Cam xy sex
Studies on the evolution of sex chromosome systems have been based during the last 40 years on the hypothesis of Susumu Ohno (1967) who observed the variable size of the W chromosome and the invariable size of the Z in different snake families.
All vertebrate males have testes that are similar in anatomy and in spermatogenesis.
It is singled out in this review because of its likely key role in sex determination both in birds and in reptiles with TSD.
DMRT1 is one of the few sex genes that have been mapped in representative species of fish, turtles, crocodiles, amphibians, birds and mammals.
Genes homologous to the mammalian X are located on chicken chromosomes 1, 4 and 12.
Nearly 40 years later the basis of Ohno’s hypothesis still stands, although it now requires some modification.(1) The marsupial X chromosome differs from the eutherian X chromosome in that genes homologous with the short arm of the human X are autosomal and do not require dosage compensation (Spencer et al., 1991).(2) The chloride channel gene (CLCN4) is X-linked and X-inactivated in humans and Mus spretus but maps to chromosome 7 in Mus musculus (Palmer et al., 1995).