Meiosis: Sources of Genetic Variation (AQA A Level Biology): Revision Note

Meiosis: sources of genetic variation

• Having genetically different offspring can be advantageous for natural selection

• Meiosis has several mechanisms that increase the genetic diversity of gametes produced

• Both crossing over and independent assortment result in different combinations of alleles in gametes, which creates genetic variation

• This means each gamete carries substantially different alleles

• During fertilisation, any male gamete can fuse with any female gamete to form a zygote

• This random fusion of gametes at fertilisation creates genetic variation between zygotes, as each will have a unique combination of alleles

• The presence of genetically diverse zygotes contributes to the genetic diversity of a species

• There is almost zero chance that individual organisms resulting from successive sexual reproduction will be genetically identical

Chromosome combinations

After meiosis

• The number of possible chromosomal combinations resulting from meiosis is equal to 2ⁿ

  • n is the number of homologous chromosome pairs

• For humans:

  • The diploid number for humans is 46, then the haploid number or number of homologous chromosomes is 23, so the calculation would be:

    • 2²³ = 8 388 608 possible chromosomal combinations

After fertilisation

• In random fertilisation, any two gametes may combine

• Therefore, the formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is (2ⁿ)²

  • n is the haploid number and ² is the number of gametes

• In humans:

  • The haploid number is 23, so the number of combinations following fertilisation is (2²³)^2­­  = 70368744177664

• This explains why relatives can differ from one another. Even with the same parents, individuals can be genetically distinct due to variation at the meiosis and fertilisation stage (as well as other possible mutations and crossing-over)