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Humans have 23 pairs of chromosomes. 1 of each pair comes from our mother, and is contributed via the egg, the other from our father, contributed via the sperm that fertilises the egg.Eggs and sperm are gametes produced by a specialised form of cell division called meiosis. Meiosis is a two-staged process which leads to the halving of the number of chromosomes in the cell so eggs and sperm contain only 23 chromosomes (1 copy of each chromosome) as opposed to the 23 pairs of chromosomes found in the rest of our cells. This means that when a sperm meets and egg they fuse their genetic material producing an embryo with 23 pairs of chromosomes again.However, eggs do not complete the second stage of their meiotic cell division until they are fertilised. As soon as a sperm penetrates the egg it completes its meiosis by spitting out half of its chromosomes (which form a polar body).
Prior to fertilisation the egg is half way through its meiotic division. It completed the first division during which it doubled its genetic material, then threw half of it away. However, that's not the same as having the normal amount of genetic material because during the first division the duplicated homologous chromosomes (i.e. the one from the mother and the one from the father) get together. One of the homologous pair is picked to go into the egg, the other is thrown away. This process is random and gives rise to Mendel's law of "independent assortment". What you end up with in the egg (before fertilisation) is half the normal normal number of chromosomes, but present in duplicate. When fertilisation occurs half of the material is ditched so that the 'mature' egg contains half the normal chromosome number (hapolidy). Add to that the haploid contribution of the sperm and you get a dipolid zygote (embryo) (containing 23 pairs of chromosomes).This does not happen in sperm. Healthy mature sperm are always haploid (have half the usual number of chromosomes).