Recall that the somatic cells in animals contain chromosomes in pairs that are similar. After replication, each chromosome looks like a pair of sausages tied together. The pairs may have varying lengths and they may be bound at different points. If bound near the middle, they resemble an X. If bound near one end, they look like a Y. However, in most animals one pair can be dissimilar. The chromosomes of one of the 23 pairs in humans are called sex chromosomes. It contains the genes that determine a person's gender, among other things. In females, both chromosomes of this pair are X's; in males one of the pair is much smaller and has the Y form. The Y chromosome contains a gene for a factor that stimulates the formation of male sexual structures, and thence the male sex hormones. The absence of this gene results in the formation of a female. Thus, each cell in a female contains an XX pair of sex chromosomes, and each cell in a male contains an XY pair. Chromosomes other than the sex chromosomes are called autosomes.
Because the chromosome pairs are separated by meiosis in the formation of gametes, the gametes from females (egg cells) can have only X chromosomes. Half of the gametes from males (sperm cells) have X and half have Y chromosomes. Thus, the sperm determines whether the individual resulting from fertilization will be male or female.
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