Meiosis & Genetic Diversity (College Board AP® Biology): Study Guide

The importance of correct chromosome separation

Separation in meiosis I

• In meiosis I, homologous chromosomes (one maternal and one paternal) pair up and then separate

• This ensures that each gamete receives one chromosome from each pair, creating a haploid (1n) set

• The chromosomes inherited are a mix of maternal and paternal, contributing to genetic variation

Separation in meiosis II

• In meiosis II, sister chromatids of each chromosome separate

• The result is gametes that still contain a haploid set, but now with one chromatid from each chromosome

• This further ensures genetic diversity

Assortment of chromosomes

• The way maternal and paternal homologues line up in meiosis I is random (independent assortment)

• This produces new combinations of alleles in gametes

Errors in separation (nondisjunction)

• Sometimes homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) fail to separate correctly, this is called nondisjunction

• This leads to gametes with the wrong number of chromosomes (aneuploidy)

• Nondisjunction during meiosis produces gametes with too many or too few chromosomes, e.g. n+1 or n–1

  • In Meiosis I: homologous chromosomes don’t separate

  • In Meiosis II: sister chromatids don’t separate (in one of the two cells)

• For example: fertilisation involving a gamete with an extra chromosome 21 can cause Down syndrome (trisomy 21).

Crossing over

• Meiosis has several mechanisms that increase the genetic diversity of gametes produced which can be advantageous for natural selection

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

• This process occurs during prophase I of meiosis I

  1. Homologous chromosomes pair up and are close to each other

  2. Sections of DNA from non-sister chromatids can cross over and get entangled

  3. These crossing points are called chiasmata

  4. 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, which increases genetic diversity among the resultant gametes

Independent assortment

• Independent assortment is another process that increases variation during gamete formation

• It involves 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 (of meiosis)

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

  • The process is random

  • Each pair can be arranged with either chromosome on top

  • The orientation of one homologous pair is 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

Random fertilization

• 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

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