(ii) how sexual reproduction can lead to genetic variation within a species

The idea

Sexual reproduction generates variation because offspring receive a new combination of alleles from two parents. Meiosis produces gametes that are haploid and genetically different: crossing over between non-sister chromatids of homologous chromosomes in prophase I, and independent assortment of homologous chromosomes in metaphase I, shuffle alleles. Independent assortment of chromatids in metaphase II adds further combinations. Random fertilisation then pairs one gamete from each parent, so many allele combinations are possible. Mutation creates new alleles in the first place. The result is that members of a species differ genetically, which is the raw material for natural selection.

For example: A parent heterozygous at two unlinked loci can produce four gamete types in equal proportions, and fertilisation multiplies the possible offspring genotypes.

What earns the marks

• Meiosis produces haploid gametes, halving the chromosome number so gametes carry one allele of each gene.
• Crossing over between non-sister chromatids of homologous chromosomes in prophase I exchanges alleles and creates recombinant chromatids.
• Independent assortment of homologous chromosomes in metaphase I, and of chromatids in metaphase II, gives many random combinations of maternal and paternal alleles.
• Random fertilisation of two gametes combines alleles from two parents, producing genetically distinct offspring.
• Mutation is the original source of new alleles, which sexual reproduction then reshuffles.
• Genetic variation within a species provides the variation on which natural selection can act.

Common mistakes and how to put them right

• Thinking sexual reproduction itself creates new alleles.

  Put it right: Mutation creates new alleles; meiosis and fertilisation reshuffle existing alleles.

• Confusing crossing over with independent assortment.

  Put it right: Crossing over exchanges sections between non-sister chromatids, whereas independent assortment is the random alignment and separation of whole chromosomes or chromatids.

• Believing offspring are identical to parents or to each other.

  Put it right: Random fertilisation and meiosis produce genetically distinct offspring.

• Assuming crossing over occurs between sister chromatids.

  Put it right: It occurs between non-sister chromatids of homologous chromosomes.

Exam tips

• Name the stage of meiosis for each source of variation (prophase I for crossing over, metaphase I and II for independent assortment).
• Use precise terms: haploid gametes, homologous chromosomes, non-sister chromatids, random fertilisation.
• Link variation explicitly to its importance for natural selection to gain credit for the 'within a species' clause.