246 ESSENTIALS OF ZOOLOGY 



In other cases that include man and Drosophila, the X-chromosome 

 will pair with a Y-chromosome which has been described as ' ' an empty 

 case " or a ' ' ghost chromosome. ' ' In cases in which a Y-chromosome is 

 present, we cannot say that the male has one less chromosome than 

 the female, but it practically amounts to that since the Y-chromosome 

 seems to contain not more than a few genes (Fig. 79). 



Linkage 



Since the number of inherited traits of an animal or plant is very 

 great while the number of chromosomes is quite small, it is apparent 

 that a single chromosome must contain many genes. This being so, 

 there is not absolutely free assortment of traits as they are passed 

 from parent to offspring, but rather the offspring must inherit his 

 traits in groups that are not, ordinarily, broken up. Therefore, when 

 two genes, A and B, are on the same chromosome of one of the par- 

 ents, if the offspring inherits gene A, he must inherit gene B. In 

 Drosophila we know that the gene which determines whether or not 

 the wing will be fringed occurs on the same chromosome as the one 

 that is responsible for the body being or not being black. Now if one 

 of the parents is fringed-winged and black-bodied, the offspring will 

 have to inherit both these traits from that parent if he receives either 

 one of them. This phenomenon is called linkage. 



Sex Linkage 



Since there are other genes than those concerned with sex on the 

 X-chromosome, it is to be expected that their inheritance will differ 

 somewhat in the two sexes. This is strikingly illustrated by certain 

 abnormalities of man in whom the abnormal condition is inherited as 

 a recessive ; for example, color blindness, blindness resulting from 

 atrophy of the optic nerves (Leber's atrophy), and hemophilia or 

 bleeder's disease, in which great loss of blood occurs even from slight 

 wounds because clotting will not take place. Since the abnormality is 

 a recessive trait produced by a gene on the X-chromosome, its effect 

 can be offset by a dominant for normal in the female, but not in the 

 male, because in the latter case there is not another X-chromosome 

 present. Therefore, the daughter of a color-blind father will not show 

 the defective trait but may pass it to her son. 



Fig. 80 shows a cross between a color-blind father and a normal 

 mother. Two kinds of male gametes are produced, half of which carry 

 the father 's gene for color blindness on the X-chromosome, and half of 



