662 GENETICS AND EVOLUTION 



egg by an X-bearing sperm results in an XX, female, zygote, and the 

 feriili/ation ol an X-bearing egg by a Y-bearing sperm results in an XY, 

 male, zygote. Since there are equal numbers ot X- and Y-bearing sperm, 

 there are equal numbers of male and female offspring. In human beings, 

 there are api)roximately 107 males born for every 100 females, and the 

 ratio at conception is said to be even higher, about 114 males to 100 

 females. One possible explanation of the numerical discrepancy is that 

 the \ chromosome is smaller than the X chromosome, and a sperm con- 

 taining a Y chromosome, being a little lighter and perhaps able to swim 

 a little faster than a sperm containing an X chromosome, would win the 

 race to the egg slightly more than half of the time. Both during the 

 period of intrauterine development and after birth, the death rate among 

 males is slightly greater than that among females, so that by the age of 

 ten or twelve there are equal numbers of males and females. In later 

 life there are more females than males in each age group. 



277. Sex-Linked Characteristics 



The X chromosome contains many genes, and the traits controlled 

 by these genes are said to be sex-linked, because their inheritance is 

 linked with the inheritance of sex. The Y chromosome contains very 

 few genes, so that the somatic cells of an XY male contain only one of 

 each kind of gene in the X chromosome instead of two of each kind as 

 in XX females. A male receives his single X chromosome, and thus all 

 of his genes for sex-linked traits, from his mother. Females receive one 

 X from the mother and one from the father. In writing the genotype 

 of a sex-linked trait it is customary to write that of the male with the 

 letter for the gene in the X chromosome plus the letter Y for the Y 

 chromosome. Thus AY would represent the genotype of a male with a 

 dominant gene for trait "A" in his X chromosome. 



The phenomenon of sex-linked traits was discovered by T. H. 

 Morgan and C. B. Bridges in the fruit fly, Drosophila. These flies nor- 

 mally have eyes with a dark red color, but Morgan and Bridges dis- 

 covered a strain with white eyes. The gene for white eye, w, proved to be 

 recessive to the gene for red eye, W, but in certain types of crosses the 

 male offspring had eyes of one color and the female offspring had eyes 

 of the other color. Morgan reasoned that the peculiarities of inheritance 

 could be explained if the genes for eye color were located in the X 

 chromosome; later work has proven the correctness of this guess. Cross- 

 ing a homozygous, red-eyed female with a white-eyed male (WW X wY) 

 produces offspring all of which have red eyes (Ww females and WY males). 

 But crossing a iiomozygous white-eyed female with a red-eyed male 

 (ww X WY) yields red-eyed females and white-eyed males (Ww and wY) 

 (Fig. :^2.5). 



In man, hemophilia (bleeder's disease) and color-blindness are sex- 

 linked traits. About 4 men in every hundred are color-blind, but some- 

 what less than one per cent of all women are color-blind. Only one gene 

 for color-blindness produces the trait in males, but two such genes (the 

 trait is recessive) are necessary to produce a color-blind female. 



