Meiosis: Sources of Genetic Variation (CIE A Level Biology)

• 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