Bio 122 Study Guide Chapter 15: The Chromosomal Basis of Inheritance

For a recessive X-linked trait to be expressed throughout the body

- a female needs two copies of the allele (homozygous) - a male needs only one copy of the allele

Polyploidy

A condition in which an organism has more than two complete sets of chromosomes. − Polyploidy is common in plants, but not animals − Polyploids are more normal in appearance than aneuploids

Imagine a human disorder that is inherited as a dominant, X-linked trait. How would the frequency of this disorder vary between males and females? A) Females would display this disorder with greater frequency than males. B) Males would display this disorder with greater frequency than females. C) Males and females would display this disorder with equal frequency.

A. Men have only one X chromosome, whereas women have two. Thus, women have twice the chance of inheriting the dominant allele that causes this disorder. Remember that the disorder is caused by a dominant allele, so an individual who inherits a single dominant allele will display the disorder. Read about the inheritance of X-linked genes.

Aneuploidy

An abnormal number of one particular chromosome (an extra chromosome or a missing chromosome).

Recombinant type

An offspring whose phenotype differs from that of the true-breeding P generation parents; also refers to the phenotype itself. When 50% of all offspring are recombinants, as in this example, geneticists say that there is a 50% frequency of recombination.

Parental types

An offspring with a phenotype that matches one of the true-breeding parental (p-generation) phenotypes.

In birds, sex is determined by ZW chromosome inheritance. Males are ZZ and females are ZW. Within pigeon populations there is a recessive, Z-linked allele that is lethal in embryos. What is the expected sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a wildtype female? A) 1:2 male to female B) 2:1 male to female C) 3:1 male to female D) 1:1 male to female

B

In cats, an X-linked locus is responsible for fur color. There are two known alleles at this locus. One results in black fur color; the other results in orange fur color. A heterozygote animal has patches of orange and black fur (tortoiseshell). Which of the following explains the patches of color in female heterozygote cats? A) Imprinting at the fur color locus B) Random X inactivation affects fur color C) Incomplete dominance of the black fur color allele D) Fur color is codominant at the organism level

B

In humans, what determines the sex of offspring, and why? A) The chromosome contribution from both parents determines sex, because the offspring uses all the parents' chromosomes. B) The male gamete determines sex, because each male gamete can contribute either an X or a Y chromosome. C) The female gamete determines sex, because only the female gametes can have one of two functional sex chromosomes. D) The male determines sex, because the sperm can fertilize either a female egg or a male egg. E) The female gamete determines sex, because only the female gamete provides cytoplasm to the zygote.

B

When Thomas Hunt Morgan crossed red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies, but all the white-eyed flies were male. Which of these best explains Morgan's result? A) The gene involved is located on the Y chromosome. B) The gene involved is located on the X chromosome. C) Other male-specific factors influence eye color in flies. D) The gene involved is located on an autosome, but only in males.

B

Gregor Mendel set up a dihybrid cross with one pea plant from the parental generation (P) producing round yellow peas and the other pea plant producing wrinkled green peas. The F2 generation included 315 plants producing round yellow peas, 108 with round green peas, 101 with wrinkled yellow peas, and 32 with wrinkled green peas. How would these results have differed if pea shape and pea color had been linked genes, located close together on the same chromosome? A) The F2 generation would have included a higher percentage of pea plants producing yellow peas. B) The F2 generation would have included a lower percentage of pea plants producing wrinkled, green peas. C) The F2 generation would have included a higher percentage of pea plants producing round, yellow peas.

C

Which of the following individuals will inherit an X-linked allele from a male parent who carries the allele? A) half of his daughters B) all of his sons C) all of his daughters D) all of his children

C

Recombination between linked genes is an adaptive advantage within a changing environment because ________. A) recombination allows genes to be shuffled along chromosomes B) recombination ensures each offspring will have an increased chance of survival C) recombination must occur or genes will not assort independently D) new allele combinations increase diversity in a population

D

Is the condition recessive or dominant?

Dominant conditions require that an affected individual have at least one affected parent. An affected offspring of two unaffected parents indicates a recessive condition

Crossing over

The reciprocal exchange of genetic material between non-sister chromatids during prophase 1 of meiosis. 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 breaks the DNA molecules of one maternal and one paternal chromatid and rejoins each to the other. In effect, when a single crossover occurs, end portions of two non-sister chromatids trade places.

The development of female gonads in humans requires a gene called

WNT4 (on chromosome 1, an autosome), which encodes a protein that promotes ovary development. An embryo that is XY but has extra copies of the WNT4 gene can develop rudimentary female gonads. Overall, sex is determined by the interactions of a network of gene products like these.

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 anatomy we associate with the "female" sex, while "male" properties are associated with the inheritance of one X chromosome and one Y chromosome ​

A deletion occurs when

a chromosomal fragment is lost. The affected chromosome is then missing certain genes.

Down syndrome is usually the result of

an extra chromosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21. ​Down syndrome​ includes characteristic facial features, short stature, correctable heart defects, and developmental delays. Individuals with Down syndrome have an increased chance of developing leukemia and Alzheimer's disease but have a lower rate of high blood pressure, atherosclerosis (hardening of the arteries), stroke, and many types of solid tumors. People with Down syndrome, on average, have a life span shorter than is typical; however, most live to middle age and beyond with proper medical care.

Because a linkage map is based strictly on recombination frequencies, it gives only an

approximate picture of a chromosome. The frequency of crossing over is not actually uniform over the length of a chromosome, as Sturtevant assumed, and therefore map units do not correspond to actual physical distances (in nanometers, for instance). A linkage map does portray the order of genes along a chromosome, but it does not accurately portray the precise locations of those genes.

Thomas Hunt Morgan's experiments with fruit flies provided convincing evidence that

chromosomes are the location of Mendel's heritable factors.

Alfred H. Sturtevant hypothesized that the percentage of recombinant offspring, the recombination frequency, calculated from experiments

dep​ends on the distance between genes on a chromosome. He assumed that crossing over is a random event, with the chance of crossing over approximately equal at all points along a chromosome. Based on these assumptions, Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. His reasoning was simple: The greater the distance between two genes, the more points there are between them where crossing over can occur. Using recombination data from various fruit fly crosses, Sturtevant proceeded to assign relative positions to genes on the same chromosomes—that is, to map genes.

A broken fragment may become reattached as an extra segment to a sister or nonsister chromatid, producing a

duplication of a portion of that chromosome.

Meiosis and random fertilization generate

genetic variation among offspring of sexually reproducing organisms due to independent assortment of chromosomes, crossing over in meiosis I, and the possibility of any sperm fertilizing any egg.

Female mammals, including human females, inherit two X chromosomes—twice the number inherited by males—so you may wonder whether females make twice as many of the proteins encoded by X-linked genes as males. 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 active 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. Most of the genes of the X chromosome that forms the Barr body are not expressed. In the ovaries, however, Barr body chromosomes are reactivated in the cells that give rise to eggs, resulting in every female gamete (egg) having an active X after meiosis.

Aneuploid conditions involving sex chromosomes appear to upset the genetic balance

less than those involving autosomes. This may be because the Y chromosome carries relatively few genes. Also, extra copies of the X chromosome simply become inactivated as Barr bodies.

A genetic map based on recombination frequencies is called a

linkage map. Sturtevant expressed the distances between genes in map units, defining one map unit as equivalent to a 1% recombination frequency.

By finding recombination frequencies for many gene pairs, we can make

linkage maps that show the order and relative distances of the genes on the chromosome.

If two genes are found on different chromosomes, or if they are far enough apart on the same chromosome that the chance of a crossover between them is very high, the genes are considered to be unlinked. Unlinked genes follow Mendel's law of independent assortment. If, however, two genes tend to "travel together" because they are near one another on the same chromosome, they are said to be

linked. Linked genes do not follow Mendel's law of independent assortment.

When genes are close together on the same chromosome, they are said to be

linked. That means the alleles, or gene versions, already together on one chromosome will be inherited as a unit more frequently than not.

In sexually reproducing species,

meiosis produces new genetic combinations in haploid gametes as a direct consequence of chromosomal behavior. Crossing over between homologs and an independent assortment of homologous pairs both lead to variation in allele combinations.

We can see if two genes are linked, and how tightly, by using data from genetic crosses to calculate the

recombination frequency.

A gene located on either sex chromosome is called a

sex-linked gene. The human X chromosome contains approximately 1,100 genes, which are called x-linked genes, while genes located on the Y chromosome are called Y-linked genes. On the human Y chromosome, researchers have identified 78 genes that code for about 25 proteins (some genes are duplicates). About half of these genes are expressed only in the testis, and some are required for normal testicular functioning and the production of normal sperm. Because there are so few Y-linked genes, very few disorders are transferred from father to son on the Y chromosome.

Large-scale chromosomal alterations in humans and other mammals often lead to

spontaneous abortion (miscarriage) of a fetus, and individuals born with these types of genetic defects commonly exhibit various developmental disorders. Plants appear to tolerate such genetic defects better than animals do.

Recessive X-linked disorders are much more common in males

than in females.

One way a triploid cell may arise is by

the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes. Tetraploidy could result from the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions would then produce a 4n embryo.

Sex linkage means that

the phenotypic ratios are different in males and females. Sex-linked inheritance lead to the discovery of sex chromosomes. The discovery of sex chromosomes was an important step in the discovery of chromosomes.

When genes are found on different chromosomes or far apart on the same chromosome, they assort independently and are said to be

unlinked.

Some disorders caused by recessive alleles on the X chromosome in humans

- Color blindness (mostly X-linked) - Duchenne muscular dystrophy - Hemophilia

Linkage

- Each chromosome has hundreds or thousands of genes (except the Y chromosome). - Genes located on the same chromosome that tend to be inherited together are called linked genes. - All of the genes on one chromosome constitute a linkage group.

The chromosome theory of inheritance

- Genes have specific locations on chromosomes, called loci. - Chromosomes undergo segregation and independent assortment (Mendel's 1st and 2nd Laws) - If genes are located on different chromosomes (autosomes), then they will follow Mendel's laws. - If genes are located close the each other on the same chromosome, then they do NOT follow Mendel's laws.

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. What are they?

- deletion - duplication - inversion - translocation

In general, what does the frequency with which crossing over occurs between two linked genes depend on? A) whether the genes are on the X or some other chromosome B) how far apart the two genes are on the chromosome C) whether the genes are dominant or recessive D) the characters the genes code for E) the phase of meiosis in which the crossing over occurs

B. The farther apart two genes are on a chromosome, the greater the probability that a crossover will occur between them, and therefore the greater the recombination frequency.

If either of the aberrant gametes during meiotic nondisjunction unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome, a condition known as

aneuploidy. (an'-yū-ploy′-dē) A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.

Sex linkage was discovered in

flies.

A chromosomal fragment may also reattach to the original chromosome but in the reverse orientation, producing an

inversion.

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.

The biochemical, physiological, and anatomical features associated with "males" and "females" are turning out to be more complicated than previously thought, with many genes involved in their development. Because of the complexity of this process,

many variations exist: Some individuals vary in the number of sex chromosomes in their cells, and ot​hers are born with intermediate sexual (intersex) characteristics, or with anatomical features that do not match an individual's sense of their own gender (transgender individuals).

Cases of extra chromosomes or missing chromosomes are caused by mistakes during

meiosis.

In mammalian testes and ovaries, the two sex chromosomes segregate during

meiosis. Each egg receives one X chromosome. In contrast, sperm fall into two categories: Half the sperm cells a male produces receive an X chromosome, and half receive a Y chromosome.

In asexually reproducing species,

mitosis, ensures faithful passage of exact genetic information to each of the two daughter cells.

Some organisms have more than two complete chromosome sets in all somatic cells. The general term for this chromosomal alteration is

polyploidy; the specific terms triploidy (3n) and tetraploidy (4n) indicate three and four chromosomal sets, respectively.

British geneticist Mary Lyon demonstrated that the selection of which X chromosome will form 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 of her cells will express one allele, while the others will express the alternate allele.

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. While there are very few Y-linked genes, many of which help determine sex, the X chromosomes have numerous genes for characters unrelated to sex. 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.

If an X-linked trait is due to a recessive allele, a female will express the phenotype only if she is homozygous for that allele. Because males have only one locus,

the terms homozygous and heterozygous lack meaning for describing their X-linked genes; the term hemizygous is used in such cases. Any male receiving the recessive allele from his mother will express the trait. For this reason, far more males than females have X-linked recessive disorders. However, even though the chance of a female inheriting a double dose of the mutant allele is much less than the probability of a male inheriting a single dose, there are females with X-linked disorders. For instance, color blindness is almost always inherited as an X-linked trait. A color-blind daughter may be born to a color-blind father whose mate is a carrie​r. Be​cause the X-linked allele for color blindness is relatively rare, though, the probability that such a man and woman will mate is low.

Rules for sex-linked inheritance:

- Males always get their sex-linked traits from their mother - Phenotype and genotype are always the same in males (no recessive or dominant) - In flies, the females have normal patterns of dominant and recessive for X-linked genes. - In mammals, females have one X inactivated, which leads to a mosaic pattern - In mammalian females there is no dominant or recessive. − Some cells will express one allele, some cells will express the other, forming mosaics.

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color blind has children with a color-blind male. What is the probability that this couple's first son will be color blind? A) 1/2 B) 2/3 C) 1/4 D) 3/4

A

Cinnabar eye color is an X-linked, recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a male having wild-type, red eyes, what percent of the F1 males will have cinnabar eyes? A) 100% B) 50% C) 25% D) 0%

A

How is the structure of chromosomes related to inheritance? A) A chromosome consists of one DNA molecule, carrying the heritable information, along with associated proteins. B) A chromosome consists of one DNA molecule, carrying the heritable information. C) A chromosome consists of two DNA molecules twisted together and carrying the heritable information. D) A chromosome consists of two DNA molecules twisted together and carrying the heritable information, along with associated proteins.

A

In cats, an X-linked locus is responsible for fur color. There are two known alleles at this locus. One results in black fur color; the other results in orange fur color. A heterozygote animal has patches of orange and black fur (tortoiseshell). Which coat color phenotypes are expected from the cross of a black female and an orange male? A) tortoiseshell females; black males B) tortoiseshell females; tortoiseshell males C) orange females; black males D) black females; orange males

A

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes a gradual deterioration of the muscles. It is seen almost exclusively in boys born to unaffected parents and usually results in death in the early teens. Is its inheritance sex-linked or autosomal? How do you know? A) The disorder's inheritance is sex-linked because it is seen almost exclusively in boys. B) The disorder's inheritance cannot be determined as sex-linked or autosomal. C) The disorder's inheritance is sex-linked because it is encoded by Y-linked genes. D) The disorder's inheritance is autosomal because sick children are born to apparently normal parents.

A

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes a gradual deterioration of the muscles. It is seen almost exclusively in boys born to unaffected parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? A) recessive B) dominant C) It cannot be determined.

A

Select the correct statement(s) about sex determination in animals. A) The mechanism of sex determination varies with different animal species. B) In all animals, males have the SRY gene and females lack this gene. C) In all animals, males are XY and females are XX.

A

Which of the following is true of an X-linked gene, but not of a Y-linked gene? A) the gene is present in both males and females B) it does not segregate like other genes C) it is expressed in half of the cells of either male or female D) it is only expressed in female offspring E) sister chromatids separate during mitosis

A

Chromosome Theory of Inheritance

According to this theory, Mendelian genes have specific loci (sites) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

If a color-blind woman married a man who had normal color vision, what would be the probable phenotypes of their children?

All the males would be color-blind, and all the females would be carriers. (Another way to say this is that ½ the offspring would be color-blind males, and ½ the offspring would be carrier females.)

Is the condition autosomal or X-linked?

An affected female offspring of two unaffected parents indicates an autosomal recessive condition. If the condition were X-linked, the male parent would be affected. To distinguish between rare X-linked and autosomal recessive conditions, choose the simpler mode for the observed pattern—that is, the mode that requires fewer unrelated individuals to carry a rare allele.

Down syndrome

An aneuploid condition that results from three copies of chromosome 21. - It is probably not lethal since chromosome 21 is the smallest human autosome, having the fewest genes. - It affects about one out of every 700 children born in the United States - The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained.

Genetic map

An ordered list of genetic loci (genes or other genetic markers) along a chromosome.

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes a gradual deterioration of the muscles. It is seen almost exclusively in boys born to unaffected parents and usually results in death in the early teens. Why is this disorder almost never seen in girls? A) For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, because such alleles are less often transmitted from parents. B) For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, since males with the recessive allele on their X chromosome die in their early teens. C) The disorder is encoded by Y-linked genes. Girls cannot have the disorder because they carry two X chromosomes and no Y chromosomes. D) The disorder is less common for girls because of features of their organisms, although they carry this allele with the same frequency as boys do.

B

The SRY gene is best described as ________. A) an autosomal gene whose product is required for the expression of genes on the X chromosome B) a gene present on the Y chromosome whose product regulates male development C) an autosomal gene whose product is required for the expression of genes on the Y chromosome D) a gene present on the X chromosome whose product regulates female development

B

When two genes are located on the same chromosome, what is the physical basis for the production of recombinant offspring in a testcross between a dihybrid parent and a double-mutant (recessive) parent?

Crossing over during meiosis I in the heterozygous parent produces some gametes with recombinant genotypes for the two genes. Offspring with a recombinant phenotype arise from fertilization of the recombinant gametes by homozygous recessive gametes from the double-mutant parent.

Why are specific alleles of two distant genes more likely to show recombination than those of two closer genes?

Crossing over results in new combinations of alleles. Crossing over is a random occurrence, and the more distance there is between two genes, the more chances there are for crossing over to occur, leading to new allele combinations.

A man with normal vision who is a dwarf due to achondroplasia has children with a woman with colorblindness who is of average height. Dwarfism caused by achondroplasia is autosomal dominant, and red-green color blindness is X-linked recessive. What proportion of their daughters are expected to be color-blind with achondroplasia? A) 1/4 B) 3/4 C) 1/2 D) 0

D

A testcross performed with F1 dihybrid flies results in more parental-type offspring than recombinant-type offspring. Which of the following statements best explains this result? A) The two genes are linked but on different chromosomes. B) Both of the characters are controlled by more than one gene. C) Recombination did not occur in the cell during meiosis. D) The two genes are linked on the same chromosome.

D

What name is given to the most common phenotype in a natural population? A) mutant phenotype B) locus C) genotype D) wild type E) autosome

D

Which of the following events will result in recombination between linked genes? A) Nonhomologous chromosomes breaking and then rejoining with one another. B) Linked genes traveling together at anaphase. C) Incomplete independent assortment. D) Crossovers between genes resulting in chromosomal exchange.

D

Which of the following statements regarding gene linkage is correct? A) Unlinked genes do not follow the law of equal segregation. B) The observed frequency of recombination of two genes that are far apart from each other has a maximum value of 100%. C) Linked genes are found on different chromosomes. D) The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.

D

During a single crossover event, how many strands of DNA must break? (recall that DNA is double stranded) A) one strand B) two strands C) eight strands D) four strands

D. After interphase, the chromosomes have been duplicated and the two members of the homologous pair associate along their length. The double-stranded DNA molecules of two non-sister chromatids--one maternal and one paternal--are broken at precisely corresponding points. During synapsis, each of the two double-stranded DNA breaks (leaving four broken ends) is closed up so that each broken (double-stranded) end is joined to the corresponding broken (double stranded) end of the non-sister chromatid, producing crossovers.

What observable cellular process explains Mendel's Law of Segregation? A) sister chromatids separating during mitosis B) the replication of DNA C) homologous chromosomes separating during meiosis II D) homologous chromosomes separating during meiosis I E) the behavior of sex-linked genes

D. Homologous chromosomes each carry a single allele for every gene. Those alleles may be the same or different from each other, but they are segregated during meiosis I and distributed to each gamete.

A cell goes through S phase, meiosis I, and cytokinesis. As the daughter cells are beginning meiosis II, which of the following is an appropriate description of the contents of each cell? A) Each has half the chromosomes and one-fourth the amount of DNA as the original cell B) Each has half the chromosomes and twice the amount of DNA as the original cell C) Each has half the chromosomes and half the amount of DNA as the original cell D) Each has half the chromosomes and the same amount of DNA as the original cell

D. Homologous pairs separate during meiosis I, reducing the number of chromosomes sets from two (diploid) to one (haploid). As each daughter cell begins meiosis II, it has one set of chromosomes-- half the chromosomes found in the original. However, since each of these chromosomes was duplicated during S phase, the daughter cell has the same amount of DNA as the original cell.

A sexually reproducing animal has two unlinked genes, one for head shape (H) and one for tail length (T) . Its genotype is HhTt. Which of the following genotypes for these two genes is possible in a gamete from this organism? A) HhTt B) Tt C) T D) HT

D. Mendel's law of segregation states that the two alleles for each heritable character (in this case, head shape or tail length) segregate during gamete formation and end up in different gametes. The law of independent assortment states that each pair of alleles from unlinked genes will assort independently of any other allele pair during gamete formation. Therefore, each gamete will receive one allele for head shape and one allele for tail length, resulting in the following possible genotypes of gametes produced by this organism: HT, hT, Ht, and ht.

A white-eyed female Drosophila is crossed with a red-eyed male Drosophila. Which statement below correctly describes the results? A) None of the females will have red eyes. B) Twenty-five percent of the females will have white eyes. C) Fifty percent of the females will have red eyes. D) Twenty-five percent of the females will have red eyes. E) None of the females will have white eyes.

E. All of the females will have red eyes because they will inherit the dominant red-eye allele from their male parent.

Consider the following family history: Bob has a genetic condition that affects his skin. Bob's wife, Eleanor, has normal skin. No one in Eleanor's family has ever had the skin condition. Bob and Eleanor have a large family. Of their eleven children, all six of their sons have normal skin, but all five of their daughters have the same skin condition as Bob. Based on Bob and Eleanor's family history, what inheritance pattern does the skin condition most likely follow? A) autosomal dominant B) X-linked recessive C) Y-linked D) autosomal recessive E) X-linked dominant

E. If the skin condition is caused by an X-linked dominant allele, a father would pass the allele on to all of his daughters, who would all have the skin condition. In contrast, the father would not pass the allele on to any of his sons because the sons would receive the father's Y chromosome, not his X chromosome. As a result, none of the sons would inherit the skin condition.

We can trace the sex of each offspring to the events of conception

If a sperm cell bearing an X chromosome fertilizes an egg, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an egg, the zygote is XY, a mal​​e. Thus, in ​​general, sex determination is a matter of chance—a fifty-fifty chance. Note that the mammalian X-Y system isn't the only chromosomal system for determining sex.

Why are inversions and reciprocal translocations less likely to be lethal than are aneuploidy, duplications, deletions, and nonreciprocal translocations?

In inversions and reciprocal translocations, the same genetic material is present in the same relative amount but just organized differently. In aneuploidy, duplications, deletions, and nonreciprocal translocations, the balance of genetic material is upset, as large segments are either missing or present in more than one copy. Apparently, this type of imbalance is very damaging to the organism. (Although it isn't lethal in the developing embryo, the reciprocal translocation that produces the Philadelphia chromosome can lead to a serious condition, cancer, by altering the expression of important genes.)

About 5% of individuals with Down syndrome have a chromosomal translocation in which a third copy of chromosome 21 is attached to chromosome 14. If this translocation occurred in a parent's gonad, how could it lead to Down syndrome in a child?

In meiosis, a combined 14-21 chromosome will behave as one chromosome. If a gamete receives the combined 14-21 chromosome and a normal copy of chromosome 21, trisomy 21 will result when this gamete combines with a normal gamete (with its own chromosome 21) during fertilization.

Why are males affected by X-linked disorders much more often than females?

Males have only one X chromosome, along with a Y chromosome, while females have two X chromosomes. The Y chromosome has very few genes on it, while the X has about 1,000. When a recessive X-linked allele that causes a disorder is inherited by a male on the X from his mother, there isn't a second allele present on the Y (males are hemizygous), so the male has the disorder. Because females have two X chromosomes, they must inherit two recessive alleles in order to have the disorder, a rarer occurrence.

Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and that between A and C is 12%. Can you determine the linear order of these genes? Explain.

No. The order could be A-C-B or C-A-B. To determine which possibility is correct, you need to know the recombination frequency between B and C.

Which one of Mendel's laws describes the inheritance of alleles for a single character? Which law relates to the inheritance of alleles for two characters in a dihybrid cross?

The law of segregation describes the inheritance of alleles for a single character. The law of independent assortment of alleles describes the inheritance of alleles for two characters.

Wild type

The phenotype most commonly observed in natural populations; also refers to the individual with that phenotype. 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.

Genetic recombination

The production of offspring with combinations of traits that differ from those found in either P generation parent.

Sex linkage usually refers to genes on the

X chromosome. Genes on the Y chromosome are called Y-linked genes; but there are very few of these. Most of these genes are related to fertility and sex determination. The X chromosome has many genes, most are unrelated to sex.

In humans, the anatomical signs of sex begin to emerge when the embryo is about 2 months old. Before then, the rudiments of the gonads are generic—they can develop into either testes or ovaries, depending on whether or not a

Y chromosome is present, and depending on what genes are active. A gene on the Y chromosome—called SRY, for sex-determining region of Y—is required for the development of testes. In the absence of SRY, the gonads develop into ovaries, even in an XY embryo.

A 50% frequency of recombination testcrosses is observed for

any two genes that are located on different chromosomes and thus cannot be linked. The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase I of meiosis, which leads to the independent assortment of the two unlinked ge​nes.

Without special signals, the human body develops as a

female (the default option). Sex determination is the way that male and female bodies get made. Sex chromosomes ultimately govern the process, but there are many steps leading to male and female bodies. The SRY gene on the Y chromosome codes for a protein that directs the development of male bodies in mammals, it overrides the default option and is the first step in male sexual development. Sex steroids have a major role in creating male and female bodies (SRY leads to testes development and male sex steroids). Different species have different sex chromosomes and different types of sex determination (different ways to make males and females).

Inactivation of an X chromosome involves

modification of the DNA and proteins bound to it called histones, including attachment of methyl groups (—CH3) to DNA nucleotides.

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.

Random mutations are the source of all

new alleles, which can lead to new phenotypic traits.

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 nondisjunction, one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally.

A fourth possible result of chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement called a

translocation.

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. Mitosis will subsequently transmit the anomaly to all embryonic cells.

Some genes on a chromosome are so far from each other that a crossover between them is

virtually certain. The observed frequency of recombination in crosses involving two such genes can have a maximum value of 50%, a result indistinguishable from that for genes on different chromosomes. In this case, the physical connection between genes on the same chromosome is not reflected in the results of genetic crosses. Despite being on the same chromosome and thus being physically connected, the genes are genetically unlinked; alleles of such genes assort independently, as if they were on different chromosomes.