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A review of the literature on avian cytogenetics reveals a long history of attempts to describe correctly the sex-determining mechanism. It was only in the last 10 or so years that technical sophistication permitted accurate chromosome identification. It is now clear that a majority of birds have a ZZ(♂)-ZW(♀) sex-determining mechanism. Males and females thus have equal chromosome numbers, but females have a block of genes, the W chromosome, peculiar to that sex.
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Stephen E. Bloom (1974) studied this question.
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