Sex-Linkage 



83 



d^^'Ox white 



c/ 



B 



I 



"^'dc/ 



9? 



d 



red 



dull 

 red 



m 



99 



white 



dull 

 red 



FIGURE 12-3. Phenotypic re- 

 sults of reciprocal matings in- 

 volving eye color. 



v.-.-/ 



cause the sex genes were located on the sex 

 chromosomes while the other genes happened 

 to be autosomal. Therefore, in all previous 

 work, we were dealing with autosomal trans- 

 mission. This always showed independent 

 segregation from the sex genes and permitted 

 the statement that sex did not influence the 

 results; that is, the results were the same 

 among the sons as among the daughters even 

 though reciprocal matings were made. 

 But consider now the results of crosses 



involving the dull red (h'+) and white {w) eye 

 color alleles. Dull red 9 X white cf (Figure 

 12-3 A) produces all dull red sons and daugh- 

 ters in Fi, as expected, w+ being dominant. 

 However, the reciprocal cross (Figure 12-3B) 

 of white 9 X dull red cf gave only white sons 

 and dull red daughters. Note that the first 

 cross produced the same result for sons as 

 for daughters, but the second, reciprocal, 

 cross gave different results, sons resembling 

 their mothers, and daughters resembling their 

 fathers. Because of this diff"erence in result 

 from reciprocal matings, we must conclude 

 that w+ and its alleles are not located auto- 

 somally. 



Let us assume that the gene for white eye 

 is located in the sex chromosomes, being 

 therefore sex-linked, and see what conse- 

 quences this would have for its transmission 

 relative to the sex phenotype.^ In the rest of 

 this Chapter we will not be particularly con- 

 cerned with learning the genetic basis of sex 

 in any greater detail, but will utilize princi- 

 pally the hypothesis that the two sexes are 

 XX and XY. 

 ^SeeT. H. Morgan (1910). 



