Meiosis Increases Genetic Diversity (College Board AP® Biology)

Crossing Over

• 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 is the process by which nonsister chromatids exchange alleles

• Process:

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

  • The nonsister 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 trading 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 pair of homologous chromosomes during meiosis

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

Crossing Over Diagram

Crossing over increases genetic diversity by recombining alleles

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

The different combinations of chromosomes following 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

Independent Assortment of Chromosomes Diagram

The random orientation of chromosomes in meiosis I

Random Fertilization

• 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

• Zygotes eventually grow and develop into adults

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

Random Fertilization of Gametes and Variation

Meiosis and the random fusion of gametes affects genetic variation

The different combinations of chromosomes following fertilization

• In random fertilization, any two gametes may combine

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

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

  • Therefore in humans, when the haploid number is 23, the number of combinations following fertilization is (2²³)^2­­ = 70 368 744 177 664

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