96 Genes in the X and Y Chromosomes 



the sperm will have neither an A nor an a gene. Similarly, 

 recessive males will produce two kinds of gametes in equal 

 numbers; one kind will have an X chromosome with the a gene 

 and the other kind will have no X chromosome and neither an 

 A nor an a gene. 



In some individuals of Drosophila melanogaster a recessive 

 gene that produces miniature wings may be found in the X chro- 

 mosome. The dominant allele of this gene acts to produce wild- 

 type wings. A homozygous wild-type female will have two X 

 chromosomes, each of which will bear one wild-type allele of 

 miniature. A miniature-winged male will have one X chromo- 

 some with the miniature gene and a Y chromosome which has 

 neither allele. If these two flies are mated together, all the Fi 

 offspring will be wild type. The females will be heterozygous 

 since they received an X chromosome with a wild-type gene from 

 their female parent and an X chromosome with a miniature 

 gene from the male parent. The Fi males, however, will have 

 only one X chromosome; since it came from the female parent, 

 it will have the wild-type gene. 



The Fi females will produce two kinds of gametes. One half 

 will have an X chromosome with the wild-type gene and the 

 other half will have an X chromosome with the miniature gene. 

 The Fi males will also produce two kinds of gametes in equal 

 numbers. One half will have an X chromosome with a wild-type 

 gene and the other half will have a Y chromosome. If the two 

 Fi flies are mated together, four different genotypes will result in 

 the F2, and they will be found in equal numbers. One half the 

 females will have two wild-type genes, one from the Fi female 

 and one from the Fi male. The other half will have a wild-type 

 gene from the Fi male and a miniature gene from the Fi female. 

 All the males will naturally have an X chromosome from the 

 Fi female and a Y chromosome from the Fi male. Since the 

 female is heterozygous for miniature, half the males will have 

 a wild-type gene and half will have a gene for miniature. Pheno- 

 typically, all the Fo females will be wild type and of the males 

 half will be wild type and half miniature (Fig. 32). 



When genes are in autosomes reciprocal crosses normally give 

 identical results, but when genes located in the X chromosome 

 are dealt with, the results of reciprocal crosses are different. 



