Genetic diversity through meiosis and fertilisation (independent assortment, crossing over): VCE Biology Unit 4

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.

Examples in context

Example 1. Tasmanian devil DFTD and lost diversity at University of Tasmania. University of Tasmania population geneticists tracked Tasmanian devil populations before and after devil facial tumour disease. The pre-disease population already had low diversity (a long-isolated island species). DFTD wiped out 80 percent of individuals, severely bottlenecking the gene pool. Survivors with rare MHC alleles (immune-recognition genes) now contribute disproportionately to the next generation. Sexual reproduction restores some variation each generation via independent assortment of about 7 chromosome pairs, crossing over and random fertilisation, but mutation alone supplies new alleles at a rate of about $10^{-8}$ per base per generation, far too slow to replace lost diversity. The Save the Tasmanian Devil Program maintains an insurance population of 600 devils with maximum allelic diversity.

Example 2. Recombination in Vitis vinifera at Australian Wine Research Institute. Australian Wine Research Institute breeders combine three sources of genetic variation when developing disease-resistant grape varieties. Crossing over during prophase I shuffles alleles within chromosomes from each parent. Independent assortment in metaphase I shuffles whole chromosomes. Random fertilisation then pairs each gamete with one of millions of possible gametes from the other parent. Mutation supplies the original variation - a rare allele for downy-mildew resistance arose in a Russian wild grape, was crossed into European wine grapes, and through several generations of recombination is now embedded in commercial Mornington Peninsula cultivars without dragging the wild plant's other characteristics with it.

Try this

Q1. Identify four sources of genetic diversity in a sexually reproducing population. [4 marks]

• Cue. Mutation (original source); crossing over during meiosis I; independent assortment of homologues; random fertilisation.

Q2. A diploid plant has 5 pairs of chromosomes. (a) Calculate the number of distinct gametes possible from independent assortment alone. (b) Estimate the total possible offspring from random fertilisation of two parents both contributing 2 to the power 5 distinct gamete types. [2+2 marks]

• Cue. (a) $2^5 = 32$. (b) $32 \times 32 = 1024$ possible distinct zygotes (ignoring crossing over).

Q3. Refer to the Tasmanian devil. (a) Define a population bottleneck. (b) Explain why recombination cannot quickly restore lost allelic diversity. (c) Outline how an insurance population is managed to maximise diversity. [2+2+2 marks]

• Cue. (a) A sharp reduction in population size, losing many alleles. (b) Recombination shuffles existing alleles; it cannot create new ones; mutation rate is too low to recover quickly. (c) Pair individuals with low mean kinship; maintain effective population size of several hundred.