Lecture 10: Sex Determination, Dosage compensation
(Old Lecture 17)
1. How do the XX/XY sex determination mechanisms differ between humans and Drosophila? How are they similar?
2. Do males of all species have Y chromosomes? Explain your answer.
3. Briefly define each of the following and explain its significance to the study of genetics.
a. Autosome.
b. Homogametic.
c. Heterogametic.
d. SRY (Sry)
e. Sxl
f. Pseudoautosomal.
g. Hemizygous
4. The human gene that codes for the testosterone receptor is carried on the human X chromosome.
a. What biological role does the testosterone receptor play?
b. What are the developmental consequences in an XY human who is hemizygous for a defective testosterone receptor gene?
c. Why is one unlikely to encounter human females who are homozygous for defective testosterone receptors?
d. What aspects of normal female development are absent in individuals with testicular feminization and why?
e. Why is it not possible to reverse testicular feminization with testosterone injections?
5. What is the Lyon hypothesis and how has its validity been demonstrated?
6. Describe the relationship among the following: Barr body, heterochromatin, late replication, dosage compensation, mosaicism.
7. Describe the major differences between dosage compensation in Drosophila and humans.
8. 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.
9. What is the only viable human monosomy? How does it arise?
10. What is a Barr body and what is its significance?
11. What mechanism makes human aneuploidies that involve sex chromosomes more viable than human aneuploidies that involve autosomes?
12. Propose a possible genetic explanation for each of the following (hint, check pages 226 and 231 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 Drosophil with no Y chromosome.
13. What is a gynandromorph and how is it formed?
14. What is the difference between an X chromosome and a Z chromosome?
15. Describe two techniques that have greatly facilitated human karyotypic analysis.
16. Describe the aneuploidy that you might expect to find in a male calico cat.
17. 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.
18. Can the gene that codes for the testosterone receptor be described as a sex-determining gene? Defend your answer.
19. Why is the designation sex lethal (Sxl) not an accurate description of the role of the gene or of its loss of function mutation?
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