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

• Having genetically different offspring can be advantageous for natural selction
• 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 Diagram

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 Diagram

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 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

Fusion of Gametes Diagram

How meiosis and the random fusion of gametes affects genetic variation