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The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes (autosomes), and are sometimes called allosomes.In some species, such as humans, organisms remain sex indifferent for a time after they're created; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.Even though the gene DMRT1 is used by birds as their sex locus, species who have XY chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their formation.

Transcaucasian mole voles (Ellobius lutescens) also have a form of XO determination, in which both sexes lack a second sex chromosome.

Human fetus genitals can sometimes develop abnormalities during maternal pregnancies due to mutations in the fetuses sex-determinism system, resulting in the fetus becoming a hermaphrodite.

The XX/XY sex-determination system is the most familiar, as it is found in humans.

This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea).

A small number of mammals also lack a Y chromosome.

Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene.

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