Genetic Causes of Variation - Meiosis and Chance

During meiosis 1 crossing over leads to each chromatid being different.During meiosis 1 the 23 pairs of homologous chromosomes (in humans) line up on the equator side by side arranged in a random order.Which chromosomes go to which daughter cell is a matter of chance.This random alignment of chromosomes leading to different combinations of paternal and maternal chromosomes in the daughter cells is called independent assortment.Independent assortment of chromosomes in Meiosis 1 and crossing over gives rise to variation.When the bivalents migrate to the equator and align on the metaphase plate they do so in a random order.There are 23 pairs of chromosomes which can be arranged in 2²³ different ways.No two chromosomes are identical due to their origin.So when the chromosomes move apart in anaphase each cell is genetically different because it has a different combination of maternal and paternal chromosomes which have been genetically recombined.Random fertilisation and random mating mean that which two gametes combine is a matter of chance.