Heterogametic Sex Chromosomes and the Development of the Dominant Gonad in Vertebrates

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Abstract
Attempts to explain sex chromosomal mechanisms in terms of male- and female-determining genes date from the early days of genetic research. The absence of major sex-determining genes in normal individuals of Drosophila melanogaster led to the balance theory of sex determination. The discovery of the male-determining function of the mammalian Y chromosome, and the localization of the determinant to a specific region on this chromosome, gave new impetus to the search for sex-determining genes. Two hypotheses were proposed: that H-Y antigen, originally thought to be the product of the hypothetical testis-determining gene on the Y chromosome, causes testicular differentiation; and that Bkm highly repeated DNA sequences may be responsible for sex determination. The gene responsible for serologically detectable H-Y antigen production is not located on the Y chromosome and therefore is present in both sexes. Since its expression must be controlled by one or more other factor, it cannot be regarded as a primary sex differentiator. H-Y antigen as well as Bkm sequences are associated with the mammalian testis and the avian or serpentine ovary. A common denominator between mammalian testes and avian ovaries is that both are dominant, precociously developing gonads. The sex-determining mechanisms in birds and mammals may be dominance determining, causing the bipotential gonadal rudiment to enter the path of early growth and development and, depending on the structural organization of the embryo, leading to testicular development in mammals and ovarian development in birds. This view is supported by data on bilateral asymmetry of gonadal growth and differentiation in humans and chicks.