Lecture 28: Dosage Compensation, Sex-Limited, Sex-Influenced Inheritance
1. What is the Lyon hypothesis and how has its validity been demonstrated?
2. Describe the relationship among the following: Barr body, heterochromatin, late replication, dosage compensation, mosaicism.
3. Describe the major differences between dosage compensation in Drosophila and humans.
4. Describe three different types of human sex chromosome trisomy. What is the sex in each case? What are the major phenotypic traits in each case? Describe the genetic mechanisms that are responsible for the trisomy in each case.
5. Turner syndrome is the only viable human monosomy?
a. How does Turner syndrome arise?
b. Speculate on why individuals with Turner syndrome are viable when all other human monosomies are lethal.
c. In view of your answer to part b, why do individuals with Turner syndrome enhibit any phenotypicdifferences from normal individuals.
6. What is a Barr body and what is its significance?
7. What mechanism makes human aneuploidies that involve sex chromosomes more viable than human aneuploidies that involve autosomes?
8. Propose a possible genetic explanation for each of the following (hint, check pages 424-426 and 539 in the textbook):
A. A human male with two X chromosomes and no Y chromosome.
B. A human female with normal testosterone receptors, who has one X and one Y chromosome.
C. A female Drosophila with a Y chromosome.
D. A male Drosophila with no Y chromosome.
E. A human female with three X chromosomes
F. A human male with one X and two Y chromosomes
9. Describe two techniques that have greatly facilitated human karyotypic analysis.
10. Describe the aneuploidy that you might expect to find in a male calico cat.
11. What would your tentative diagnosis be in each of the following cases:
A. A human female with no Barr bodies.
B. A human female with one Barr body
C. A human female with two Barr bodies,
D. A human male with no Barr bodies.
E. A human male with one Barr body.
12. Briefly define each of the following and explain its significance to the study of genetics.
a. Autosome.
b. Homogametic.
c. Heterogametic.
d. Hemizygous
e. Sex-limited
f. Sex-influenced
13. How will the following differ in a ZZ/ZW system, compared to an XX/XY system?
a. Which will be the homogametic sex in each?
b. Which sex will exhibit hemizygous expression of recessive genes in each?
c. Would you expect large numbers of essential genes to be carried on the W chromosome? Relate your answer to the Y chromosome.
d. If a male is heterozygous for a Z-linked trait, how will it be expressed in his female progeny.
e. Describe a comparable phenomenon in an XX/XY system.
14. Distinguish among traits that are sex-linked, sex-limited, and sex-influenced.
15. Explain how the use of attached X stocks of Drosophila allows direct trnasmission of X-linked mutations from father to son.
16. Describe as many alternative types of nondisjuntion as you can that could give rise to each of the following. Be specific about which meiotic division in which parent the nondisjunction occurred in (you should be able to envision more than one possibility in most of the cases).
a. Turner syndrome
b. Kleinfelter syndrome
c. Triple X female.
d. XYY male.
17. Using non-lethal sex-linked markers, such as color blindness, devise genetic tests that will distinguish among the possibilities in each part of your answer to question 16.
18. A white eyed male fly from a true-breeding stock is mated with a white eyed female from a true-breeding stock. All of the F1 progeny of both sexes have wild-type eyes. (To understand what is involved, you will have to draw on information about possible reasons for white eyes presented at different times during the semester. If you are having problems, see problem 34 at the end of chapter 14 for clues. If that is not enough, see Drosophila eye color in lecture 25 notes and white eyes in lecture 27 notes. Also be sure to take into account any possible epistatic effects that might reasonably be expected.)
a. Propose a feasible genetic explanation for this outcome?
b. Based on the explanation you proposed in part a, what phenotypic distribution would you expect in the F2 generation.
c. If you exchanged causes of white eyes between the male and female parental strains that you used in part a, would the F1 phenotype remain the same?
d. What phenotypic distribution would you expect in the F2 from part c?
19. Rooster feathering is an autosomal recessive that is sex-limited to males in its expression (see figure 14.23). Non-barred is recessive to barred feathers and is sex-linked in chickens (see figure 14.19). A rooster from a true-breeding non-barred and rooster-feathered line is mated to a hen from a true-breeding barred and hen-feathered line. What will be the phenotypic distribution of the F2 generation?
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