THE investigation of the chromosomes in a large number of insects and other animals has shown that the so-called X- and Y-chromosomes furnish a mechanical basis for the determination of sex in the fertilised egg, its inheritance in later generations, and the usual occurrence of approximate equality of the two sexes when one of them is heterozygous (XY or XO). The fundamental character of this relation between the X- and Y-chromosomes and sex is now generally admitted. It would appear that the difference in the chromosome content of the nuclei in the two sexes affects the, metabolism during development in such a way as to produce one sex or the other, and in some groups to affect the secondary sexual characters as well. Combined cytological and breeding investigations have shown further that in most insects and mammals, including man, the male is the heterozygous sex, while in the Lepidoptera and birds the female is heterozygous.
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Biographical Memoirs of Fellows of the Royal Society (1964)