Since the discovery of SRY/Sry as a testis-determining gene on the mammalian Y chromosome in 1990, extensive studies have been carried out on the immediate target of SRY/Sry and genes functioning in the course of testis development. Comparative studies in non-mammalian vertebrates including birds have failed to find a gene equivalent to SRY/Sry, whereas they have suggested that most of the downstream factors found in mammals including SOX9 are also involved in the process of gonadal differentiation. Although a gene whose function is to trigger the cascade of gene expression toward gonadal differentiation has not been identified yet on either W or Z chromosomes of birds, a few interesting genes have been found recently on the sex chromosomes of chickens and their possible roles in sex determination or sex differentiation are being investigated. It is the purpose of this review to summarize the present knowledge of these sex chromosome-linked genes in chickens and to give perspectives and point out questions concerning the mechanisms of avian sex determination.
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