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Chromosomes are thread-like molecules that carry hereditary information for everything from height to eye color. They are made of protein and one molecule of DNA, which contains an organism's genetic instructions, passed down from parents. In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell. Humans have 22 of these chromosome pairs, called autosomes.
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XY sex-determination system

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The XY sex-determination system is the sex-determination system found in humans , most other mammals , some insects Drosophila , some snakes, some fish guppies , and some plants Ginkgo tree. In this system, the sex of an individual is determined by a pair of sex chromosomes. Females typically have two of the same kind of sex chromosome XX , and are called the homogametic sex. Males typically have two different kinds of sex chromosomes XY , and are called the heterogametic sex. In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development.
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