obtain, through breeding, doubly mutant flies with black bodies and wings much smaller than normal, called vestigial wings. The mutant alleles were recessive to the wild-type alleles, and neither gene was on a sex chromosome. In his investigation of these two genes, Morgan carried out the crosses shown in this figure: The first was a P generation cross to generate F1 dihybrid flies, and the second was a testcross. The resulting flies had a much higher proportion of the combinations of traits seen in the P generation flies (called parental phenotypes) than would have been expected if the two genes had assorted independently. Morgan thus concluded that body color and wing size are usually inherited together in specific (parental) combinations because the genes for these characters are near each other on the same chromosome.

Sister chromatids differ from nonsister chromatids in that sister chromatids __________.

are products of the S phase of the cell cycle and are two copies of one chromosome -Recall that sister chromatids are two copies of one chromosome, closely associated all along their lengths; this association is called sister chromatid cohesion. Together, the sister chromatids make up one duplicated chromosome. In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents. Homologs appear alike when viewed under a microscope, but they may have different versions of genes, each called an allele, at corresponding loci. Homologs are not associated with each other in any obvious way except during meiosis.

A clone is the product of __________.

asexual reproduction and mitosis -Parents endow their offspring with coded information in the form of hereditary units called genes. The transmission of hereditary traits has its molecular basis in the precise replication of DNA, which produces copies of genes that can be passed from parents to offspring. In asexual reproduction, a single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes. An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals through the process of mitosis. Sexual reproduction produces zygotes, not clones. Meiosis results in gametes, which fuse to produce zygotes during sexual reproduction.

When crossing over occurs between two linked genes in an F1 dihybrid testcross, the frequency of the parental types will always __________ the frequency of the recombinant types when compared to Mendel's unlinked genes.

be greater than -In Morgan's genetic linkage experiment, most of the offspring from the testcross for body color and wing size had parental phenotypes. That suggested that the two genes were on the same chromosome since the occurrence of parental types with a frequency greater than 50% indicates that the genes are linked. About 17% of offspring, however, were recombinants. Seeing these results, Morgan proposed that some process must occasionally break the physical connection between specific alleles of genes on the same chromosome. Later experiments showed that this process, now called crossing over, accounts for the recombination of linked genes. In crossing over, which occurs while replicated homologous chromosomes are paired during prophase of meiosis I, a set of proteins orchestrates an exchange of corresponding segments of one maternal and one paternal chromatid. In effect, when a single crossover occurs, end portions of two nonsister chromatids trade places. The figure below shows how crossing over in a dihybrid female fly resulted in recombinant eggs and, ultimately, recombinant offspring in Morgan's testcross. Most eggs had a chromosome with either the b+ vg+ or the b vg parental genotype, but some had a recombinant chromosome (b+ vg or b vg+). Fertilization of all classes of eggs by homozygous recessive sperm (b vg) produced an offspring population in which 17% exhibited a nonparental recombinant phenotype, reflecting combinations of alleles not seen before in either P generation parent.

Mendel studied __________, heritable features that vary among individuals; each variant is called a __________.

characters; trait -Mendel probably chose to work with peas because there are many varieties of peas. For example, one variety has purple flowers, while another variety has white flowers. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white color for flowers, is called a trait.

The __________ states that genes have specific positions on chromosomes and that chromosomes undergo both __________ and __________.

chromosomal theory of inheritance; segregation; independent assortment -Around 1902, Walter S. Sutton, Theodor Boveri, and others independently noted these parallels and began to develop the chromosomal theory of inheritance. According to this theory, Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.As you can see in the figure above, the separation of homologs during anaphase I accounts for the segregation of the two alleles of a gene into separate gametes, and the random arrangement of chromosome pairs at metaphase I accounts for the independent assortment of the alleles for two or more genes located on different homolog pairs. This figure traces the same path as the dihybrid pea cross you learned about previously, illustrated in the figure below. By carefully studying the figure above, however, you can see how the behavior of chromosomes during meiosis in the F1 generation and the subsequent random fertilization give rise to the F2 phenotypic ratio observed by Mendel.

__________ are a group of genetically identical individuals produced by a process called __________.

clones; asexual reproduction -Only organisms that reproduce asexually have offspring that are exact genetic copies of themselves. In asexual reproduction, a single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes. For example, single-celled eukaryotic organisms can reproduce asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells. The genomes of the offspring are virtually exact copies of the parent's genome. Some multicellular organisms are also capable of reproducing asexually. Because the cells of the offspring are derived by mitosis in the parent, the "chip off the old block" is usually genetically identical to its parent. An individual that reproduces asexually gives rise to clones, a group of genetically identical individuals. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations.

In addition to the ABO system, humans have other blood groups, including the MN system. Individuals who have alleles for both M and N show __________ for the M and N red blood cell surface antigens.

codominance -A variation in dominance relationships between alleles is called codominance; in this variation, the two alleles each affect the phenotype in separate, distinguishable ways. For example, the human MN blood group is determined by codominant alleles for two specific molecules located on the surface of red blood cells, the M and the N molecules. A single gene locus, at which two allelic variations are possible, determines the phenotype of this blood group. Individuals homozygous for the M allele (MM) have red blood cells with only M molecules; individuals homozygous for the N allele (NN) have red blood cells with only N molecules. But both M and N molecules are present on the red blood cells of individuals heterozygous for the M and N alleles (MN). Note that the MN phenotype is not intermediate between the M and N phenotypes, a fact that distinguishes codominance from incomplete dominance. Rather, both M and N phenotypes are exhibited by heterozygotes since both molecules are present.

A red bull is crossed with a white cow and all of the offspring are roan, an intermediate color that is caused by the presence of both red and white hairs. This is an example of genes that are __________.

codominant -Alleles can show different degrees of dominance and recessiveness in relation to each other. In Mendel's classic pea crosses, the F1 offspring always looked like one of the two parental varieties because one allele in a pair showed complete dominance over the other. In such situations, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable. Another variation on dominance relationships between alleles is called codominance; in this variation, the two alleles each affect the phenotype in separate, distinguishable ways. Roan is a codominant trait and caused by the presence of both red and white hairs. The red and white alleles are present at equivalent loci on homologous chromosomes. There is no indication that a second gene pair is involved, so epistasis is not an option, nor would the trait be polygenic.

Somatic cells in humans differs from gametes in that human somatic cells __________.

contain two sets of each of the 23 chromosome types -In humans, each somatic cell has 46 chromosomes. During mitosis, the chromosomes become condensed enough to be visible under a light microscope. At this point, they can be distinguished from one another by their size, the positions of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains. Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest chromosomes.The resulting ordered display is called a karyotype. The two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called homologous chromosomes, or homologs. Both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye color is situated at a particular locus on a certain chromosome, then the homolog of that chromosome will also have a version of the same gene specifying eye color at the equivalent locus. Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the haploid number is 23 (n = 23). The set of 23 consists of the 22 autosomes plus a single sex chromosome. An unfertilized egg contains an X chromosome, but a sperm may contain an X or a Y chromosome.

Regardless of whether an organism is an animal, a plant, a fungus, or an algal cell, all zygotes are __________ and are formed during the __________ of two __________ gametes.

diploid; fertilization; haploid -In general, the steps of the human life cycle are typical of many sexually reproducing animals. Indeed, the processes of fertilization and meiosis are the hallmarks of sexual reproduction in plants, fungi, and protists as well as in animals. Fertilization and meiosis alternate in sexual life cycles, maintaining a constant number of chromosomes in each species from one generation to the next. Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies depending on the species. These variations can be grouped into three main types of life cycles. In the type that occurs in humans and most other animals, gametes are the only haploid cells. Meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization. After fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid.

Achondroplasia, a type of dwarfism, and Huntington's disease are examples of __________, with the exception that the Huntington's allele is __________.

dominant inherited disorders; lethal -Although many harmful alleles are recessive, a number of human disorders are due to dominant alleles. One such example is achondroplasia, a form of dwarfism that occurs in one out of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Therefore, all people who are not achondroplastic dwarfs—99.99% of the population—are homozygous for the recessive allele. Like the presence of extra fingers or toes mentioned earlier, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele. Dominant alleles that cause lethal diseases are much less common than recessive alleles that have lethal effects. All lethal alleles arise by mutations (changes to the DNA) in cells that produce sperm or eggs; presumably, such mutations are equally likely to be recessive or dominant. A lethal recessive allele can be passed from one generation to the next by heterozygous carriers because the carriers themselves have normal phenotypes. A lethal dominant allele, however, often causes the death of afflicted individuals before they can mature and reproduce, and thus the allele is not passed on to future generations. In cases of late-onset diseases, however, a lethal dominant allele may be passed on. If symptoms first appear after reproductive age, the individual may already have transmitted the allele to his or her children. For example, a degenerative disease of the nervous system called Huntington's disease is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal. As with other dominant traits, a child born to a parent with the Huntington's disease allele has a 50% chance of inheriting the allele and the disorder. In the United States, this disease afflicts about one in 10,000 people.

Genomic imprinting in mammals appears to primarily affect genes involved in __________.

embryonic development -In recent years, geneticists have identified two to three dozen traits in mammals that depend on which parent passed along the alleles for those traits. Such variation in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting. Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes. Because these genes are imprinted differently in sperm and eggs, a zygote expresses only one allele of an imprinted gene, which it inherited from either the female or the male parent. Genomic imprinting is thought to affect only a small fraction of the genes in mammalian genomes, but most of the known imprinted genes are critical for embryonic development. The Igf2 gene in mice is an example of a growth factor required for normal prenatal development. Experiments have also shown that organisms with improperly imprinted genes often die before birth.

Darwin realized the importance of heritable variation to evolution __________.

even though he never read any of Gregor Mendel's work -Sexual reproduction greatly increases the genetic variation present in a population. Although Darwin realized that heritable variation is what makes evolution possible, he could not explain why offspring resemble—but are not identical to—their parents. Ironically, Gregor Mendel, a contemporary of Darwin, published a theory of inheritance that helps explain genetic variation, but his discoveries had no impact on biologists until 1900, more than 15 years after Darwin (1809-1882) and Mendel (1822-1884) had died.

In X-linked patterns of inheritance, __________.

fathers pass X-linked alleles only to their daughters, and mothers pass X-linked alleles to both their daughters and their sons -While most Y-linked genes help determine sex, the X chromosomes have genes for many characters unrelated to sex. X-linked genes in humans follow the same pattern of inheritance that Morgan observed for the eye-color locus he studied in Drosophila. Fathers pass X-linked alleles to all of their daughters but to none of their sons.In contrast, mothers can pass X-linked alleles to both sons and daughters, as shown below, for the inheritance of a mild X-linked disorder, red-green color blindness.

What is the result when a diploid cell undergoes meiosis?

four haploid cells -Meiosis is preceded by the duplication of chromosomes. However, this single duplication is followed by not one but two consecutive cell divisions, called meiosis I and meiosis II. These two divisions result in four haploid daughter cells each with only half as many chromosomes as the parent cell which are not genetically identical to the diploid parent cell. Two diploid cells would be the result of a diploid cell undergoing mitosis. Four cells are produced, but two of them are not diploid.

A life cycle in which the only multicellular form is haploid is most typical of __________.

fungi -Plants and some species of algae exhibit a type of life cycle called alternation of generations. This type includes both diploid and haploid stages that are multicellular. Gametes are produced during the haploid stage. A similar type of life cycle occurs in most fungi and some protists, including some algae. After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing. Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism. Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into gametes. The only diploid stage found in these species is the single-celled zygote. In most fungi, the multicellular form is haploid, and fertilization (and creation of a zygote) is immediately followed by meiosis. Primates and fish, like all animals, represent a multicellular diploid form. Plants follow the life cycle called "alternation of generations." Many protists are unicellular and do not typically go through an alternation of generations.

The chromosome theory of inheritance states that __________.

genes occupy specific positions on chromosomes, homologous chromosomes segregate from each other during meiosis, and chromosomes assort independently during meiosis -Using improved techniques of microscopy, cytologists worked out the process of mitosis in 1875 and meiosis in the 1890s. Cytology and genetics converged when biologists began to see parallels between the behavior of chromosomes and the behavior of Mendel's proposed hereditary factors during sexual life cycles: Chromosomes and genes are both present in pairs in diploid cells; homologous chromosomes separate and alleles segregate during the process of meiosis; and fertilization restores the paired condition for both chromosomes and genes. Around 1902, Walter S. Sutton, Theodor Boveri, and others independently noted these parallels, and the chromosome theory of inheritance began to take form. According to this theory, Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

Allelic variation is an important source of __________ in a population.

genetic diversity -Mutations are the original source of genetic diversity. These changes in an organism's DNA create the different versions of genes known as alleles. In meiosis I, crossing over produces chromosomes with new combinations of maternal and paternal alleles. At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alternative, nonequivalent ways with respect to other chromosomes because their sister chromatids are no longer identical. The different possible arrangements of nonidentical sister chromatids during meiosis II further increase the number of genetic types of daughter cells that can result from meiosis. The important point for now is that crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation in sexual life cycles.

Gene silencing is called __________.

genomic imprinting -Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes. Because these genes are imprinted differently in sperm and in eggs, the offspring expresses only one allele of an imprinted gene, the allele that has been inherited from either the female or the male parent. The imprints are then transmitted to all body cells during development. In each generation, the old imprints are "erased" from gamete-producing cells, and the chromosomes of the developing gametes are newly imprinted according to the sex of the individual forming the gametes. In a given species, the imprinted genes are always imprinted in the same way. For instance, a gene imprinted for maternal allele expression is always imprinted this way, generation after generation. Genomic imprinting thus seems to consist of methyl (-CH3) groups that are added to cytosine nucleotides of one of the alleles. Such methylation may silence the allele, an effect consistent with the evidence that heavily methylated genes are usually inactive.

A man who can roll his tongue and a woman who cannot roll her tongue have a son who can roll his tongue (R = can roll tongue; r = can't roll tongue). The son is curious about whether his father is homozygous or heterozygous for the tongue-rolling trait. Which of the following facts would allow him to know?

his fathers mother cannot roll her tongue -Unable to manipulate the mating patterns of people, geneticists must analyze the results of matings that have already occurred. They do so by collecting information about a family's history for a particular trait and assembling this information into a family tree describing the traits of parents and children across the generations—the family pedigree. By examining phenotypes of the son's grandparents, he can determine information regarding his parents' genotypes. If his father's mother cannot roll her tongue, then his father must be heterozygous because this trait is recessive. If the father's mother cannot roll her tongue, the father had to have inherited a recessive allele from her. Because tongue-rolling ability is dominant, knowing that his paternal grandfather and his paternal grandmother can both roll their tongues would not tell you whether his father is homozygous or heterozygous. His sister must have inherited the tongue-rolling allele from her father, but this would not help determine whether the father is homozygous or heterozygous for the trait. Knowing that the son's own daughter cannot roll her tongue would tell him that he himself is heterozygous, but would say nothing about his father. It was already clear that he was heterozygous, because his mother is homozygous recessive.

A pair of genetic structures carrying genes that control the same inherited characters are called __________.

homologous chromosomes -In humans, each somatic cell has 46 chromosomes. During mitosis, the chromosomes become condensed enough to be visible under a light microscope. At this point, they can be distinguished from one another by their size, the positions of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains. Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest chromosomes. The resulting ordered display is called a karyotype. The two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called homologous chromosomes, or homologs. Both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye color is situated at a particular locus on a certain chromosome, then the homolog of that chromosome will also have a version of the same gene specifying eye color at the equivalent locus.

Which of the following occurs in meiosis, but not mitosis?

homologous chromosomes separate -A number of key differences between meiosis and mitosis exist. Basically, meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis conserves the number of chromosome sets. Therefore, meiosis produces cells that differ genetically from their parent cell and from each other, whereas mitosis produces daughter cells that are genetically identical to their parent cell and to each other. In addition, homologous chromosomes separate during anaphase I. This event follows synapsis, an event unique to prophase I. Remember that independent assortment and crossing over occur during meiosis, producing daughter cells that are genetically unique from the parent cells. The nuclear envelope disappears during both mitosis and meiosis. Sister chromatids undergo disjunction during both mitosis and meiosis. A spindle apparatus forms during both mitosis and meiosis.

If a plant variety is true-breeding for a dominant trait, then __________.

if the plant were allowed to self-pollinate, all of the progeny would have the dominant trait -Around 1857, Mendel began breeding garden peas in the abbey garden to study inheritance. Although the question of heredity had long been a focus of curiosity at the monastery, Mendel's fresh approach allowed him to deduce principles that had remained elusive to others. One reason Mendel probably chose to work with peas is that they are available in many varieties. For example, one variety has purple flowers, while another variety has white flowers. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white color for flowers, is called a trait. Mendel chose to track only those characters that occurred in two distinct, alternative forms. For example, his plants had either purple flowers or white flowers; there were no colors intermediate between these two varieties. Mendel also made sure that he started his experiments with varieties that, over many generations of self-pollination, had produced only the same variety as the parent plant. Such plants are said to be true-breeding. For example, a plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all give rise to plants that also have purple flowers. In this case, the parent shows the dominant trait, so the progeny would as well. All the progeny would show the same phenotype, so a 3:1 phenotype ratio would not be observed among the progeny. A true breeding can also be described as homozygous, but not heterozygous. Even the genes of true-breeding varieties can mutate.

Characteristic of the bdelloid rotifer is that it __________.

is an example of an animal that has not reproduced sexually in 40 million years -The ability of sexual reproduction to generate genetic diversity is the most commonly proposed explanation for the evolutionary persistence of this process. Consider the rare case of the bdelloid rotifer. This group has apparently not reproduced sexually throughout the 40 million years of its evolutionary history. Does this mean that genetic diversity is not advantageous to this species? It turns out that bdelloid rotifers are an exception that proves the rule: This group has mechanisms other than sexual reproduction for generating genetic diversity. For example, they live in environments that can dry up for long periods of time, during which they can enter a state of suspended animation. In this state, their cell membranes may crack in places, allowing entry of DNA from other rotifers and even other species. Evidence suggests that this DNA can become incorporated into the genome of the rotifer, leading to increased genetic diversity. This supports the ideas that genetic diversity is advantageous and that sexual reproduction has persisted because it generates such diversity.

The major contribution of sex to evolution is that __________.

it provides a method to increase genetic variation -Darwin recognized that a population evolves through the differential reproductive success of its variant members. On average, those individuals best suited to the local environment leave the most offspring, thereby transmitting their genes. Thus, natural selection results in the accumulation of genetic variations favored by the environment. As the environment changes, the population may survive if, in each generation, at least some of its members can cope effectively with the new conditions. Mutations are the original source of different alleles, which are then mixed and matched during meiosis. New and different combinations of alleles may work better than those that previously prevailed. The ability of sexual reproduction to generate genetic diversity is one of the most commonly proposed explanations for the evolutionary persistence of this process. Sexual life cycles produce enormous genetic variation among offspring. Because there are asexual organisms, sex is not the only mechanism by which organisms reproduce. Sex does not allow for the inheritance of somatic mutations.

Compared to peas, fruit flies were thought to have no variations, until the discovery of a __________ fly by __________.

male white-eyed; Morgan -Although Mendel could readily obtain different pea varieties from seed suppliers, Morgan was probably the first person to search for different varieties of the fruit fly. He faced the tedious task of carrying out many matings and then microscopically inspecting large numbers of offspring in search of naturally occurring variant individuals. After many months of this, he grumbled, "Two years' work wasted. I have been breeding those flies for all that time and I've got nothing out of it." Morgan persisted, however, and was finally rewarded with the discovery of a single male fly with white eyes instead of the usual red. The phenotype for a character most commonly observed in natural populations, such as red eyes in Drosophila, is called the wild type. Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant phenotypes because the alternatives are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.

Individuals with an extra X chromosome __________.

may have subnormal intelligence or be at risk for learning disabilities - Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally. If either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome, a condition known as aneuploidy. Fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (so that the cell has 2n ‒ 1 chromosomes); the aneuploid zygote is said to be monosomic for that chromosome. If a chromosome is present in triplicate in the zygote (so that the cell has 2n + 1 chromosomes), the aneuploid cell is trisomic for that chromosome. Nondisjunction of sex chromosomes produces a variety of aneuploid conditions. Having an extra X chromosome affects males much more than females. In males, the extra X chromosome causes Klinefelter syndrome: abnormally small testes and sterility, some breast enlargement and other female body characteristics, and possible subnormal intelligence. Trisomy X females are normal for the most part, although they may be taller than average and may demonstrate learning disabilities. Trisomy X females are healthy and have no distinguishing physical features. Although males with an extra X chromosome have abnormally small testes and are sterile, trisomy X females are fertile. Individuals with XXY are males with Klinefelter syndrome.

The process called __________ reduces the chromosome number by __________.

meiosis; two consecutive cell divisions -Many of the steps of meiosis closely resemble corresponding steps in mitosis. Meiosis, like mitosis, is preceded by the duplication of chromosomes. However, this single duplication is followed by not one but two consecutive cell divisions, called meiosis I and meiosis II. These two divisions result in four daughter cells (rather than the two daughter cells of mitosis), each with only half as many chromosomes as the parent cell—one set, rather than two. The overview of meiosis in this figure shows, for a single pair of homologous chromosomes in a diploid cell, that both members of the pair are duplicated and that the copies are sorted into four haploid daughter cells. Recall that sister chromatids are two copies of one chromosome that are closely associated all along their lengths; this association is called sister chromatid cohesion. Together, the sister chromatids make up one duplicated chromosome. In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents. Homologs appear alike when viewed under a light microscope, but they may have different versions of genes, each called an allele, at corresponding loci. Homologs are not associated with each other in any obvious way except during meiosis.

Which life cycle stage is found in plants but not animals?

multicellular haploid -Plants and some species of algae exhibit a type of life cycle called alternation of generations. This type includes both diploid and haploid stages that are multicellular. The multicellular diploid stage is called the sporophyte. Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides mitotically, generating a multicellular haploid stage called the gametophyte. Gametes and zygotes are found in all life cycles. Multicellular and unicellular diploid cells are found in animals as well as plants.

The effect of the environment on a phenotype is referred to as __________.

multifactorial -Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. A single tree, locked into its inherited genotype, has leaves that vary in size, shape, and greenness, depending on their exposure to wind and sun. For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests. Even identical twins, who are genetic equals, accumulate phenotypic differences as a result of their unique experiences. Whether human characters are influenced more by genes or by the environment—in everyday terms, nature versus nurture—is a debate that we will not attempt to settle here. We can say, however, that a genotype generally is not associated with a rigidly defined phenotype but, rather, with a range of phenotypic possibilities due to environmental influences. For some characters, such as the ABO blood group system, the phenotypic range has no breadth whatsoever; that is, a given genotype mandates a very specific phenotype. Other characters, such as a person's blood count of red and white cells, vary quite a bit, depending on such factors as the altitude, the customary level of physical activity, and the presence of infectious agents. Generally, the phenotypic range is broadest for polygenic characters. Environment contributes to the quantitative nature of these characters, as we have seen in the continuous variation of skin color. Geneticists refer to such characters as multifactorial, meaning that many factors, both genetic and environmental, collectively influence phenotype.

Human ABO groups are best described as an example of __________.

multiple alleles -Only two alleles exist for the pea characters that Mendel studied, but most genes exist in more than two allelic forms. The ABO blood groups in humans, for instance, are determined by three alleles of a single gene: IA, IB, and i. A person's blood group may be one of four types: A, B, AB, or O. These letters refer to two carbohydrates—A and B—that may be found on the surface of red blood cells. A person's blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O). Matching compatible blood groups is critical for safe blood transfusions.

Which of the following is a function of mitosis in humans?

multiplication of body cells -Mitosis is the type of cell division that occurs in somatic cells. In this type of cell division, daughter cells have the same number of chromosomes as the parent cell. Somatic cells that go through mitosis are usually diploid. The multiplication of body cells is a primary function of mitosis in humans. Mitosis also enables a multicellular adult to form from a fertilized egg and produces cells for growth and tissue repair. Mitosis keeps the same number of chromosomes, so it does not produce gametes. Mitosis distributes a copy of all of the chromosomes to the two new daughter cells, keeping the chromosome number constant. Mitosis produces two identical copies of the chromosomes, so it does not contribute to genetic variability.

__________ is the failure of __________ to separate in meiosis I or the failure of __________ to separate in meiosis II.

nondisjunction; homologous chromosomes; sister chromatids -A mishap called a nondisjunction occurs when members of a pair of homologous chromosomes do not move apart properly during meiosis I or when sister chromatids fail to separate during meiosis II. In nondisjunction, one gamete receives two of the same type of chromosome, and another gamete receives no copy of that chromosome. The other chromosomes are usually distributed normally.

In meiosis, __________ of __________ cross over and form __________.

nonsister chromatids; homologous pairs; chiasmata -After interphase, the chromosomes have been duplicated, and the sister chromatids are held together by proteins called cohesins. Early in prophase I, the two members of a homologous pair associate loosely along their lengths. Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. The DNA of two nonsister chromatids—one maternal and one paternal—is broken by specific proteins at precisely corresponding points. Next, the formation of a zipper-like structure called the synaptonemal complex holds one homolog tightly to the other. During this association, called synapsis, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa. These points of crossing over become visible as chiasmata (singular, chiasma) after the synaptonemal complex disassembles and the homologs move slightly apart from each other. The homologs remain attached because sister chromatids are still held together by sister chromatid cohesion even though some of the DNA may no longer be attached to its original chromosome. At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate.

A linkage map __________.

orders genes on a chromosome based on recombination frequencies -A genetic map based on recombination frequencies is called a linkage map. An example is Sturtevant's linkage map of three genes: the body-color (b) and wing-size (vg) genes and a third gene, called cinnabar (cn). Cinnabar is one of many Drosophila genes affecting eye color. Cinnabar eyes, a mutant phenotype, are a brighter red than the wild-type color. The recombination frequency between cn and b is 9%; that between cn and vg, 9.5%; and that between b and vg, 17%. In other words, crossovers between cn and b and between cn and vg are about half as frequent as crossovers between b and vg. Only a map that locates cn about midway between b and vg is consistent with these data, as you can prove to yourself by drawing alternative maps. Sturtevant expressed the distances between genes in map units, defining one map unit as equivalent to a one percent recombination frequency. A linkage map presents the correct order of genes on the chromosome, although it is not necessarily an accurate representation of the actual distances between genes. Linkage maps can be constructed for both sex chromosomes and autosomes. The order of genes on a chromosome based on their location with respect to a stained band is a feature of a cytological map. A linkage map does not showsthe ordering and exact spacing of genes on a chromosome. However, DNA technology has made the construction of such a map possible. A linkage map cannot predict the probability that you will have a male or female child.

Unless the chromosomes were stained to show band patterns, a karyotype would be least likely to show which of the following?

part of a chromosome turned around -In humans, each somatic cell has 46 chromosomes. During mitosis, the chromosomes become condensed enough to be visible under a light microscope. At this point, they can be distinguished from one another by their size, the positions of their centromeres, and the pattern of colored bands produced by certain stains. Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. This becomes clear when images of the chromosomes are arranged in pairs, starting with the longest chromosomes. The resulting ordered display is called a karyotype. A karyotype is a display of condensed chromosomes arranged in pairs. Karyotyping can be used to screen for defective chromosomes or abnormal numbers of chromosomes associated with certain congenital disorders. An inversion, where part of a chromosome is turned around, affects neither the number of chromosomes nor the length of a chromosome, and is generally difficult to detect without staining to show chromosomal bands. An extra or missing chromosome can be detected by counting the number of chromosomes displayed. By noting the size of the chromosomes, you could see if extra genetic material is present. The attachment of a large part of a chromosome to another chromosome could be detected by looking at chromosomal length.

A family tree that describes traits in families is called a __________.

pedigree analysis -Because scientists are unable to manipulate the mating patterns of people, geneticists instead analyze the results of matings that have already occurred. They do so by collecting information about a family's history for a particular trait and assembling this information into a family tree describing the traits of parents and children across the generations—the family pedigree. The figure gelow shows a three-generation pedigree that traces the occurrence of a pointed contour of the hairline on the forehead. This trait, called a widow's peak, is due to a dominant allele, W. Because the widow's peak allele is dominant, all individuals who lack a widow's peak must be homozygous recessive (ww). The two grandparents with widow's peaks must have the Ww genotype, since some of their offspring are homozygous recessive. The offspring in the second generation who do have widow's peaks must also be heterozygous because they are the products of Ww × ww matings. The third generation in this pedigree consists of two sisters. The one who has a widow's peak could be either homozygous (WW) or heterozygous (Ww), given what we know about the genotypes of her parents (both Ww).

In humans, height and skin color have continuous variation in the population because of __________.

polygenic inheritance -Mendel studied characters that could be classified on an either-or basis, such as purple- versus white-flower color. But many characters, such as human skin color and height, are not one of two discrete characters but instead vary in the population in gradations along a continuum. These are called quantitative characters. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character. (In a way, this is the converse of pleiotropy, where a single gene affects several phenotypic characters.) Height is a good example of polygenic inheritance: A recent study using genomic methods identified at least 180 genes that affect height. Skin pigmentation in humans is also controlled by many separately inherited genes. Here, we'll simplify the story in order to explain the concept of polygenic inheritance. Let's consider three genes, with a dark-skin allele for each gene (A, B, or C) contributing one "unit" of darkness (also a simplification) to the phenotype and being incompletely dominant to the other allele (a, b, or c). In our model, an AABBCC person would be very dark, whereas an aabbcc individual would be very light. An AaBbCc person would have skin of an intermediate shade. Because the alleles have a cumulative effect, the genotypes AaBbCc and AABbcc would result in the same genetic contribution (three units) to skin darkness. There are seven skin color phenotypes that could result from a mating between AaBbCc heterozygotes. In a large number of such matings, the majority of offspring would be expected to have intermediate phenotypes (skin color in the middle range).

The zipper-like complex that forms in meiosis occurs during __________ and is called the __________.

prophase I; synaptonemal complex -After interphase, the chromosomes have been duplicated, and the sister chromatids are held together by proteins called cohesins. Early in prophase I, the two members of a homologous pair associate loosely along their lengths. Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. The DNA of two nonsister chromatids—one maternal and one paternal—is broken by specific proteins at precisely corresponding points. Next, the formation of a zipper-like structure called the synaptonemal complex holds one homolog tightly to the other. During this association, called synapsis, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa. These points of crossing over become visible as chiasmata (singular, chiasma) after the synaptonemal complex disassembles and the homologs move slightly apart from each other. The homologs remain attached because sister chromatids are still held together by sister chromatid cohesion even though some of the DNA may no longer be attached to its original chromosome. At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate.

In a cross between wild-type (red-eyed) female flies and mutant white-eyed male flies, the F1 generation should always produce females with __________ eyes and males with __________ eyes.

red-red -Morgan mated his white-eyed male fly with a red-eyed female. All the F1 offspring had red eyes, suggesting that the wild-type allele is dominant. When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio among the F2 offspring. However, there was a surprising, additional result: The white-eye trait showed up only in males. All the F2 females had red eyes, while half the males had red eyes and half had white eyes. Therefore, Morgan concluded that somehow, a fly's eye color is linked to its sex. (If the eye-color gene were unrelated to sex, half of the white-eyed flies would have been male and half would have been female.)

The ability to observe meiosis in gamete production supported Mendel's first law of __________, which applies when __________ separate, and supported Mendel's second law of __________, which applies when __________ sort __________.

segregation; homologous pairs; independent assortment; alleles; independently -Using improved techniques of microscopy, cytologists worked out the process of mitosis in 1875 (see the drawing at the lower left) and the process of meiosis in the 1890s. Cytology and genetics converged as biologists began to see parallels between the behavior of Mendel's proposed hereditary factors during sexual life cycles and the behavior of chromosomes: As shown in the figure below, chromosomes and genes are both present in pairs in diploid cells, and homologous chromosomes separate—and alleles segregate—during the process of meiosis. Furthermore, after meiosis, fertilization restores the paired condition for both chromosomes and genes.

Besides the fact that pea plants have a short generation time, the key to Mendel's successful plant-breeding experiments was that pea plants usually __________.

self-pollinate -In addition to having many varieties, peas have a short generation time and a large number of offspring from each mating. Furthermore, Mendel could strictly control mating between plants. Each pea flower has both pollen-producing organs (stamens) and an egg-bearing organ (carpel). In nature, pea plants usually self-fertilize: Pollen grains from the stamens land on the carpel of the same flower, and sperm released from the pollen grains fertilize eggs present in the carpel. To achieve cross-pollination of two plants, Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Each resulting zygote then developed into a plant embryo encased in a seed (pea). Mendel could thus always be sure of the parentage of new seeds.

Genetic diversity requires __________.

sexual reproduction, independent assortment, crossing over between nonsister chromatids of homologous chromosomes, and random fertilization -Mutations are the original source of genetic diversity. These changes in an organism's DNA create the different versions of genes known as alleles. Once these differences arise, reshuffling of the alleles during sexual reproduction produces the variation that results in each member of a sexually reproducing population having a unique combination of traits. In species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation. Three mechanisms contribute to the genetic variation arising from sexual reproduction: independent assortment of chromosomes, crossing over, and random fertilization.

With a microscope, you examine some somatic cells from a woman and notice that each nucleus has two Barr bodies. What can you infer about the sex chromosomes in this individual?

she is XXX -Female mammals, including humans, inherit two X chromosomes—twice the number inherited by males—so you may wonder whether females make twice as much as male of the proteins encoded by X-linked genes. In fact, most of one X chromosome in each cell in female mammals becomes inactivated during early embryonic development. As a result, the cells of females and males have the same effective dose (one copy) of most X-linked genes. The inactive X in each cell of a female condenses into a compact object called a Barr body that lies along the inside of the nuclear envelope. Most of the genes of the X chromosome that forms the Barr body are not expressed. This individual is XXX. Inactive X chromosomes in the cells of a female condense into compact objects known as Barr bodies. All X chromosomes in excess of one are inactivated.

How many genes are present in the human genome?

tens of thousands -Parents endow their offspring with coded information in the form of hereditary units called genes. The genes we inherit from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as shared eye color or freckles. Our genes program the specific traits that emerge as we develop from fertilized eggs into adults. The genome is an organism's entire complement of DNA, the molecule that carries the genes. Each chromosome has hundreds or thousands of genes; the entire human genome has on the order of 20,000 to 25,000 genes.

The term "true-breeding plants" means __________.

that self-pollinating plants will always produce the same trait of a particular character -Mendel chose to track only those characters that occurred in two distinct, alternative forms, such as purple or white flower color. He also made sure that he started his experiments with varieties that, over many generations of self-pollination, had produced only the same variety as the parent plant. Such plants are said to be true-breeding. For example, a plant with purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all give rise to plants that have purple flowers.

Wild type refers to __________.

the most common phenotype thought to be found in the natural population -The phenotype for a character most commonly observed in natural populations, such as red eyes in Drosophila, is called the wild type. Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.

In an X-linked, or sex-linked, trait, it is the contribution of __________ that determines whether a son will display the trait.

the mother -In humans and other mammals, there are two varieties of sex chromosomes, designated X and Y. The Y chromosome is much smaller than the X chromosome. A person who inherits two X chromosomes, one from each parent, usually develops as a female. A male develops from a zygote containing one X chromosome and one Y chromosome. Each egg contains one X chromosome. In contrast, sperm fall into two categories: Half the sperm cells a male produces contain an X chromosome, and half contain a Y chromosome. We can trace the sex of each offspring to the events of conception: If a sperm cell bearing an X chromosome happens to fertilize an egg, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an egg, the zygote is XY, a male. The human X chromosome contains approximately 1,100 genes, which are called X-linked genes. The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes. Fathers pass X-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass X-linked alleles to both sons and daughters. The son receives his single X chromosome from his mother and the Y from his father.

In people with sickle-cell disease, red blood cells break down, clump, and clog the blood vessels. The blood vessels and the broken cells accumulate in the spleen. Among other things this leads to physical weakness, heart failure, joint pain, and brain damage. Such a suite of symptoms can be explained by __________.

the pleiotropic effects of the sickle-cell allele -Sickle-cell disease is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells; in homozygous individuals, all hemoglobin is of the sickle-cell (abnormal) variety. When the oxygen content of an affected individual's blood is low (at high altitudes or under physical stress, for instance), the sickle-cell hemoglobin molecules aggregate into long rods that deform the red cells into a sickle shape. Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, pain, organ damage, and even paralysis. Pleiotropy refers to the ability of certain genes to affect multiple characters, which sickle-cell disease exhibits. Sickle-cell disease is not polygenic. The sickle-cell allele is sufficient to cause these symptoms without interacting with another gene. The sickle-cell allele is sufficient to cause these symptoms without any other interactions. Drugs do not create this symptom complex.

Mendel's law of segregation states that __________.

the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes -In terms of chromosomes, this segregation corresponds to the distribution of the two members of a pair of homologous chromosomes to different gametes in meiosis. Note that if an organism has identical alleles for a particular character—that is, the organism is true-breeding for that character—then that allele is present in all gametes. But if different alleles are present, as in the F1 hybrids, then 50% of the gametes receive the dominant allele and 50% receive the recessive allele.

When an individual has an additional chromosome, that chromosome set is called __________, and the condition is known as __________.

trisomic; aneuploidy -If either of the aberrant gametes unites with a normal gamete at fertilization, the zygote will also have an abnormal number of a particular chromosome, a condition known as aneuploidy. On the other hand, fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (and thus the cell has 2n - 1 chromosomes); the aneuploid zygote is said to be monosomic for that chromosome. If a chromosome is present in triplicate in the zygote (and thus the cell has 2n + 1 chromosomes), the aneuploid cell is trisomic for that chromosome.

X inactivation occurs when there is/are at least __________ X chromosomes present in an individual, forming a structure called a __________ by a __________ event.

two; Barr body; random -Female mammals, including human females, inherit two X chromosomes—twice the number inherited by males—so you may wonder whether females make twice as much of the proteins encoded by X-linked genes as males make. In fact, almost all of one X chromosome in each cell in female mammals becomes inactivated during early embryonic development. As a result, the cells of females and males have the same effective dose (one copy) of most X-linked genes. The inactive X in each cell of a female condenses into a compact object called a Barr body (discovered by Canadian anatomist Murray Barr), which lies along the inside of the nuclear envelope. British geneticist Mary Lyon demonstrated that the selection of which X chromosome forms the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation.

n humans, a normal male's somatic cells will have __________ set(s) of chromosomes and sex chromosomes designated as __________ and __________, while in bees and ants, a normal male's somatic cells have __________ set(s) of chromosomes, which are __________.

two; X; Y; one; haploid -Humans and other mammals have two types of sex chromosomes, designated X and Y. The Y chromosome is much smaller than the X chromosome. A person who inherits two X chromosomes, one from each parent, usually develops as a female; a male inherits one X chromosome and one Y chromosome. Bees and ants are in the haplodiploid system, where there are no sex chromosomes. Males develop from unfertilized eggs and are haploid.

Somatic cells in humans contain __________ set(s) of chromosomes and are therefore termed __________.

two; diploid -In humans, each somatic cell has 46 chromosomes. Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are two chromosomes of each of 23 types. Any cell with two chromosome sets is called a diploid cell and has a diploid number of chromosomes, abbreviated 2n. Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). Usually, human cells are not triploid.

At the end of mitosis, __________ daughter cells that are genetically __________ are formed, while at the end of meiosis, __________ daughter cells that are genetically __________ are formed.

two; identical; four; distinct -At the end of mitosis, two daughter cells that are genetically identical are formed, while at the end of meiosis, four daughter cells that are genetically distinct are formed. The four daughter cells are genetically distinct from one another and from the parent cell at the end of meiosis, while at the end of mitosis, two daughter cells, each genetically identical to the parent cell and with the same number of chromosomes, are produced.

You perform a testcross using F1 dihybrid flies. If, in the resulting offspring, the percentages of parental and recombinant offspring are about the same, this would indicate that the two genes are __________.

unlinked -Genes located near each other on the same chromosome tend to be inherited together in genetic crosses; such genes are said to be genetically linked and are called linked genes. When geneticists follow linked genes in breeding experiments, the results deviate from those expected from Mendel's law of independent assortment. To see how linkage between genes affects the inheritance of two different characters, let's examine another of Morgan's Drosophila experiments. In this case, the characters are body color and wing size, each with two different phenotypes. Wild-type flies have gray bodies and normal-sized wings. In addition to these flies, Morgan had managed to obtain, through breeding, doubly mutant flies with black bodies and wings much smaller than normal, called vestigial wings. The mutant alleles are recessive to the wild-type alleles, and neither gene is on a sex chromosome. In his investigation of these two genes, Morgan carried out a variety of crosses. The first was a P generation cross to generate F1 dihybrid flies, and the second was a testcross. The resulting flies had a much higher proportion of the combinations of traits seen in the P generation flies (called parental phenotypes) than would be expected if the two genes assorted independently. The genes in this case would be unlinked and on different chromosomes because the percentage of parental and recombinant offspring are about the same. An excess number of parental over recombinant offspring would have indicated linkage. The relative proportions of parental and recombinant offspring give clues regarding linkage, not DNA sequence. The relative proportions of parental and recombinant offspring give clues regarding linkage, not whether the genes are mutant or not. Imprinting refers to the sex-specific inactivation of certain genes during gametogenesis.

In incomplete dominance, the offspring __________.

will have an appearance that's intermediate between those of the two parental types -For some genes, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties. This phenomenon, called incomplete dominance, is seen when red snapdragons are crossed with white snapdragons: All the F1 hybrids have pink flowers. This third, intermediate phenotype results from flowers of the heterozygotes having less red pigment than the red homozygotes.

If the two traits that Mendel looked at in his dihybrid cross of smooth yellow peas with wrinkled green peas had been controlled by genes that were located near each other on the same chromosome, then the F2 generation __________.

would have deviated from the 9:3:3:1 phenotypic ratio that is predicted by the law of independent assortment -Mendel identified his second law of inheritance by following two characters at the same time, such as seed color and seed shape. Seeds (peas) may be either yellow or green. They also may be either round (smooth) or wrinkled. From single character crosses, Mendel knew that the allele for yellow seeds is dominant (Y), and the allele for green seeds is recessive (y). For the seed-shape character, the allele for round is dominant (R), and the allele for wrinkled is recessive (r). Imagine crossing two true-breeding pea varieties that differ in both of these characters—a cross between a plant with yellow round seeds (YYRR) and a plant with green-wrinkled seeds (yyrr). The F1 plants will be dihybrids, individuals heterozygous for the two characters being followed in the cross (YyRr). This type of cross, called a dihybrid cross, is a cross between F1 dihybrids, and can determine whether the different traits are inherited together or independently. The F1 plants, of genotype YyRr, exhibit both dominant phenotypes, yellow seeds with round shapes, no matter which hypothesis is correct. The key step in the experiment is to see what happens when F1 plants self-pollinate and produce F2 offspring. If the hybrids must transmit their alleles in the same combinations in which the alleles were inherited from the P generation, then the F1 hybrids will produce only two classes of gametes: YR and yr. This "dependent assortment" hypothesis predicts that the phenotypic ratio of the F2 generation will be 3:1, just as in a monohybrid cross. If the two characters are located on the same chromosome, they will not segregate independently and would not produce the 9:3:3:1 ratio that Mendel actually observed. If four phenotypes were observed in the F2 in a 9:3:3:1 ratio, then the genes would have segregated independently of each other.

Color in squash is controlled by epistatic interactions in which color is recessive to no color. At the first locus white squash (W) is dominant to colored squash (w). At the second locus yellow (Y) is dominant to green (y). What is the phenotype of a squash with the genotype wwYy?

yellow -In epistasis (from the Greek for "standing upon"), the phenotypic expression of a gene at one locus alters that of a gene at a second locus. The genotype at the first locus is homozygous recessive for colored squash, and the genotype at the second locus is heterozygous. Yellow is dominant. Therefore, the squash is yellow. The genotype is homozygous recessive for colored squash so it won't be white. This is a system of dominant/recessive alleles. There is no incomplete dominance or codominance so the squash wouldn't have multiple colors or patterns. The genotype is heterozygous for yellow, so the dominant yellow color will be expressed.

In Labrador retrievers, a dog that has the genotype BBee, where BB produces black-pigmented fur and ee produces yellow-pigmented fur, would have __________ fur and would exhibit __________.

yellow; epistasis -In epistasis (from the Greek for "standing upon"), the phenotypic expression of a gene at one locus alters the expression of a gene at a second locus. An example will help clarify this concept: In Labrador retrievers (commonly called "Labs"), black coat color is dominant to brown coat color. Let's designate B and b as the two alleles for this character. For a Lab to have brown fur, its genotype must be bb; these dogs are called chocolate Labs. But there is more to the story. A second gene determines whether or not pigment will be deposited in the fur. The dominant allele, symbolized by E, results in the deposition of either black or brown pigment, depending on the genotype at the first locus. But if the Lab is homozygous recessive for the second locus (ee), then the coat is yellow, regardless of the genotype at the black/brown locus (so-called golden Labs). In this case, the gene for pigment deposition (E/e) is said to be epistatic to the gene that codes for black or brown pigment (B/b). What happens if we mate black Labs that are heterozygous for both genes (BbEe)? Although the two genes affect the same phenotypic character (coat color), they follow the law of independent assortment. Thus, our breeding experiment would represent an F1 dihybrid cross like those that produced a 9:3:3:1 ratio in Mendel's experiments. We can use a Punnett square to represent the genotypes of the F2 offspring. As a result of epistasis, the phenotypic ratio among the F2 offspring would be 9 black to 3 chocolate to 4 golden Labs. Other types of epistatic interactions produce different ratios, but all are modified versions of 9:3:3:1.