16.1.8—Random fertilisation and variation

Meiosis produces haploid gametes that already differ because of chromosome behaviour. Random fertilisation adds a further source of variation: any genetically different male gamete may fuse with any genetically different female gamete, so the zygote receives a new combination of alleles from the two parents.

• Each parent contributes one gamete, but the particular sperm and egg that fuse are not predetermined. Their random fusion samples from two varied gamete pools.
• The resulting zygote is diploid and combines one haploid chromosome set from each gamete; different pairings can therefore produce different allele combinations in offspring.

Meiosis creates variation within each parent through crossing over and independent assortment. Random fertilisation combines that parental variation, so offspring from the same parents can differ even when no new mutation is being considered.

Random fertilisation describes the genetic sampling principle, not a claim that every pairing has equal population frequency. This card completes the meiosis-to-offspring variation chain; it does not replace the neighbouring cards’ mechanisms for crossing over, independent assortment or image-stage identification.