CONTINUITY OF THE RACE 



605 



@) @ 





Fig. 24-15. Genes that ore located on separate chromosomes give the typical Mendelian ratios, as in the case of 

 four genes located on four different chromosomes in the left above. The typical 9:3:3:1 ratio results in the F2. 

 If the dominant genes are located on one chromosome and the recessives on another, they will behave as single 

 genes as shov\/n in the figures to the right. 



one-half defective and one-half normal 

 genes for eye color. The Y chromosome, of 

 course, plays no part except in determining 

 the sex. Therefore, the ratio is 1:1 for each 

 of the sexes and also for the entire F2 

 generation. 



The explanation of sex-linked genes in 

 Drosophila suffices for hemophilia and red- 

 green color blindness in man. For example, 

 in the latter instance, if a woman hetero- 

 zygous for color blindness marries a nor- 

 mal-visioned man, all of the daughters of 

 this combination will have normal vision 

 but half of the sons will be color blind 

 (Fig. 24-14). One-half of the daughters, 

 however, will be heterozygous for the de- 

 fect, whereas the normal sons will show no 

 trace of the anomaly and therefore will 



never transmit it to their children. The 

 heterozygous daughters can have color- 

 blind sons while the homozygous daughters 

 will never pass the trait on to either sons 

 or daughters. Obviously, this type of criss- 

 cross inheritance explains the early obser- 

 vation concerning hemophilia in royal fami- 

 lies. Many other organisms, both plants and 

 animals, exhibit this type of inheritance. 



Characters other than the sex organs are 

 controlled by genes on the autosomes but, 

 even so, they are definitely associated with 

 the sex. Their effect is probably on the pro- 

 duction of hormones which in turn control 

 the appearance or absence of the trait. Such 

 traits are said to be sex-limited because 

 they do not follow the pattern of sex-linked 

 genes. The presence of horns in one sex 



