SEX-LINKAGE 



the same although their fathers are of the two kinds. They obtain 

 the gene only from their mothers. Thus we see that the gene follows 

 the path of the X chromosome. At the same time we see that the Y 

 has no effect. Where the X chromosome carries the red-producing 

 form or allelomorph of the gene, the male fly is red-eyed; where the 

 X carries the white allelomorph the male fly is white-eyed. Figure lo 

 makes this clear. 



In mammals, like flies, the male is the heterozygous sex, but in 

 birds it is the female. Sex-linkage is of peculiar and practical interest 

 in both. The gold-silver difference used in sexing crossed chicks 

 depends on a gene in the X chromosome, and so shows criss-cross 

 inlieritance like the red- white eye in the fly. Haemophilia and colour- 

 blindness depend on recessive genes in the X of man and hence 

 appear less often in the XX female. Their inheritance follows the 

 same rules as that of white-eye in flies and their frequencies in the 

 population can be deduced from these rules. Thus Pickford found 

 that populations with 7 • 8 per cent of colour-blind men have • 65 

 per cent of affected women. The tortoiseshell cat is heterozygous for 

 the X-linked gene, one allelomorph of which gives black, the other 

 yellow, when homozygous. In mammals a normal male cannot be 

 heterozygous for an X-linked gene and tortoiseshell males occur 

 only as rare and sterile abnormalities. 



In Drosophila the red-white eye difference showed that for the 

 purpose of this sex-linked experiment the Y chromosome might as 

 well be empty. No gene in the Y appeared to correspond with this 

 gene, for no gene interfered with its action. This is a general property 

 of the Y chromosome for, with the sole exception of the gene for 

 "bobbed," the Y simply does not correspond. Nor does it show 

 recognizable differences of its own. That is to say, different males 

 do not carry Y chromosomes determining differences comparable 

 with that between red and white eyes. What differences they can 

 carry we shall see later. 



The fly is typical of a great many insects in the emptiness of 

 the Y chromosome. In the vertebrates, however, the Y chromo- 

 some shows more correspondence with the X. The corresponding 

 genes are of two kinds. Some of them do not cross-over from one 

 chromosome to the other. For example the Y chromosome in 

 poultry seems to carry the normal allelomorph of the barring gene 



Elements of Genetics AQ D 



