16.1.7—Genetic variation in meiosis

Meiosis generates genetically varied gametes through two distinct chromosome processes: crossing over changes allele combinations within homologues, while independent assortment changes which whole homologues enter each gamete. Random fertilisation adds variation when gametes fuse.

• During meiosis I, homologous chromosomes pair and non-sister chromatids can exchange corresponding sections at chiasmata. A section can break and rejoin with the other chromatid, creating new combinations of alleles.
• Crossing over is recombination between homologous non-sister chromatids; it is not exchange between sister chromatids and does not change chromosome number.

• At metaphase I, each homologous pair can align in either orientation independently of the other pairs. Which maternal or paternal homologue moves to each pole therefore changes the whole-chromosome combinations in the resulting gametes.
• Independent assortment changes combinations of whole chromosomes, whereas crossing over changes combinations within chromosome segments.

During fertilisation, any male gamete can fuse with any female gamete. Combining two independently varied haploid gamete pools produces zygotes with different allele combinations.

Crossing over, independent assortment and random fertilisation are distinct sources of variation. They do not imply a fixed probability for one family, and this card does not cover image-stage identification or detailed inheritance calculations.