How does meiosis contribute to variation in offspring?
Meiosis produces great genetic diversity among an organism?s sex cells for two reasons. To see how, let?s consider the eggs produced by a human female. Like all humans, this woman has two of each kind of chromosome, one that she inherited from her mother and one that she inherited from her father. First, as we have seen, crossing over during meiosis I causes the woman?s homologous chromosomes to exchange parts, so that most of the chromosomes in her eggs are unique composites of the chromosome she inherited from her mother and the chromosome she inherited from her father. Even if there were no such thing as crossing over, however, the woman would still produce a huge number of genetically different eggs. This is because each pair of homologous chromosomes separates independently during meiosis I. If we ignore crossing over, each egg would receive either the chromosome the woman inherited from her mother or the chromosome she inherited from her father. One egg could receive chromosomes 1, 3, 4, 5, 7, 10, 13, etc., from her mother and chromosomes 2, 6, 8, 9, 11, 12, etc., from her father. A second egg she produces is almost certain to receive a different set of chromosomes, perhaps chromosomes 2, 3, 5, 6, 7, etc., from her mother and chromosomes 1, 4, 8, 9, etc., from her father. It is clear that the independent separation of homologous chromosomes alone produces a huge number of possible egg cells. Crossing over only expands the possibilities. It?s no wonder that no two eggs or sperm produced by a single individual are alike. This genetic diversity produced during meiosis is crucial to evolution.