- Sexual reproduction combines genes from two organisms. Gametes are produced by meiosis. In this type of cell division each of the cells formed contains only one of each pair of homologous chromosomes, and therefore only one copy of each gene.
- Cells with only one chromosome from each pair are called haploid; cells with pairs of homologous chromosomes are diploid.
- In most organisms gametes are haploid while body cells are diploid.
- Formation of gametes by meiosis, followed by fertilisation, maintains a constant chromosome number from generation to generation.
- After the ovum is released from the ovary, it moves slowly along the oviduct. The sperms, which have limited energy stores because of their tiny cytoplasm, must swim up the oviduct to reach and fertilise the ovum. Many sperms fail to complete the journey.
- Mammalian sperms release digestive enzymes that break down the coating of the ovum and allow one sperm to reach and penetrate its membrane.
- Fertilisation is fusion of the nuclei of male and female gametes. It produces a diploid zygote.
- In most organisms there is a clear difference between male and female gametes. Male gametes are smaller, produced in much larger numbers and are motile. In mammals, ova have cytoplasm that contains nutrient reserves.
- The main advantage of sexual reproduction is the creation of variation in the offspring.
- Variation in a species provides a survival advantage. When environmental conditions change, it is more likely that there will be some individuals that are adapted to the changed conditions, and so the species will not be wiped out.
- Some species include both asexual and sexual reproduction in their life cycle. This has the advantage that they can
reproduce and spread rapidly in the asexual stage and introduce variation in the sexual stage.