In many organisms including vertebrates sex is determined by the possession of a particular combination of chromosomes. In some cases, presence or absence of one special chromosome, known as the accessory or X-chromosome, is the determining factor, eg in the insect Pyrrhocoris apterus males and females have 13 and 14 chromosomes respectively. In many species there are two sex chromosomes, and the sex of the individual depends on whether it has two identical chromosomes, the homogametic sex, or one of each of the two types, the heterogametic sex. Where the female is homogametic, as in mammals, the two chromosomes are designated XX, and the male

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