Random Fertilisation

Fertilisation is the fusion of a male gamete and a female gamete to form a zygote. Each gamete is haploid, so fertilisation restores the diploid chromosome number (46 in humans). The zygote receives one chromosome of each homologous pair from each parent.Fertilisation is random: any male gamete can fuse with any female gamete. Which sperm reaches the egg first is a matter of chance and does not depend on the alleles it carries.This greatly increases genetic variation. Meiosis already makes each parent's gametes genetically different. Random fertilisation then combines one of these unique gametes from each parent.Worked example: independent assortment alone lets a human make 2²³ different gametes. The number of different zygotes two parents could produce is therefore:2²³ × 2²³ = 2⁴⁶ ≈ 7.0 × 10¹³That is about 70 trillion combinations, before crossing over is even counted. Crossing over makes the true number far larger, which is why siblings (apart from identical twins) are never genetically identical.So there are three sources of variation in sexual reproduction: crossing over and independent assortment during meiosis, and random fertilisation. The first two create new combinations of alleles within gametes; random fertilisation creates new combinations between two individuals. Together they give each offspring a new genotype, which gives natural selection a range of individuals to act on.