- the law of segregation says that alleles occur in pairs and that each gamete receives only one allele of each pair. The alleles 'segregate' or separate into different gametes. eg heterozygous person (Aa) has one copy of A allele and one copy of the a allele. After meiosis 50% of gametes carry the A allele and 50% carry a.If individuals was heterozygous for 2 genes AaBb, equal numbers of AB,Ab,aB, ab gametes are produced due to segregation
- The fact that each gametes receives only one randomly selected copy of any pair of alleles increases the chance of variation after fertilisation
Independent assortment as a source of variation
- Independent assortment occurs when homologous chromosome pairs line up in random order at the centre of the cell during metaphase I of meiosis. This results in variation due to the large number of possible combinations produced by different arrangement of chromosomes. eg in a human cell with 23 pairs, there can be 8 million different combinations of alleles.
Recombination during meiosis as a source of variation
- Recombination involves the production of new combinations of alleles. Recombination of genes is called independent assortment. The recombination of alleles of genes on the same chromosome is called crossing over. Crossing over occurs when two homologous chromosomes line up together at the beginning of meiosis. The ends of the non-sister chromatids lie over each other, break and then rejoin onto the other chromatid. The site where the chromatids cross over each other is the chiasma. This increases genetic variation because it produces new combinantions of alleles not possible through independent assortment alone.
Fertilisation
- Both ovum and sperm have a unique combination of alleles due to independent assortment, segragation and crossing over. It is also chance which sperm gets to fertilise the ovum so the fertilised egg would have a unique set of alleles.