- Reproduction is classified into asexual and sexual types. In asexual reproduction, equivalent progeny is created by cytokinesis or other means, and no change occurs at the gene level.
- In sexual reproduction, paternal and maternal haploid gametes fuse together to form a diploid zygote. In multicellular organisms, this zygote divides repeatedly and develops into a new individual.
- The mechanism of creating haploid cells from diploid cells is called meiosis.
- During the meiotic process, crossover occurs between homologous paternal and maternal chromosomes. Gene recombination takes place at the crossover points, which changes the combination of genes. Many combinations of homologous chromosomes are distributed to gametes. As a result, diverse gametes with varied mixtures of paternal and maternal genes are produced.
- In higher organisms, gametes become increasingly specialized for fertilization. As an example, mammalian male gametes differentiate into sperms with a flagellum, spermatogenesis and well-developed mitochondria.
- Fertilization occurs through the fusion of specialized male and female gametes.
- Fertilization is precisely controlled to avoid multiple fertilizations and to make the process species-specific. As an example, for the fertilization of a sperm and an ovum to occur in mammals, the glycoproteins in the zona pellucida on the ovum surface must species-specifically bind to the receptors on the sperm membrane.