The Chromosomal Basis of Inheritance

1. Chapter 15 The Chromosomal Basis of Inheritance Questions prepared by Janet LanzaUniversity of Arkansas at Little Rock Louise PaquinMcDaniel College

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 genes and chromosomes. • It allowed scientists to see the DNA present within chromosomes. • 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 are representative? • a) AaBb × AaBb • b) 46, XY or 46, XX • c) vg+vgse+se × vgvgsese • d) +2 × +3

4. Morgan’s Experimental EvidenceImagine that Morgan had chosen a different organism for his genetics experiments. What kind of species would have made a better choice than fruit flies?

5. Morgan’s Experimental EvidenceImagine that Morgan had used a grasshopper (2n = 24 and an XX, XO sex determination system). 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

6. The Chromosomal Basis of SexThink about bees and ants, groups in which males are haploid. Which of the following are accurate statements about bee and ant 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 that human females have. • 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

7. The Chromosomal Basis of SexIn some Drosophila species 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.

8. The Chromosomal Basis of SexImagine that a deleterious recessive allele occurs on the W chromosome of a chicken (2n = 78). Where would it be most likely to appear first in a genetics experiment? • in a male because there is no possibility of the presence of a normal, dominant allele • in a male because it is haploid • in a female because there is no possibility of the presence of a normal, dominant allele • in a female because all alleles on the W chromosomes are dominant to those on the Z chromosome • none of the above

9. Inheritance of Sex-Linked GenesIn 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.

10. X Inactivation in Female MammalsImagine two species of mammals 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.

11. Mapping the Distance Between GenesImagine 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.

12. Triploid species are usually sterile (unable to reproduce), whereas tetraploids are often fertile. Which of the following are likely good explanations of these facts? • 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.

13. Chromosomal rearrangements can occur after chromosomes break. Which of the following statements are 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 is one that is homozygous for a duplication of that same gene because loss of a function can be lethal.

14. Imagine that you could create medical policy for a country. In this country it is known that the frequency of Down syndrome babies increases with increasing age of the mother and that the severity of characteristics varies enormously and unpredictably among affected individuals. Furthermore, financial resources are severely limited, both for testing of pregnant women and for supplemental training of Down syndrome children. What kind of policy regarding fetal testing would you implement?

15. The lawyer for a defendant in a paternity suit asked for DNA testing of a baby girl. Which of the following set of results would demonstrate that the purported father was not actually the genetic father of the child? • The mitochondrial DNA of the child and “father” did not match. • DNA sequencing of chromosome 5 of the child and “father” did not match. • The mitochondrial DNA of the child and mother did not match. • DNA sequencing of chromosome 5 of the child and mother did not match. • The mitochondrial DNA of the child and “father” matched but the mitochondrial DNA of the child and mother did not.