4IO EVOLUTION, GENETICS, AND EUGENICS 



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 X-chromosome and a Y-chromosome unite to produce a white- 

 eyed male. This is the detailed procedure followed by all sex- 

 linked characters of this sort, and is shown diagrammatically in Fig- 

 ure 80. 



We have seen that white eyes seem to be purely a male character, 

 inasmuch as it does not seem to express itself in females even when 

 present in the germ plasm. Why is this not just a secondary sexual 

 character like the differences in size and shape of the body that char- 

 acterize the two sexes? The answer to this query is that, if we perform 

 the proper breeding experiment, it is possible to transfer the white- 

 eye character to the female. For example, let us take one of the daugh- 

 ters of a white-eyed male and mate her with a white-eyed male. 

 The female is a hybrid carrying the white-eye gene in one of her 

 X-chromosomes and the red-eye gene in the other X-chromosome. 

 She will produce equal numbers of gametes with the two eye-color 

 genes. The male will also have two kinds of gametes, one with a white- 

 eye-bearing X-chromosome and one with a Y-chromosome. Random 

 pairing of the types of gametes of the two parents will produce 

 four classes of individuals in equal numbers: one female with a red- 

 eyed X and a white-eyed X (phenotypically red-eyed) ; one female with 

 two white-eyed X-chromosomes, and therefore white-eyed; one 

 male with a red-eyed X, and therefore red-eyed; and one male 

 with a white-eyed X, and therefore white-eyed. It is clear, then, 

 that the white-eye character is not limited to one sex, but merely 

 closely linlced to the male sex under normal breeding conditions. AH 

 sex-linked characters are recessive, for were they dominant they would 

 express themselves somatically when either one dose or two doses of 



