Sex-Linkage 93 



QUESTIONS FOR DISCUSSION 



12.1. Would you have phrased differently the statement on p. 84 that "sons receive their 

 single X from their mother"? Explain. 



12.2. What would you expect to be the genotypes of the zygotes produced, after sex chro- 

 mosome nondisjunction occurs in both male and female Drosophila, in the cross 

 X"'*Y by X"X"? What is the phenotypic outcome in each case? 



12.3. If a trait is proved to be due to a gene unlinked to any autosome, does this mean 

 the gene is linked to a sex chromosome? Explain. 



12.4. A husband and wife both have normal vision, although both their fathers are red-green 

 color-bHnd. What is the chance that the first child this couple has will be: 



a. A normal son? 



b. A normal daughter? 



c. A red-green color-blind son? 



d. A red-green color-blind daughter? 



12.5. One twin is hemophilic while the twin brother is not. 



a. What is the probable sex of the hemophilic twin? 



b. Are the twins monozygotic? Explain. 



c. Give the genotypes of both twins, and of their mother. 



12.6. A hemophilic father has a hemophilic son. Give the most probable genotypes of 

 the parents and child. 



12.7. A Drosophila male with cubitus interruptus wing venation, ebony body color, and 

 white eye color is mated to a wild type female (normal wing venation, gray body 

 color, and dull red eyes). The Fi females are crossed to males like their father. 



Give the kinds and relative frequencies of genotypes and of phenotypes expected 

 among the offspring of the last cross. 



12.8. Are you convinced that all genes have their material basis in the chromosomes? 

 Explain. 



12.9. What reason can you give for believing in Drosoplula that the Y chromosome is 

 missing a gene present in the X chromosome? that the X is missing a gene present 

 in the Y? 



12.10. List evidences supporting the theory that chromosomes contain the material basis 

 for genes. 



12.11. Have we presented any evidence that a chromosome carries more than one gene? 

 Explain. 



12.12. What proportion of all genes for hemophilia type A are found in human males? 

 Justify your answer. 



