Sex Linkage 



99 



Rocks and several other types of domestic chickens (Fig. 34). 

 Barred feathers (B) are dominant over nonbarred (b) , and these 

 genes are on the Z chromosome. In the chicken, the male is 

 ZZ, having eighteen pairs of chromosomes, and the female is ZO, 

 having only one sex chromosome in addition to the seventeen 

 pairs of autosomes. 



Daltonism. Several striking sex-linked characters have been 

 found in human beings. One of them is a type of color blindness 



Fig. 34. A nonbarred Rhode Island Red (left) and a barred Plymouth 

 Rock. Barred is the result of a dominant gene B and is located on the Z 

 chromosome. (Courtesy of Dr. F. A. Hays.) 



known as Daltonism which is caused by a recessive gene. In- 

 dividuals not possessing the dominant allele cannot distinguish 

 between red and green. As human beings follow the XY type 

 of sex inheritance, a cross between a heterozygous woman and a 

 normal man w^ould produce normal daughters; half the sons 

 would be normal but the other half would be color blind. Thus 

 men inherit color blindness from their mother and not from 

 their father. This red-green color blindness is very much rarer 

 in women than in men. The frequency of men is 8 per cent and 

 that of women 0.64 per cent, or the frequency of women equals 

 the frequency of men squared. 



It is interesting to consider why this is so. Since men have 

 only one X chromosome and therefore only one gene at that 

 locus, they will be color blind if this single gene is recessive. 

 Women, however, will not be color blind unless they have two 



