... for a normal characteristic, such as human height. Discontinuous variation is where characteristics are fit into separate categories with no intermediates such as human blood groups. During the process of meiosis each homologous pair of chromosomes aligns independently along the equator and whether the maternal or paternal chromosome faces a particular pole is quite random. This is known as independent assortment and is one of the causes of variation in the gametes. Unlinked genes are transmitted independently of each other, leading to random distribution of genes. Alleles can also be separated and reformed if crossing over occurs. This is where the paternal and maternal chromatids of chromosomes intertwine and cross over each other at points called chiasmata. The chromatids break at these points and rejoin so that the alleles from one chromatid join alleles on another. An exchange of chromatids takes place between chromatids. Although sexual reproduction produces variation it ...

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