• 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

Crossing over of non-sister chromatids leading to the exchange of genetic material

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 8 324 608 different combinations

Independent assortment of homologous chromosomes leading to different genetic combinations in daughter cells

• 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 fertilization any male gamete can fuse with any female gamete to form a?zygote
• This?random fusion of gametes?at fertilization 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

How meiosis and the random fusion of gametes affects genetic variation