3.4.3 Genetic diversity can arise as a result of mutation of during meiosis

Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. Mutagenic agents can increase the rate of gene mutation. Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.

Meiosis
Meiosis produces four daughter cells each with half the number of chromosomes as the parent cell. It is important as it (usually) produces gametes which have half the number of chromosomes which means the chromosome number of offspring does not double when two gametes fuse. During meiosis homologous pairs of chromosomes separate so only one chromosome from each pair enters a daughter cell (the haploid number). when two haploid gametes fuse during fertilisation the diploid/full number of chromosomes is restored. Meiosis involves two nuclear divisions (it's a bit like mitosis, but twice if you get me):

1. meiosis 1: homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged in a process known as crossing over. By the end of the is division the homologous pairs have separated with one chromosome from each pair going into one of two daughter cells
2. meiosis 2: the chromatids move apart and form 2 new cells. At the end of meiosis 4 cells have formed each with a diploid number of chromosomes

Meiosis also produces genetic variation among the offspring which may lead to adaptations that improve survival chances. It brings about genetic variation in the following ways:

• independent segregation of homologous chromosomes
• during meiosis 1 each chromosome lines up alongside its homologous partner at the equator, this arrangement is random. Which pair goes into each daughter cell with which other pairs depends on how they are lined up in the parent cell. Since the line up is random the combination of chromosomes that go into each cell is a matter of chance. This is called independent segregation.
• To further this, the alleles of each member of a homologous pair may differ. The independent assortment of these chromosomes produces new genetic combinations
• new combinations of maternal and paternal alleles (crossing over and recombination)
• after each chromosome lines up alongside its homologous partner the chromatids of each pair become twisted around one another. During this time tensions are created and portions of the chromatids break off. They then rejoin with the chromatids of its homologous pair. In this way new genetic combinations of maternal and paternal alleles are produced.
• If there is no recombination by crossing over only two different types o cell are produced. If recombination occurs four different types are produced. This means that crossing over further increases genetic variety.

As we know, homologous pairs line up at the equator during meiosis 1. Each of one pair can pass into each daughter (independent segregation) so there is a large number of possible combinations of chromosomes in any daughter cell. we can use the formula 2ⁿ where n is the number of pairs of homologous chromosomes to determine the number of possible combinations of chromosomes for each daughter cell.

Variety further increases through the random pairing of male and female gametes. We can calculate the number of combinations using the equation (2ⁿ)².

NOTE: It is important to realise that these calculations are based on chromosomes that do not undergo crossing over. If recombination occurs it will greatly increase the number of possible combinations in the gametes.