SEX DETERMINATION AND SEX-LINKED HEREDITY 



40Q 



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 blindness, 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 melanogaster. 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 

 male was mated to a normal red-eyed female. The offspring of this 

 mating were all red-eyed in appearance (phenotypically), but tlie 

 females were obviously all genotypically hybrid red-and- white eyed, 

 for when mated with normal red-eyed males half of their sons were 

 white eyed and half red eyed, but all daughters were red eyed. Sub- 

 sequent experiments showed that half of the daughters were pure red 

 eyed and half hybrid red-and-white eyed. Now what sort of mech- 

 anism in the germ cells could account for this peculiar but very uniform 

 type of hereditary behavior? 



Professor Morgan explained the whole thing in a beautifully simple 

 way by assuming that the gene of the sex-Hnked character was situated 

 in the X-chromosome of the mutant male, for the male has but one 

 X-chromosome along with a Y-chromosome (see Fig. 78). In the 

 reduction division of the germ ceUs of this individual two kinds of male 

 gametes (spermatozoa) are formed in equal numbers, one carrying the 

 X-chromosome with the white-eyed gene and the other the Y-chromo- 

 somes. Now, whenever a female gamete (egg) of the normal red-eyed 

 female used as a mate is fertilized by a sperm with the X-chromosome, 

 an XX individual or female will result, and aU of these females will 

 get the white-eye gene along with the X-chromosome from their 

 white-eyed father. But whenever an egg is fertilized by a sperm- with 

 the Y-chromosome, a male wiU be produced, and all of these will be red 

 eyed because they get their X-chromosome from their mothers. Why 

 are not these female offspring possessing the white-eye factor white 

 eyed? Because they have also inherited an X-chromosome contain- 

 ing the red-eyed factor from their mothers, and red eye is dominant 

 over white eye. These red-and-white-eyed hybrid daughters are now 



