the sources of genetic diversity within a sexually reproducing population, including independent assortment of chromosomes, crossing over during meiosis, random fertilisation, and the role of mutation as the original source of variation

What this dot point is asking

VCAA wants the four sources of genetic variation in a sexually reproducing population, with the stage of meiosis or fertilisation for each, and an understanding of why mutation is the ultimate source.

The answer

A sexually reproducing population carries a pool of genetic variation that is essential for evolution and species survival. Variation arises from four sources, three of which operate during meiosis or fertilisation and shuffle existing alleles, plus mutation, which creates new ones.

Independent assortment (metaphase I of meiosis)

During metaphase I, homologous chromosome pairs (bivalents) line up at the cell equator. The orientation of each bivalent is random and independent of every other pair. Either homologue can face either pole.

For an organism with n chromosome pairs, the number of possible chromosome combinations in a gamete is 2^n. Humans have 23 pairs, giving 2^23 (about 8.4 million) combinations per gamete from independent assortment alone, before any crossing over.

This is why siblings (who share the same parents) inherit different mixes of maternal and paternal chromosomes.

Crossing over (prophase I of meiosis)

During prophase I, homologous chromosomes pair tightly along their length (synapsis), forming bivalents or tetrads. Non-sister chromatids exchange segments at points called chiasmata.

The result is recombinant chromatids: chromatids that carry a mixture of maternal and paternal alleles on a single chromosome. Without crossing over, alleles on the same chromosome would always travel together. Crossing over breaks linkage and produces new allele combinations.

Crossing over typically occurs one to three times per chromosome arm. The chance of recombination between two loci is roughly proportional to the distance between them, which is the basis of genetic mapping.

Random fertilisation

In humans, one ejaculation releases hundreds of millions of sperm, each genetically unique because of the variation produced in meiosis. Any one of these sperm can fertilise the egg, which is itself one of a unique set of possible egg genotypes.

Even ignoring crossing over, a single couple can produce 2^23 x 2^23 (about 70 trillion) genotypically distinct zygotes from random fertilisation alone. With crossing over, the number is effectively infinite.

Mutation: the original source

Independent assortment, crossing over and random fertilisation only shuffle existing alleles. They do not create new ones.

A mutation is a change in the DNA sequence. New alleles arise only by mutation. Although mutations occur at a low rate, over many generations they introduce the new genetic material that meiosis and fertilisation then shuffle.

Mutation is therefore described as the ultimate or original source of variation, while meiosis and fertilisation are the proximate sources of variation between siblings within a generation.

For mutation to contribute to evolution it must occur in germline cells (sperm or egg precursors) so that it can be passed to offspring. Somatic mutations affect only the individual.

Why genetic diversity matters

Genetic diversity in a population is the raw material for natural selection. A diverse population is more likely to contain individuals whose phenotype suits a new selection pressure (drought, disease, predation), increasing the chance that the population survives and adapts. Small, inbred populations have low diversity and are at higher risk of extinction.

Worked example

A pea plant is heterozygous at two unlinked genes: Aa Bb. During meiosis:

• Independent assortment at metaphase I lets either A or a travel with either B or b, producing four possible gametes (AB, Ab, aB, ab) in equal proportions.
• Crossing over between linked loci (if they were on the same chromosome) would produce the same four gametes, with the recombinant types appearing less often than the parental types.
• Random fertilisation with a similarly heterozygous pollen grain produces nine offspring genotypes in a 9:3:3:1 phenotypic ratio.
• Mutation in one gamete could replace an A allele with a brand new A* allele that gives a phenotype no parent has.

The first three processes are predictable in their statistical outcomes; only the fourth introduces something genuinely new.

Common traps

Saying mutation "happens during meiosis" only. Mutations occur whenever DNA is copied or damaged, in any cell. Only germline mutations are heritable.

Confusing independent assortment with crossing over. Independent assortment shuffles whole chromosomes at metaphase I. Crossing over exchanges segments of chromatids during prophase I.

Forgetting random fertilisation. Many students name only meiotic sources. Random fertilisation roughly squares the variation produced by meiosis.

Saying meiosis "creates new alleles". Meiosis recombines existing alleles. Only mutation creates new alleles.

Calling 2^23 a small number. 2^23 is about 8.4 million; combined with crossing over and random fertilisation it is effectively limitless.

In one sentence

Sexually reproducing populations gain genetic diversity from independent assortment of homologues at metaphase I (about 8.4 million combinations in humans), crossing over between non-sister chromatids at prophase I, random fertilisation of two genetically unique gametes (over 70 trillion possible zygotes per couple), and mutation, which is the only source of genuinely new alleles and is therefore the ultimate origin of all variation.