6.2.3 Variation: Sexual Reproduction

• Organisms of the same species will have very similar genotypes, but two individuals will have differences between their DNA base sequences
  • Considering the size of genomes, these differences are small between individuals of the same species

• The small differences in DNA base sequences between individual organisms within a species is called genetic variation
• During sexual reproduction, genetic variation is transferred from one generation to the next and this generates phenotypic variation within a species
• During sexual reproduction, genetic variation is caused by the following processes, as they result in a new combination of alleles in a gamete or individual:
  • Crossing over of non-sister chromatids during prophase I of meiosis
  • Independent assortment of homologous chromosomes during metaphase I of meiosis
  • Random fusion of gametes during fertilisation

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

Random fusion of gametes

• 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 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

Mutations

• Although not specific to sexual reproduction, genetic variation can also be caused by mutations, as mutations can result in the generation of new alleles
  • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (due to the fact that the genetic code is degenerate
  • New alleles are not always seen in the individual that they first occur in
  • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

Sources of Genetic Variation Table