Every cell in your body carries two complete sets of DNA that form an X. Each cell has the same exact sets. Half of the X is from your mother and half of the X is from your father. Together, the two halves make a complete chromosome. But when a sperm cell is created, only half of the father's chromosomes are used, and the choice of which bits of DNA that the cell picks is a random one. The same thing goes for a woman's eggs. When a sperm and an egg join to form a new person, the genes are from a random mix of all four grandparents. That's why it's very unlikely that siblings will look exactly the same.
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