What Causes Genetic Diversity? Mutation and Meiosis
Learn how mutation creates new alleles and meiosis reshuffles them through independent segregation and crossing over in AQA A Level Biology, with an example.
Genetic diversity comes from mutation creating new alleles and meiosis producing different combinations of alleles in gametes. An allele is a version of a gene; a gamete is a reproductive cell, such as a sperm or egg.
Mutation creates new alleles
A gene mutation changes the base sequence of DNA. For example, a base may be substituted or deleted. This can produce a new allele and sometimes change the amino acid sequence of a protein. Mutations arise spontaneously during DNA replication, and mutagens can increase their rate.
Not every mutation changes a protein or has a harmful effect. Because the genetic code is degenerate, more than one DNA base triplet can code for the same amino acid. A substitution may therefore leave the amino acid sequence unchanged.
Meiosis reshuffles alleles
Meiosis makes gametes with different combinations of alleles in two main ways:
- Independent segregation: pairs of homologous chromosomes separate independently during the first division of meiosis. Each gamete receives a different mix of chromosomes inherited from the two parents.
- Crossing over: homologous chromosomes exchange sections of DNA, producing new combinations of alleles on a chromosome.
For example, with just 222 pairs of homologous chromosomes, independent segregation alone can produce 22=42^2=422=4 chromosome combinations in gametes. Crossing over can increase the variety further. Random fertilisation then combines any one sperm with any one egg, adding another source of variation among offspring.
In a population, genetic diversity means the number of different alleles present. Mutation can increase that number if a new allele persists. Meiosis does not necessarily increase the number of alleles in the population, but it changes which alleles occur together in individual gametes. Offspring can therefore inherit combinations different from either parent.
Exam mistake to avoid: meiosis usually does not create new alleles. It rearranges alleles already present; mutation is the original source of new alleles.
