It is thought that a
temperature-dependent amniote was the common ancestor of animals with sex chromosomes.
A sex-determination system is a biological system that determines the development of sexual characteristics in anorganism.
Most sexual organisms have two sexes.
Occasionally there are hermaphrodites in place of one or both sexes. There are
also some species that are only one sex due to parthenogenesis, the act of a female reproducing
without fertilization.
In many
species, sex determination is genetic: males and females have different alleles or even different genes that
specify their sexual morphology.
In animals this is often accompanied by chromosomal differences, generally through combinations
ofXY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is
generally triggered by a main gene (a "sex locus"), with a multitude
of other genes following in a domino effect.
In
other cases, sex is determined by environmental variables (such as temperature)
or social variables (e.g. the size of an organism relative to other members of
its population). Environmental sex determination preceded the genetically
determined system.
Chromosomal
determination
XX/XY sex chromosomes
The XX/XY sex-determination system is the most familiar, as it is found in humans. In the
system, females have two of the same kind of sex chromosome (XX), while males
have two distinct sex chromosomes (XY).
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