CHAPTER XXII 



SEX-LINKED HEREDITY 



Up to this point we have been dealing with Mendelian experiments 

 in which no more than one gene in any given chromosome was con- 

 cerned. The various Mendelian ratios, including the modified ratios 

 presented in connection with the factor hypothesis, depend upon each 

 of the different allelomorphic factors involved being in a different pair 

 of chromosomes. If two or more genes were in the same chromosome, 

 one would not expect them to follow the laws of random assortment 

 and random recombination. The study of sex-linked heredity opened 

 the way to the discovery that particular genes are in particular chromo- 

 somes, that they tend to remain together in heredity, but that genes 

 in homologous chromosomes are frequently exchanged in a mutual 

 fashion. This discovery has led to a knowledge of linkage and cross- 

 ing-over, and to the discovery of the detailed architecture of the germ 

 plasm. 



Nearly twenty years ago a peculiar kind of heredity, known as sex- 

 linked heredity, was discovered and explained by Morgan. We had 

 previously known of this kind of heredity in man, as in the case of 

 color-blindness, free-bleeding, etc., but its mechanism was not even 

 guessed at. It had long been known that color-blind individuals are 

 almost invariably males, that such males marrying normal women 

 never have any color-blind offspring, but that their daughters when 

 mated with normal men have some color-blind sons, but never color- 

 blind daughters. Thus color blindness shows a strong predilection for 

 males, and is called a sex-linked character. Free bleeding, night blind- 

 ness, and several other human characters are known to be inherited in 

 the same fashion. 



The mechanics of this form of heredity was worked out by Pro- 

 fessor T. H. Morgan as the result of his work on the classic fruit fly 

 Drosophila mdanogaster. In this valuable little insect the eyes are 

 typically bright red. In a stock of typical red-eyed flies Morgan one 

 •day noted one white-eyed male. This had been born of typical red- 

 eyed ancestry, so the white-eye character in addition to being sex- 

 linked was a mutant, appearing suddenly without any preliminary 

 steps. To test the heritability of this new character, the white-eyed 



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