The Chromosomal Basis of Inheritance

Why did the improvement of microscopy techniques in the late 1800s set the stage for the emergence of modern genetics? It revealed new and unanticipated features of Mendel’s pea plant varieties. It allowed the study of meiosis and mitosis, revealing parallels between behaviors of the Mendelian concept of the gene and the movement/pairing of chromosomes. It allowed scientists to see the nucleotide sequence of DNA. It led to the discovery of mitochondria. It showed genes functioning to direct the formation of enzymes. Answer: B 2

Why did the improvement of microscopy techniques in the late 1800s set the stage for the emergence of modern genetics? It revealed new and unanticipated features of Mendel’s pea plant varieties. It allowed the study of meiosis and mitosis, revealing parallels between behaviors of the Mendelian concept of the gene and the movement/pairing of chromosomes. It allowed scientists to see the nucleotide sequence of DNA. It led to the discovery of mitochondria. It showed genes functioning to direct the formation of enzymes. 3

Morgan and his colleagues worked out a set of symbols to represent fly genotypes. Which of the following is representative? AaBb  AaBb 46 or 46w w or w on X 2  3 Answer: C 4

Morgan and his colleagues worked out a set of symbols to represent fly genotypes. Which of the following is representative? AaBb  AaBb 46 or 46w w or w on X 2  3 5

Imagine that Morgan had used a grasshopper (2n  24, and sex is determined as follows: male has X, and female has XX) to study sex linkage. Predict where the first mutant would have been discovered. on the O chromosome of a male on the X chromosome of a male on the X chromosome of a female on the Y chromosome of a male Answer: B This question is designed to show students that there are a variety of sex determination mechanisms and to help students understand the significance of Morgan’s work. Answer A is incorrect because there is no O chromosome—the X chromosome has no pairing partner. Answer C is incorrect because a mutant is likely recessive and is likely to be masked by a dominant on the other X chromosome in a female. Answer D is incorrect because grasshopper males do not have a Y chromosome. Answer B is correct because a mutant on the male’s single X chromosome would not be masked by a normal allele on a second X chromosome. 6

Imagine that Morgan had used a grasshopper (2n  24, and sex is determined as follows: male has X, and female has XX) to study sex linkage. Predict where the first mutant would have been discovered. on the O chromosome of a male on the X chromosome of a male on the X chromosome of a female on the Y chromosome of a male 7

Think about bees, which have no X and Y sex chromosomes Think about bees, which have no X and Y sex chromosomes. Males are haploid, whereas fertilization results in females, as diploid cells become females. Which of the following are accurate statements about bee males when they are compared to species in which males are XY and diploid for the autosomes? Bee males have half the DNA of bee females, whereas human males have nearly the same amount of DNA as human females. Considered across the genome, harmful (deleterious) recessives will negatively affect bee males more than Drosophila males. Human and Drosophila males have sons, but bee males do not. Inheritance in bees is like inheritance of sex-linked characteristics in humans. none of the above Answer: A, B, or C The point of this question is that there are a variety of sex determination mechanisms among different species. Answers A, B, and C are correct. Answer A is correct because bee males are haploid, but the only difference in the amount of DNA in human males and females is that the Y chromosome is slightly smaller than the X chromosome. Answer B is correct because deleterious alleles from the whole genome will affect male bees, but deleterious recessives on the autosomes in Drosophila males can be masked by a dominant allele. Answer C is correct because, in humans and Drosophila, half of a male’s sperm carry a Y chromosome and cause the production of male offspring, but in bees and ants males are haploid and any egg the sperm fertilizes develops into a female. Answer D is incorrect because male bees have no sons. 8

Think about bees, which have no X and Y sex chromosomes Think about bees, which have no X and Y sex chromosomes. Males are haploid, whereas fertilization results in females, as diploid cells become females. Which of the following are accurate statements about bee males when they are compared to species in which males are XY and diploid for the autosomes? Bee males have half the DNA of bee females, whereas human males have nearly the same amount of DNA as human females. Considered across the genome, harmful (deleterious) recessives will negatively affect bee males more than Drosophila males. Human and Drosophila males have sons, but bee males do not. Inheritance in bees is like inheritance of sex-linked characteristics in humans. none of the above 9

This mutation occurs in all offspring of a male with the mutation. Determination of sex in Drosophila is similar to that in humans. In some species of Drosophila, there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation? This mutation occurs in all offspring of a male with the mutation. This mutation occurs in all male but no female offspring of a male with the mutation. This mutation occurs in all offspring of a female with the mutation. This mutation occurs in all male but no female offspring of a female with the mutation. This mutation occurs in all offspring of both males and females with the mutation. Answer: B The point of this question is to help students understand sex linkage by looking at the effect of genes on the Y chromosome. Answer A is incorrect because half of the sperm of the mutant male will contain an X chromosome and produce normal size daughters. Answer C is incorrect because females do not have a Y chromosome and cannot carry the mutation. Answer D is incorrect because, again, females do not have a Y chromosome and thus cannot carry the mutation. Answer E is incorrect because females cannot carry the mutation (no Y chromosome) and only the half of the eggs that are fertilized by a Y-bearing sperm will receive the mutation (i.e., only half of the male offspring). Answer B is correct because all the sons of the mutant male receive his Y chromosome. 10

This mutation occurs in all offspring of a male with the mutation. Determination of sex in Drosophila is similar to that in humans. In some species of Drosophila, there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation? This mutation occurs in all offspring of a male with the mutation. This mutation occurs in all male but no female offspring of a male with the mutation. This mutation occurs in all offspring of a female with the mutation. This mutation occurs in all male but no female offspring of a female with the mutation. This mutation occurs in all offspring of both males and females with the mutation. 11

In cats, a sex-linked gene affects coat color In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate? The phenotype of o-Y males is black/brown because the nonfunctional allele o does not convert eumelanin into phaeomelanin. The phenotype of OO and Oo males is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of Oo males is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the nonfunctional allele o does not convert eumelanin into phaeomelanin. The phenotype of O-Y males is orange because the nonfunctional allele O does not convert eumelanin into phaeomelanin, while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin. Answer: A This focuses on the color of males and the action of the enzyme that converts eumelanin (brown/black pigment) to phaeomelanin (orange pigment). Male genotypes will be either O-Y or o-Y, with phenotypes of either orange or black/brown, respectively. In O-Y males, the eumelanin is converted to phaeomelanin, and in o-Y males, the eumelanin is unchanged. To answer this question, a student must know that males have only one copy of the gene and must understand that a functional allele produces an enzyme that catalyzes the chemical reaction. 12

In cats, a sex-linked gene affects coat color In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate? The phenotype of o-Y males is black/brown because the nonfunctional allele o does not convert eumelanin into phaeomelanin. The phenotype of OO and Oo males is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of Oo males is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the nonfunctional allele o does not convert eumelanin into phaeomelanin. The phenotype of O-Y males is orange because the nonfunctional allele O does not convert eumelanin into phaeomelanin, while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin. 13

Imagine two species of cats that differ in the timing of Barr body formation during development. Both species have genes that determine coat color, O for the dominant orange fur and o for the recessive black/brown fur, on the X chromosome. In species A, the Barr body forms during week 1 of a 6-month pregnancy, whereas in species B, the Barr body forms during week 3 of a 5-month pregnancy. What would you predict about the coloration of heterozygous females (Oo) in the two species? Both species will have similar sized patches of orange and black/brown fur. Species A will have smaller patches of orange or black/brown fur than will species B. The females of both species will show the dominant fur color, orange. Answer: B This question relates genetics, embryonic development, and Barr body formation. The sizes of the patches should be related to the timing of Barr body formation in development. Early formation of the Barr body would mean that larger clusters of cells are affected by which X chromosome is inactivated compared to later Barr body formation. 14

Imagine two species of cats that differ in the timing of Barr body formation during development. Both species have genes that determine coat color, O for the dominant orange fur and o for the recessive black/brown fur, on the X chromosome. In species A, the Barr body forms during week 1 of a 6-month pregnancy, whereas in species B, the Barr body forms during week 3 of a 5-month pregnancy. What would you predict about the coloration of heterozygous females (Oo) in the two species? Both species will have similar sized patches of orange and black/brown fur. Species A will have smaller patches of orange or black/brown fur than will species B. The females of both species will show the dominant fur color, orange. 15

Imagine a species with three loci thought to be on the same chromosome Imagine a species with three loci thought to be on the same chromosome. The recombination rate between locus A and locus B is 35%, and the recombination rate between locus B and locus C is 33%. Predict the recombination rate between A and C. The recombination rate between locus A and locus C is either 2% or 68%. The recombination rate between locus A and locus C is probably 2%. The recombination rate between locus A and locus C is either 2% or 50%. The recombination rate between locus A and locus C is either 2% or 39%. The recombination rate between locus A and locus C cannot be predicted. Answer: C The recombination rate between loci A and B is 35%. Locus C can be either between A and B or on the opposite side of B from A. If locus C is in between locus A and locus B (ACB), the distance between locus A and C would be 2%. Locus C cannot be on the other side of A from B (CAB) because the recombination rate would have to be higher than 35%. Thus far, answers A, B, C, and D could be correct. If locus C is on the other side of locus B from locus A (ABC), adding the two recombination rates gives a prediction of 68%, but the maximum recombination rate between two loci is 50%, the same frequency of recombinants that are on different chromosomes. Using this information, answer A cannot be correct because a recombination rate cannot be greater than 50%. Answer B could be right, but it is not the only possible placement and so answer B is incomplete. Answer C is the best answer (2% if C is between A and B, and 50% if C is on the other side of B than A). 16

Imagine a species with three loci thought to be on the same chromosome Imagine a species with three loci thought to be on the same chromosome. The recombination rate between locus A and locus B is 35%, and the recombination rate between locus B and locus C is 33%. Predict the recombination rate between A and C. The recombination rate between locus A and locus C is either 2% or 68%. The recombination rate between locus A and locus C is probably 2%. The recombination rate between locus A and locus C is either 2% or 50%. The recombination rate between locus A and locus C is either 2% or 39%. The recombination rate between locus A and locus C cannot be predicted. 17

Triploid species are usually sterile (unable to reproduce), whereas tetraploids are often fertile. Which of the following is likely a good explanation of these facts? (Hint: Synapsis.) In mitosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In meiosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In mitosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. In meiosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. Answer: B The point of this question is to make students think about mitosis and meiosis in relation to polyploids. To answer this question, students should draw chromosomes of a triploid and a tetraploid as they go through mitosis and meiosis. Answers A and C are incorrect because chromosomes do not synapse during mitosis. Answer D is incorrect because tetraploids do have partners at synapsis but triploids do not. Answer B is correct—one-third of the chromosomes do not have a partner. 18

Triploid species are usually sterile (unable to reproduce), whereas tetraploids are often fertile. Which of the following is likely a good explanation of these facts? (Hint: Synapsis.) In mitosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In meiosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In mitosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. In meiosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. 19

Chromosomal rearrangements can occur after chromosomes break Chromosomal rearrangements can occur after chromosomes break. Which of the following statements is most accurate with respect to alterations in chromosome structure? Chromosomal rearrangements are more likely to occur in mammals than in other vertebrates. Translocations and inversions are not deleterious because no genes are lost in the organism. Chromosomal rearrangements are more likely to occur during mitosis than during meiosis. An individual that is homozygous for a deletion of a certain gene is likely to be more damaged than one that is homozygous for a duplication of that same gene because loss of a function can be lethal. Answer: D Chromosomal rearrangements are important in evolution. For example, duplications provide raw material on which natural selection can act (e.g., the globin genes are thought to have arisen via gene duplication). This question will make students think about the consequences of chromosomal rearrangements. Answer A is not correct and should be nixed by students because there is no information in the chapter that would lead to this conclusion. Answers B and C directly contradict material in the text and are therefore incorrect. 20

Chromosomal rearrangements can occur after chromosomes break Chromosomal rearrangements can occur after chromosomes break. Which of the following statements is most accurate with respect to alterations in chromosome structure? Chromosomal rearrangements are more likely to occur in mammals than in other vertebrates. Translocations and inversions are not deleterious because no genes are lost in the organism. Chromosomal rearrangements are more likely to occur during mitosis than during meiosis. An individual that is homozygous for a deletion of a certain gene is likely to be more damaged than one that is homozygous for a duplication of that same gene because loss of a function can be lethal. 21

Which of the following statements about crossing over is false? It accounts for the recombination of linked genes. It occurs while replicated homologus are paired during prophase I of meiosis. Portions of sister chromatids change places. It breaks the physical connection between specific alleles on the same chromosome. Recombinants may result. Answer: C

Which of the following statements about crossing over is false? It accounts for the recombination of linked genes. It occurs while replicated homologus are paired during prophase I of meiosis. Portions of sister chromatids change places. It breaks the physical connection between specific alleles on the same chromosome. Recombinants may result.

Which of the following is a type of chromosomal alteration that differ from all of the others? aneuploidy polyploidy triploidy tetraploidy octaploidy Answer: A

Which of the following is a type of chromosomal alteration that differs from all of the others? aneuploidy polyploidy triploidy tetraploidy octaploidy Answer: A