HEREDITY 



331 



been widely used in the study of inheritance. It is a sex-hmited char- 

 acter and is recessive to Red, which is the normal eye color. The 

 manner in which this recessive character is segregated and the 

 explanation of the fact that it is usually found only in males are 

 clear from the diagram (Fig. 207). A male with White eyes is 



PARENTS 



^ 



MALE 



FEMALE 



XY xr yx XX 



MALES 



FEMALES 



MATING 



^2 



Fig. 207. — Chart showing the behavior of genes that are associated with the sex 

 chromosomes. The example given is that of White eye, a sex-Unked character in the 

 fruit fly. It is recessive to the normal eye color, Red. See also Fig. 201, Fig. 202, and 

 Fig. 203. 



mated with a normal Red-eye female, homozygous for Red. The 

 Fi generation then, as usual, are male and female in equal num- 

 bers. The females derive one of their X chromosomes from the 

 father, which contains the gene for White eye, and one from the 

 mother, which contains the normal allelomorph. Red. The females 

 are therefore heterozygous for eye color; since Red is dominant, 

 the phenotype is Red eye. The males of the Fi generation derive 

 their X chromosome from the mother and are therefore Red eye. 



