Genetics 309 



side a male. This is frequently startling observed in butterflies 

 where one pair of wings will be black the other pair yellow. 



The rare appearance of these abnormal individuals indicates 

 that sex determination is not simply a matter of the X and Y chromo- 

 somes, but actually represents a balance between the sex chromosomes 

 and the autosomes. 



Sex-Linked Characters. — The genes for certain somatic char- 

 acters are carried on the X chromosomes, and are called sex-linked 

 genes. Because of this fact, the female has the usual allelomorphic 

 pair of genes for a character, while the male with but a single X chromo- 

 some can have only one gene for these characters. The Y does not carry 

 these genes. This makes it possible for a recessive, sex-linked character 

 to show up more frequently in the male. In the female, this same reces- 

 sive factor carried on one X chromosome might not express itself due 

 to the fact that the dominant allelomorphic factor may be on the other 

 X chromosome. In man, an example of a sex-linked character is a 

 type of color blindness in which red and green are confused. This color 

 blindness is determined by a recessive gene carried on the X chromo- 

 some. 



If c designates a gene for color blindness and C a gene for its 

 normal allelomorph, the following genotypes could occur in each sex. 

 In these crosses, the allelomorph in the male is, of course, absent; in 

 its place the Y chromosome is listed. 



Male Female 



X°Y Normal X°X° Normal 



X"=Y Color blind X^X'= Normal 



X'^X'' Color blind 



From this, it will be observed that in sex-linked inheritance, the 

 female may act as a carrier, having the recessive character but not 

 showing it. Also it can be seen that in sex-linked inheritance, the 



Fig. 98.— A cross involving a sex-linked character; in this case, color blindness. 

 This character is carried on the X chromosome, so there are two genes in the 

 female and only one in the male. The factor for color blind is recessive to the 

 normal condition. 



