SEX-LINKED HEREDITY 279 



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

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

 acters of this sort, and is shown diagrammatically in Figure 69. 



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 linked 

 to the male sex under normal breeding conditions. All sex-linked 

 characters are recessive, for were they dominant they would express 

 themselves somatically when either one dose or two doses of the gene 

 are present. The reason why the character appears normally in males 

 only is that males have only one X-chromosome, a situation which 

 makes it possible for any recessive gene located in the X-chromosome 

 to express itself. The female, however, has always two X-chromo- 

 somes, and unless she inherits the recessive gene from both parents 

 — a condition that would rarely occur in nature — she would always 

 have the corresponding dominant character in one X-chromosome 

 to mask or offset the recessive character in the other X-chromosome. 

 In man it is also the unfortunate male that falls heir to all of the rather 

 detrimental sex-linked characters, while the female, though inheriting 

 the character more often than the male, practically never shows the 

 effects of it. 



An interesting variant upon the usual type of sex-linked breeding 

 experiment is the so-called reciprocal cross, starting out with a white- 



