406 THE NEW INDIVIDUAL Part IV 



spermatogenesis of the long-horned grasshopper. In 1905, Nettie Stevens 

 published an account of the sex chromosomes in a beetle (Tenebrio) and 

 showed that the male had 19 large chromosomes (18 autosomes plus an X) 

 and one small one (Y). In the same year, Edmund B. Wilson announced 

 similar discoveries in insects; one of them, the common squash bug {Anasa 

 tristis), has 22 chromosomes in the body cells of the female and 21 in those 

 of the male. 



Sex-linked and Sex-influenced Inheritance 



Sex-linked. The sex chromosomes, chiefly the X-chromosomes, carry other 

 genes besides those associated with sex. These are known as sex-linked genes. 

 Among the best known of human sex-linked characters are color blindness 

 and hemophilia or "bleeding." 



Color blindness varies in degrees from a weakened sense of red-green to 

 the absolute loss of color as in late twilight. Red-green color blindness and 

 hemophilia have long been known to be inheritable in the same peculiar 

 criss-cross way (Fig. 20.15). A color blind man may transmit color blindness 

 through his daughters who have normal vision to half of his grandsons; a color 

 blind woman transmits color blindness to her sons and to her daughters who 

 become carriers. The gene for color blindness (c) is carried on the X-chromo- 

 some and is recessive to normal vision (C); females have two X-chromosomes, 

 males an X and Y-chromosome. A woman may be a carrier producing eggs 

 half of which carry the gene for color blindness though she herself has normal 

 vision. Color blindness shows that genes for sex and for other characters may 

 be associated in the same chromosome. It also emphasizes the fact that genes 

 on the X-chromosomes are not transmitted by a father to his sons and so 

 reduces the importance ascribed to a direct male line of inheritance. 



Eight out of 100 persons are color blind and it is likely that accidents are 

 sometimes due to misinterpretation of red and green traffic signals. These 

 colors are an unfortunate choice for signals, red and blue would have been 

 distinguishable by almost everybody. Engineers and pilots and other officers 

 on railways, steamships, and airplanes are tested for color blindness; in some 

 states, automobile drivers are not. 



Hemophilia, the abnormal tendency to bleed, has been widely publicized 

 because of its distribution in the royal families of Europe. The most famous 

 pedigree of hemophilia is that of Queen Victoria who jwas a carrier (Fig._ (T^^ 

 20.16). Of her four sons, only Leopold (II. 8) who lived to be 31 was 

 affected. The other three sons were free from it including Edward VII (II. 2) 

 from whom George VI was descended. One of Queen Victoria's carrier 

 daughters, Alice (II. 3) was the mother of Alexandra of Russia (III.6Xwhose„ 

 son Alexis (IV. 12) suffered severely from hemophilia. Victoria's vQther'car- 

 rier daughter, Beatrice (II.9), was the mother of Victoria Eugenie (III. 16) 



