Meiosis: Sources of Genetic Variation (Cambridge (CIE) 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 (random orientation) result in different combinations of alleles in gametes

Crossing over

• Crossing over is the process by which non-sister chromatids exchange alleles

• Process:

  • During meiosis I homologous chromosomes pair up and are in very close proximity to each other

  • The non-sister chromatids can cross over and get entangled

  • These crossing points are called chiasmata

  • The entanglement places stress on the DNA molecules

  • As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

• This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes

• There is usually at least one, if not more, chiasmata present in each bivalent during meiosis

• Crossing over is more likely to occur further down the chromosome away from the centromere

Independent assortment

• Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I

• The different combinations of chromosomes in daughter cells increases genetic variation between gametes

• In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle

  • Each pair can be arranged with either chromosome on top, this is completely random

  • The orientation of one homologous pair is independent (unaffected by the orientation of any other pair)

• The homologous chromosomes are then separated and pulled apart to different poles

• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up

• To work out the number of different possible chromosome combinations the formula 2ⁿ can be used, where n corresponds to the number of chromosomes in a haploid cell

  • For humans this is 2²³ which calculates as 8 388 608 different combinations

Fusion of gametes

• Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment

• 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

• There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical