Bio 122 Chapter 14 Study Guide: Mendel and the Gene Idea
Epistasis
A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.
Monohybrids
An organism that is heterozygous with respect to a single gene of interest. All of the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa. We refer to a cross between such heterozygotes as a monohybrid cross.
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.
How do we determine the probability that two or more independent events will occur together in some specific combination? For example, what is the chance that two coins tossed simultaneously will both land heads up?
The multiplication rule states that to determine the probability of one event and the other occurring, we multiply the probability of one event (one coin coming up heads) by the probability of the other event (the other coin coming up heads). By the multiplication rule, then, the probability that both coins will land heads up is 1/2*1/2= 1/4.
Mendel's model
- alternative versions of genes account for variations in inherited characters. - for each character, an organism inherits two versions (that is, two alleles) of a gene, one from each parent. - if the two alleles at a locus 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 law of segregation, states that the two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes.
How would you determine whether the curl allele is dominant or recessive? A) Matings of the original mutant cat with true-breeding noncurl cats will produce both curl and noncurl F1 offspring if the curl allele is dominant, but only noncurl offspring if the curl allele is recessive. B) The allele can only be determined by methods of molecular genetics. C) Matings of the original mutant cat with true-breeding noncurl cats will produce both curl and noncurl F1 offspring if the curl allele is recessive, but only noncurl offspring if the curl allele is dominant. D) Matings of the original mutant cat with true-breeding noncurl cats will produce only curl F1F1 offspring if the curl allele is dominant, but both curl and noncurl offspring if the curl allele is recessive.
A
An individual heterozygous for eye color, skin color, and number of eyes mates with an individual who is homozygous recessive for all three characters; what would be the expected phenotypic ratio of their offspring? [Hint: B = black eyes, b = orange eyes; G = green skin, g = white skin; C = two eyes, c = one eye]
1 black eyes, green skin, two eyes : 1 black eyes, green skin, one eye : 1 black eyes, white skin, two eyes : 1 black eyes, white skin, one eye : 1 orange eyes, green skin, two eyes : 1 orange eyes, green skin, one eye : 1 orange eyes, white skin, two eyes : 1 orange eyes, white skin, one eye. 1:1:1:1:1:1:1:1 ratio is the expected outcome of a BbGgCc x bbggcc cross.
The probability scale ranges from 0 to 1. An event that is certain to occur has a probability of
1, while an event that is certain not to occur has a probability of 0. With a coin that has heads on both sides, the probability of tossing heads is 1, and the probability of tossing tails is 0. With a normal coin, the chance of tossing heads is 1/2, and the chance of tossing tails is 1/2.
Quantitative characters vary in a population along a continuum. How do such characters differ from the characters investigated by Mendel in his experiments on peas? A) Quantitative characters are due to polygenic inheritance, the additive effects of two or more genes on a single phenotypic character. A single gene affected all but one of the pea characters studied by Mendel. B) Environment and genes affect quantitative characters, whereas only genes determined the pea characters studied by Mendel. C) The nature of inheritance of quantitative characters is poorly understood, and Mendel understood the nature of inheritance for the characters he studied in his peas.
A
When Mendel set up a Parental (P) cross between true breeding purple and white flowered plants to generate the F1 and then allowed the F1 to self-pollinate to generate the F2 he saw a dominant to recessive ratio of 3:1. What phenotypic ratio would be expected if he crossed the F1 with the original purple parent? A) 4:0 B) 9:3:3:1 C) 1:1 D) 3:1
A
Which of the following statements correctly describes the terms monohybrid cross and dihybrid cross? A) A dihybrid cross involves organisms that are heterozygous for two characters that are being studied, and a monohybrid cross involves organisms that are heterozygous for only one character being studied. B) A monohybrid cross involves a single parent, whereas a dihybrid cross involves two parents. C) A monohybrid cross is performed for one generation, whereas a dihybrid cross is performed for two generations. D) A monohybrid cross results in a 9:3:3:1 ratio, whereas a dihybrid cross gives a 3:1 ratio
A
Which of the following statements correctly explains the fact that all seven of the pea plant traits studied by Mendel obeyed the principle of independent assortment? A) All of the genes controlling the traits were on different chromosomes. B) None of the traits obeyed the law of segregation. C) The diploid number of chromosomes in the pea plants was seven. D) All of the genes controlling the traits had only two different alleles.
A
What number and types of chromosomes are found in a somatic cell in an animal with a diploid number of 48? A) 48 chromosomes (24 from each parent, including one sex chromosome from each) B) 24 chromosomes (12 from each parent, including one sex chromosome from each) C) 23 chromosomes (11 from each parent, plus one sex chromosome) D) 96 chromosomes (48 from each parent, including one pair of sex chromosomes from each)
A. All somatic cells in animals have two sets of chromosomes, one haploid set (n) from each parent, and each set includes one sex chromosome. For an animal with a diploid number of 48 (2n=48), the number of chromosomes in the somatic cells is 48 and the number of chromosomes in each haploid set is 24 (n=24). Humans have a diploid number of 46 (2n=46), and the number of chromosomes in each set is 23 (n=23), including one sex chromosome.
Two mice are heterozygous for albinism (Aa). The dominant allele (A) codes for normal pigmentation, and the recessive allele (a) codes for no pigmentation. What percentage of their offspring would have an albino phenotype? A) 25 B) 75 C) 100 D) 50
A. The offspring would be in a 3:1 ratio of normally pigmented mice to albino mice.
In order to determine the genotype of a MendAlien with black eyes and green skin, you would cross this individual with a(n) _____ individual. A) bbgg B) bbGG C) BBGG D) BBgg
A. This is an extension of a testcross for a single character.
A tall, purple-flowered pea plant (TtPp) is allowed to self-pollinate. (The recessive alleles code for short plants and white flowers.) The phenotypic ratio of the resulting offspring is 9:3:3:1. What is the genotype of the plant whose phenotype appeared once out of every 16 offspring (the "1" in the 9:3:3:1 ratio)? A) TtPp B) ttpp C) TTpp D) TTPP
B
Assuming independent assortment at all loci, what is the probability that a cross between the following parents, AABbCc × AaBbCc, will produce an AaBbCc offspring? A) 1/2 B) 1/8 C) 1/16 D) 3/4
B
How could you be sure the cats are true-breeding? A) You know that cats are true-breeding when curl × noncurl matings produce only curl offspring. B) You know that cats are true-breeding when curl × curl matings produce only curl offspring. C) You know that cats are true-breeding when curl × curl matings produce curl offspring. D) It can only be checked by methods of molecular genetics.
B
How would you obtain true-breeding curl cats? A) You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F2 cats resulting from the F1 cats × F1 cats crosses no matter whether the curl trait is dominant or recessive. B) You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F1 cats resulting from the original curl × noncurl crosses no matter whether the curl trait is dominant or recessive. C) You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F1 cats resulting from the original curl × noncurl crosses but only if the curl trait is recessive. D) You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F1 cats resulting from the original curl × noncurl crosses but only if the curl trait is dominant.
B
In his breeding experiments, Mendel first crossed true-breeding plants to produce a second generation, which were then allowed to self-pollinate to generate the offspring. How do we name these three generations? A) F, P1, and P2 B) P, F1, and F2 C) P1, P2, and F D) P1, P2, and P3 E) F1, F2, and F3
B
When constructing a Punnett square, the symbols on the outside of the boxes represent _______, while those inside the boxes represent _______. A) parents, gametes B) gametes, progeny C) gametes, parents D) progeny, gametes
B
Which of the following describes why Mendel continued some of his experiments into the F2 or F3 generation? A) Collecting data from more generations allowed him to obtain a larger number of offspring. B) Following multiple generations allowed him to determine whether a recessive trait would reappear. C) Data from the F1 generation did not allow him to distinguish which alleles were segregating. D) Determining whether a dominant trait would reappear required multiple dihybrid crosses.
B
You know that alleles are alternative versions of a gene. What makes alleles different from each other? A) They are located at different loci on the chromosome. B) They have different sequences of DNA nucleotides. C) They are situated on non-homologous chromosomes. D) They are homozygous.
B
How could the botanist best determine whether the genotype of the green-pod plant is homozygous or heterozygous? A) Cross the green-pod plant with another green-pod plant. B) Cross the green-pod plant with a yellow-pod plant. C) Self-pollinate the green-pod plant.
B. A cross between a plant of unknown genotype and one that is known to be homozygous recessive is called a test cross because the recessive homozygote tests whether there are any recessive alleles in the unknown. Because the recessive homozygote will contribute an allele for the recessive characteristic to each offspring, the second allele (from the unknown genotype) will determine the offspring's phenotype.
Black eyes are dominant to orange eyes, and green skin is dominant to white skin. A male MendAlien with black eyes and green skin, has a parent with orange eyes and white skin. A female MendAlien with orange eyes and white skin. If these male and female MendAliens were to mate, the predicted phenotypic ratio of their offspring would be _____. A) 3 black eyes, green skin : 3 black eyes, white skin : 9 orange eyes, green skin : 1 orange eyes, white skin B) 1 black eyes, green skin : 1 black eyes, white skin : 1 orange eyes, green skin : 1 orange eyes, white skin C) 1 black eyes, green skin : 3 black eyes, white skin : 3 orange eyes, green skin : 9 orange eyes, white skin D) There is insufficient information to determine Sam's genotype. E) 9 black eyes, green skin : 3 black eyes, white skin : 3 orange eyes, green skin : 1 orange eyes, white skin
B. Sam's genotype is BbGg, and Carole's genotype is bbgg.
A phenotypic ratio of 9:3:3:1 in the offspring of a cross indicates that _____. A) both parents are homozygous dominant B) both parents are heterozygous for both genes C) one parent is heterozygous and one parent is homozygous recessive D) one parent is homozygous dominant and one parent is homozygous recessive E) one parent is homozygous dominant and one parent is heterozygous
B. Such a result indicates that the genes assort independently and that, for each gene, the alleles exhibit a dominant/recessive relationship.
Alleles
Different forms of a gene. The purple-flower allele and the white-flower allele are two DNA sequence variations possible at the flower-color locus on a pea plant's chromosomes. The purple-flower allele sequence allows synthesis of purple pigment, and the white-flower allele sequence does not.
Normal hemoglobin is a tetramer, consisting of two molecules of B-globin and two molecules of a-globin; normal hemoglobin molecules do not associate with each other. In sickle-cell disease, the change in a single amino acid results in mutant hemoglobin tetramers, which associate with each other and assemble into large fibers. Based on this information alone, what can we conclude about the changes in the structure of sickle cell hemoglobin as compared to normal hemoglobin? A) altered secondary and tertiary structure B) altered primary and quaternary structure C) altered primary structure only D) altered quaternary structure only
B. The primary structure of a protein is its sequence of amino acids, secondary structure is the result of hydrogen bonds between the repeating constituents of the polypeptide backbone, tertiary structure is the three-dimensional shape stabilized by interactions between side chains, and quaternary structure results when two or more polypeptides associate with each other. Normal hemoglobin tetramers do not associate with each other. The single amino acid change that causes sickle-cell disease alters the primary and quaternary structure of the protein, and as a result, the abnormal hemoglobin tetramers aggregate into chains. Not enough information is given to determine whether secondary and tertiary interactions are affected.
A diploid animal is dihybrid at the Head shape (H) and Tail length (T) loci. Which of the following gamete genotypes can it produce? A) HhTt B) Hh C) Ht D) HHTT
C
An obstetrician knows that her patient's fetus is at risk for a serious disorder that is detectable biochemically in fetal cells. The obstetrician would most reasonably offer which of the following procedures to her patient? A) blood transfusions B) sonogram C) amniocentesis or CVS D) karyotyping of the woman's somatic cells
C
In a situation in which genes assort independently, what is the ratio of the gametes produced by an AaBB individual? A) 1 A : 1 B B) 3 A : 1 B C) 1 AB : 1 aB D) 3 AB : 1 ab E) 3 AA : 1 BB
C
Pea plants produce either purple or white flowers with purple showing complete dominance. A gardener was given plants with purple flowers. Which of the following types of crosses would best allow her to determine the genotype of her plant in one generation? A) Self-pollination B) A dihybrid cross C) A testcross D) A monohybrid cross
C
If an organism with the genotype AaBb produces gametes, what proportion of the gametes would be Bb? A) 1/4 B) 1/2 C) None D) 3/4
C. Alleles of the same gene must separate during gamete formation; thus, the two B alleles would be distributed to different gametes.
Which of the following do you expect if an individual is heterozygous for the sickle-cell trait? A) they will have full-blown sickle-cell disease because the allele is dominant B) they will be more apt to acquire a serious case of malaria C) they will not develop sickle-cell disease D) they will show some symptoms of the disease because the allele that causes sickle-cell disease is not really recessive E) none of the listed choices are correct
C. Two recessive alleles are required for someone to manifest full-blown sickle-cell disease. Heterozygotes will be healthy. However, because these individuals produce both normal and abnormal hemoglobin ,they may suffer from some symptoms of the disease.
Folk singer Woody Guthrie died of Huntington's disease, an autosomal dominant disorder. Which statement below must be true? A) His daughters will die of Huntington's disease but not his sons. B) All of his children will develop Huntington's disease. C) It is very likely that at least one of Woody Guthrie's parents also had the allele for Huntington's disease. D) His sons will develop Huntington's disease but not his daughters. E) There is not enough information to answer the question.
C. Unless the disease is caused by a new mutation, which is quite rare, individuals with a dominant condition must have inherited the dominant allele from one of their parents. As it happens, Guthrie's mother also died of Huntington's disease.
What is the difference between heterozygous and homozygous individuals? A) Heterozygotes carry two copies of a gene while homozygotes only carry one. B) The homozygote will express the dominant trait and the heterozygote will express the recessive trait. C) Homozygotes have one chromosome while heterozygotes have two similar chromosomes. D) All of the gametes from a homozygote carry the same version of the gene while those of a heterozygote will differ.
D
The information contained in DNA is used to make which of the following products? A) tRNA B) mRNA only C) proteins only D) proteins, mRNA, and tRNA
D. The information that programs all of a cell's activities is encoded in the sequence of DNA nucleotides. DNA directs RNA synthesis and, through mRNA, directs the order of amino acids during protein synthesis; this entire process is called gene expression. Two types of RNA required for protein synthesis are mRNA and tRNA. (Other RNAs, such as rRNAs, are also necessary, and these too are encoded by genes in DNA.
Every gene is a sequence of
DNA nucleotides at a specific position along a chromosome called a locus.
If a plant variety is true-breeding for a dominant trait, then __________. A) if the plant were allowed to self-pollinate, the dominant and recessive traits would consistently appear in a 3:1 ratio among the progeny B) the plant is heterozygous for the trait C) if the plant were crossed with a heterozygote, one-half of the progeny would show the dominant trait, and one-half would show the recessive trait D) the variety is unable to mutate E) if the plant were allowed to self-pollinate, all of the progeny would have the dominant trait
E
You cross a true-breeding, red-flowered snapdragon with a true-breeding, white-flowered one. All of the F1 are pink. What does this say about the parental traits? A) pink is dominant, and red and white are recessive B) red and white are codominant C) both red and white are pleiotropic D) red is completely dominant E) red shows incomplete dominance over white
E
In the following cross the genotype of the female parent is BbGg. She has green skin and black eyes. The father has green skin and black eyes. The phenotypes of the F1 generation is 300 (black eyes/green skin: 100 black eyes/white skin. What is the genotype of the male parent? [Hint: B = black eyes, b = orange eyes, G = green skin, g = white skin] A) BbGg B) BbGG C) bbGG D) BBGG E) BBGg
E. All of the offspring have black eyes, and there is a 3:1 ratio of skin color.
A BbGg x bbgg cross yields a phenotypic ratio of approximately 5 black eyes, green skin : 5 orange eyes, white skin: 1 black eyes, white skin : 1 orange eyes, green skin. Which of the following best explains these results? A) Mendel's law of segregation is being violated. B) The genes for eye color and skin color are co-dominant. C) The heterozygous individual is male, and the homozygous individual is female. D) Mendel's laws of segregation and independent assortment are being violated. E) Mendel's law of independent assortment is being violated.
E. If the genes for eye color and skin color assorted independently, then the outcome of this cross would have been a 1:1:1:1 ratio.
A cross between two individuals that are heterozygous for eye and skin color would be an example of a _____ cross. A) test B) difficult C) trihybrid D) monohybrid E) dihybrid
E. This is a cross of heterozygotes for two characters.
True or false? In diploid organisms, a dominant phenotype will only be expressed if the individual is homozygous dominant for that trait.
False. A dominant phenotype is indeed expressed if the individual is homozygous dominant for that trait, but the dominant phenotype is also expressed if the individual is heterozygous for the trait. In fact, heterozygous expression is the definition of dominant.
Allowing F1 hybrids to self-pollinate (or to cross-pollinate with other F1 hybrids) produces an
F2 generation (second filial generation).
Modern genetics began during the mid-1800s with a monk named
Gregor Mendel, who discovered the basic principles of heredity by breeding garden peas in carefully planned experiments..
Carrier
In genetics, an individual who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring.
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 from the Latin word for "son").
In some pea plant crosses, the plants are self-pollinated. Is self-pollination considered asexual or sexual reproduction? Explain.
Self-pollination is sexual reproduction because meiosis is involved in forming gametes, which unite during fertilization. As a result, the offspring in self-pollination are genetically different from the parent.
Pleiotropy
The ability of a single gene to have multiple effects. 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. In the garden pea, the gene that determines flower color also affects the color of the coating on the outer surface of the seed, which can be gray or white.
A rooster with gray feathers and a hen of the same phenotype produce 15 gray, 6 black, and 8 white chicks. What is the simplest explanation for the inheritance of these colors in chickens? What phenotypes would you expect in the offspring of a cross between a gray rooster and a black hen?
The black and white alleles are incompletely dominant, with heterozygotes being gray in color. A cross between a gray rooster and a black hen should yield approximately equal numbers of gray and black offspring.
The law of segregation
The two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes. Mendel's law states that the pairs of homologous chromosomes separate in meiosis so that only one chromosome from each pair is present in each gamete. The gene for flower color in pea plants, for example, exists in two versions, one for purple flowers and the other for white flowers.
True or false? The same phenotype can be produced by more than one genotype.
True. Since there exist dominant and recessive versions of many genes, a phenotype that is based upon the dominant version will be expressed in both homozygous (AA) and heterozygous (Aa) genotypes.
Law of independent assortment
Two or more genes assort independently—that is, each pair of alleles segregates independently of any other pair of alleles—during gamete formation. This law applies only to genes (allele pairs) located on different chromosomes (that is, on chromosomes that are not homologous) or, alternatively, to genes that are very far apart on the same chromosome.
Variations in inherited characteristics is due to the presence of
alleles, which are alternative versions of genes.
A heritable feature that varies among individuals, such as flower color, is called a
character.
Alleles can show different degrees of dominance and recessiveness in relation to each other. In Mendel's classic pea crosses, the F1F1 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.
Imagine crossing two true-breeding pea varieties that differ in both see color and see shape 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).
Only two alleles exist for the pea characters that Mendel studied, but most genes
have more than two alleles. The ABO blood groups in humans, for instance, are determined by the two alleles a person has of the blood group gene; the three possible alleles are I^A, I^B, 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 attached to specific cell-surface molecules on red blood cells. An individual's blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O), along with the relevant genotypes.
An organism that has two different alleles for a gene is called a
heterozygote and is said to be heterozygous for that gene. Unlike homozygotes, heterozygotes produce gametes with different alleles, so they are not true-breeding. For example, P- and p-containing gametes are both produced by our F1 hybrids.
Diploid cells have two sets of chromosomes, one set inherited from each parent, that form
homologous pairs.
An organism that has a pair of identical alleles for a gene encoding a character is called a
homozygote and is said to be homozygous for that gene. Homozygous plants "breed true" because all of their gametes contain the same allele—either P or p in this example. If we cross dominant homozygotes with recessive homozygotes, every offspring will have two different alleles—Pp.
For some genes, neither allele is completely dominant, and the F1F1 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. (This is unlike the case of Mendel's pea plants, where the Pp heterozygotes make enough pigment for the flowers to be purple, indistinguishable from those of PP plants.)
During which part of meiosis (meiosis I or meiosis II) do the two alleles of a gene separate? During which phase does the separation occur?
meiosis I, anaphase
When a disease-causing recessive allele is rare, it is relatively unlikely that two carriers of the same harmful allele will meet and mate. The probability of passing on recessive traits increases greatly, however, if the man and woman are close relatives (for example, siblings or first cousins). This is because people with recent common ancestors are
more likely to carry the same recessive alleles than are unrelated people. Thus, these consanguineous ("same blood") matings, indicated in pedigrees by double lines, are more likely to produce offspring homozygous for recessive traits—including harmful ones. Such effects can be observed in many types of domesticated and zoo animals that have become inbred.
In place of breeding experiments, geneticists analyze the results of human matings that have already occurred. They collect information about a family's history for a particular trait and assemble this information into a family tree describing the trait across the generations—a family
pedigree.
we distinguish between an organism's appearance or observable traits, called its
phenotype, and its genetic makeup, its genotype.
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.
True-breeding
referring to organisms that produce offspring of the same variety over many generations of self-pollination. 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.
Breeding an organism of unknown genotype with a recessive homozygote is called a
testcross because it can reveal the genotype of that organism. Given a purple-flowered pea plant, we cannot tell if it is homozygous (PP) or heterozygous (Pp) because both genotypes result in the same purple phenotype. To determine the genotype, we can cross this plant with a white-flowered plant (pp), which will make only gametes with the recessive allele (p). The allele in the gamete contributed by the purple-flowered plant of unknown genotype will therefore determine the appearance of the offspring. If all the offspring of the cross have purple flowers, then the purple-flowered mystery plant must be homozygous for the dominant allele, because a PP×pp cross produces all Pp offspring. But if both the purple and the white phenotypes appear among the offspring, then the purple-flowered parent must be heterozygous. The offspring of a Pp×pp cross will be expected to have a 1:1 phenotypic ratio.
If an individual has a genetic condition that neither parent has, then that condition must be recessive. Dominant conditions require
that every affected individual have at least one affected parent. In situations where the inheritance mode of a rare condition cannot be definitely determined, the most likely mode is the one that requires the fewest unrelated individuals to have the condition-causing allele.
We refer to phenomena such as coin tosses as independent events. Each toss of a coin, whether done sequentially with one coin or simultaneously with many, is independent of every other toss. And like two separate coin tosses,
the alleles of one gene segregate into gametes independently of another gene's alleles (the law of independent assortment).
Mendel's quantitative analysis of the F2F2 plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity, now called
the law of segregation and the law of independent assortment.
According to the addition rule,
the probability that any one of two or more mutually exclusive events (one event or the other) will occur is calculated by adding their individual probabilities. As we have just seen, the multiplication rule gives us the individual probabilities that we will now add together. The probability for one possible way of obtaining an F2 heterozygote—the dominant allele from the egg and the recessive allele from the sperm—is 1/4. The probability for the other possible way—the recessive allele from the egg and the dominant allele from the sperm—is also 1/4. Using the rule of addition, then, we can calculate the probability of an F2 heterozygote as 1/4+1/4=1/2.
Another variation on 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 N molecules. A single gene (L), for which two allelic variations are possible (LMLM or LNLN), determines the phenotype of this blood group. Individuals homozygous for the LMLM allele (LMLMLMLM) have red blood cells with only M molecules; individuals homozygous for the LNLN allele (LNLNLNLN) 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 (LMLNLMLN). Note that the MN phenotype is not intermediate between the M and N phenotypes, which distinguishes codominance from incomplete dominance. Rather, both M and N phenotypes are exhibited by heterozygotes, since both molecules are present.
The law of segregation states that
the two alleles for a gene separate during gamete formation, and end up in different gametes. In the case of the heterozygous green-pod plant (Gg), one gamete will receive the dominant allele (G), and the other gamete will receive the recessive allele (g). The law of segregation accounts for the prediction that 50% of the offspring of the test cross will have green pods and 50% will have yellow pods.
A diploid organism carries two alleles for each autosomal gene. The two alleles are found at comparable locations (loci) on homologous chromosomes. The alleles may be identical or slightly different, but
they affect the same genetic character.
Each variant for a character, such as purple or white color for flowers, is called a
trait.
The homologs of a chromosome pair contain the same genetic loci. Therefore, each genetic locus is represented
twice in a diploid cell.
Incomplete dominance and epistasis are both terms that define genetic relationships. What is the most basic distinction between these terms?
Incomplete dominance describes the relationship between two alleles of a single gene, whereas epistasis relates to the genetic relationship between two genes (and the respective alleles of each).
