THE GREAT RELAY RACE 



469 



In mankind there are twenty-four pairs of chromosomes, of which 

 twenty-three pairs, common to both sexes, are called autosomes, and 

 to these is added one pair of sex chromosomes, designated XF in the 

 male and XX in the female. A curious fact about F-chromosomes in 

 general is that, with few exceptions, breeding experiments prove them 

 to be devoid of genes. They play a dummy hand. Thus the 

 F-chromosome exerts the same non-contributory role in heredity as 

 the element does in the XO combination. The X-chromosome, on 

 the other hand, not only plays a part in sex determination, but it also 

 harbors additional genes that control the appearance of other traits 

 and characters. These are called sex-linked traits. Their existence 

 is demonstrated in the male because there is nothing in the F-chromo- 

 some mate to conceal them. 



This point may be made clear by citing Morgan's now famous case 

 of the white-eyed Drosophila. Many years ago in one of his cultures 

 of normal red-eyed flies, there appeared a single white-eyed male 

 mutant individual. The conjunction of Professor Morgan's seeing 

 eye with the white eye of this particular tiny fly marks an event in the 

 history of genetics comparable to what happened to the science of 

 physics when the falhng apple and Sir Isaac Newton's head came 

 together. In both cases an exceptional brain was fortunately 

 stimulated, with far-reaching benefits to science. When Morgan's 

 unique white-eyed male fly was mated with a normal red-eyed female, 

 all the offspring were red-eyed, thus showing the dominance of the 

 red-eyed character over white-eye. When these red-eyed hybrids 

 were mated together, the expected Mendelian ratio of three reds to 

 one white resulted, but all the males were white-eyed. Omitting the 

 autosomes and representing only the sex chromosomes, the matter 

 may be diagrammed as foUow^s. (The underscored A" indicates that 

 red-eye color is linked with the sex chromosome. The absence of 

 underscoring means white-eye.) 



