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Modern Genetic Analysis Answers to Selected Problems
Chapter 16*

3. | In humans, a single copy of the Y chromosome is sufficient to shift development toward normal male phenotype. The extra copy of the X chromosome is simply inactivated. Both mechanisms seem to be all-or-none rather than to be based on concentration levels. See question

5. | The concentration of Sxl is crucial for female development and dispensable for male development. The dominant SxlM male-lethal mutations may not actually kill all males but simply produce an excessive amount of gene product, so only females (fertile XX and XY) result. The reversions may eliminate all gene product, resulting in XX (sterile) and XY males. The reversions would be recessive because, presumably, a single normal copy of the gene may produce enough gene product to "toggle the switch" in development to female. See question

7.

a. The results suggest that ABa and ABp are not determined at this point in their development. Future determination and differentiation of these cells depend on their position within the developing organism.
b. Because an absence of EMS cells leads to a lack of determination and differentiation of AB cells, the EMS cells must be at least in part responsible for AB-cell development, either through direct contact or by the production of a diffusible substance.
c. Most descendants of the AB cells do not become muscle cells when P2 is present; all descendants of the AB cells become muscle cells when P2 is absent. Therefore, P2 must prevent some AB descendants from becoming muscle cells.
See question

12. | Proper ftz expression requires Kr in the fourth and fifth segments and kni in the fifth and sixth segments. See question

15. | The anteriorposterior axis would be reversed. See question

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