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

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.

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

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.

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

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.

Mutations occur whenever DNA is copied or damaged, in any cell. Only germline mutations are heritable.

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

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

Meiosis recombines existing alleles. Only mutation creates new alleles.

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