Some fish, crustaceans, insects such as butterflies and mothsand reptiles use this system. Recent comparative genomic studies support this hypothesis as the gene content of the primate Y chromosome has been stable over the last 25 million years, suggesting that an equilibrium gene content has been reached in humans .
Cambridge: University of Cambridge. It is perhaps the smallest gene occupying only 14 base pairs. Dacks J, Kasinsky H Nuclear condensation in protozoan gametes and the evolution of anisogamy. In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology.
It is exactly opposite the condition found in cockroaches and grasshoppers.
Sex portal. Females also experience a selective advantage from haplodiploidy but not paternal genome elimination because unfertilized eggs can develop and contribute to fitness when mating opportunities are rare. Social factors can act as primary sex-determining cues: sexually undifferentiated larvae of the marine green spoonworm that land on unoccupied sea floor develop into females and grow up to 15 cm longwhile larvae that come into contact with females develop into tiny males 1—3 mm long that live inside the female.
Other unusual systems include those of the swordtail fish [ clarification needed ] ;  the Chironomus midges [ clarification needed ] [ citation needed ] ; the juvenile hermaphroditism of zebrafishwith an unknown trigger;  and the platyfishwhich has W, X, and Y chromosomes.
Haplodiploidy: males develop from unfertilized, haploid eggs, and females from fertilized, diploid eggs e. Nature : —
In this system, the sex of an individual is determined by a pair of sex chromosomes. As Y-chromosome determines the male sex of the individual, it is also called androsome. An example of an X-linked gene is red-green colorblindness in humans. All animals have a set of DNA coding for genes present on chromosomes.
PLoS Biol 3 : e Steinemann S, Steinemann M Y chromosomes: born to be destroyed. Kuwabara; Peter G. New England Journal of Medicine. Located at : commons. Washington, DC: Smithsonian Books.