16.1.5—Meiosis I and II stages

Meiosis generates genetic variation in gametes through crossing over and independent assortment, and fertilisation adds further variation when gametes fuse at random. These processes change allele combinations without changing the basic haploid chromosome-set number of a gamete.

• During meiosis I, homologous chromosomes pair and non-sister chromatids can exchange corresponding sections at chiasmata. The exchanged DNA creates new combinations of alleles on the homologues.
• Crossing over is recombination between homologous non-sister chromatids; it is not exchange between sister chromatids or a change in 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.
• During fertilisation, any male gamete can fuse with any female gamete. Combining two independently varied haploid gamete pools gives zygotes with different allele combinations.

Crossing over changes chromosome segments, whereas independent assortment changes which whole homologues enter a gamete; random fertilisation combines two gametes. These are sources of variation associated with sexual reproduction, not the clonal outcome of ordinary mitosis, and no probability value is implied.