278 EVOLUTION, GENETICS, AND EUGENICS 



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

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

 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-wbite 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-linked 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. 53). In the 

 reduction division of the germ cells 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 all 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 will 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 

 bred to normal red-eyed males, whose X-chromosome carries the red- 

 eye factor. The females will produce two kinds of gametes in equal 

 numbers, one with the X-chromosome carrying the red-eye gene, the 

 other with the X-chromosome carrying the white-eye gene; while the 

 male will produce two kinds of gametes, one with an X-chromosome 

 carrying the red-eye gene and the other with only a Y-chromosome. 

 Each kind of male gamete will unite equally often with each kind 

 of female gamete, and the result will be four kinds of zygotes in equal 

 numbers: one in which two red-eyed X-chromosomes come together 

 and produce a pure red-eyed female; one in which a red-eyed and a 

 white-eyed X-chromosome come together and produce a hybrid female; 

 one in which a red-eyed X-chromosome and a Y-chromosome unite 

 to produce a red-eyed male: and finally, one in which a white-eyed 



