Abstract
Why are there two chromosomal sex-determining mechanisms in vertebrates; ZW/ZZ, meaning female heterogamety, and XX/XY, meaning male heterogamety? We propose an evolutionary explanation. Transition from environmental sex determination to genetic sex determination can result when an allele that accelerates embryonal or larval growth takes over the role of environment in bringing about differential growth. Species with environmental sex determination showing female size advantage, are predisposed to evolve the ZW/ZZ mechanism, whereas species with male size advantage are predisposed to evolve the XX/XY mechanism.
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