520 



THE PRINCIPLES OF HEREDITY 



Finally, mention should be made of a few forms of insects, such 

 as the grasshopper, which have what is called the XO method of sex 

 determination. In these forms the method is exactly like the XY 

 method except that the Y-chromosome is lacking. The female grass- 

 hopper has 24 chromosomes as a diploid number which includes a pair 

 of X-chromosomes, but male grasshoppers have only 23 chromosomes, 

 including only one X-chromosome. All eggs carry the X, but only one 

 half of the sperms carry the X — the other half carry only 11 chromo- 



X- chromosome bearing 

 gene for color blindness 



X-chromosome bearing 

 gene for normal vision 



^ Y-chromosome 



Color-blind vision 



<2><S3| Normal color vision 



Normal husband 



Normal wife 

 Normal children 



One-half of sons are color-blind 



Fig. 32.10. Sex-linked inheritance. When a color-blind man marries a woman who 

 does not carry the gene for color blindness, all of their children will be normal in their 

 color vision. The daughters of such a couple, however, will carry the X-chromosome 

 from their father, and this chromosome bears the recessive gene for color blindness. 

 When such a daughter marries a normal-visioned man, one-half of her sons will be 

 color-blind and one-half of her daughters will carry the gene, but will have normal 

 vision. Note that the men cannot carry the gene unless they are color-blind. 



