106 Genes in the X and Y Chromosomes 



Incomplete Sex Linkage 



If a gene is located in a chromosomal segment which is found 

 in both the X and Y chromosomes, it is incompletely sex-linked. 

 The behavior of a dominant gene located in the homologous part 

 of the X chromosome only is similar in many respects to sex 

 linkage. If that dominant gene is located only in the Y chromo- 

 some the results are comparable in part to Y chromosome in- 

 heritance. However, the results are always complicated by the 

 fact that the X and Y chromosomes in the male may exchange 

 segments during meiosis. This exchange of segments is known 

 as ''crossing over," and is discussed in Chapter 10. 



QUESTIONS AND PROBLEMS 



1. A red-eyed female is mated with a white-eyed male. The offspring 

 (Fi) were 32 red-eyed females, 29 white-eyed females, 31 red-eyed males, 

 and 27 white-eyed males. What are the genotypes of the two parents? 



2. A normal woman married to a normal man has two sons. One is 

 normal and the other is a "bleeder." What do we know of the genotypes 

 of the parents? 



3. Mrs. A, who is normal, married Mr. B, who is also normal, and 

 they have one daughter, C. Mrs. L, who is normal, has a normal son, 

 P, by her hemophilic husband, Mr. M. C and P marry and have one son 

 R, who is a bleeder. From which grandparent did R get his gene for 

 hemophilia ? 



4. In poultry, barred feathers are dominant over nonbarred, and are 

 sex linked. A nonbarred hen is mated with a barred cock and three off- 

 spring are produced which are respectively a barred female, a nonbarred 

 female, and a barred male. What are the genotypes of the hen and cock? 



5. A breeder has a nonbarred hen and a rooster heterozygous for 

 barred. He wants to estabhsh a true-breeding race of barred poultry. 

 How would he do it, and how many generations would he have to raise 

 before he was sure his poultry would breed true? Would it take him a 

 longer or a shorter time to establish a true-breeding nonbarred stock? 



6. Give the Fi and F2 of the following crosses of which the parental 

 genotypes are given: the symbols refer to eye color in Drosophila mela- 

 yiogaster: TTTF X wY; Ww^ X wY; Ww X wY; w^w^ X wY; w^w X wY] 

 WW X w^Y; Ww^ X w^Y; Ww X w^Y; wHv X w'Y] ww X WY; Ww^ X 

 WY; WwX WY] wHd^ X TFY; w^w X TFY; ww X WY. 



7. A red-eyed female Drosophila is crossed with an eosin-eyed male. 

 The F2 consisted of 5 red females ; 2 eosin females ; 1 white female ; 2 red 

 males; 4 eosin males; 2 white males. What are the genotypes of the 

 parents and the phenotypes and genotypes of the Fi ? 



