Sperm and Egg Cells

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In body cells (all your cells apart from sex cells), chromosomes come in pairs because we have two parents). One chromosome in every pair has come from each parent.

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The sex cells (the sperm and egg) are different from ordinary body cells because they contain just 23 single chromosomes - that's one chromosome from each pair.

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When the sperm fertilises the egg (during sexual reproduction), the 23 chromosomes in the sperm combine with the 23 chromosomes in the egg.

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The fertilised egg then has 23 pairs of chromosomes, just like an ordinary body cell.

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The two chromosomes in a pair always carry the same genes and each gene is always found in the same place on the two chromosomes. Because the two chromosomes in a pair came from different parents, they might have different alleles of these genes. Alleles are different versions of the same gene.

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