The production of offspring by the fusion of genetically closely related gametes. Self fertilization is the most intense form of inbreeding. It is widespread in the plant kingdom, being inevitable in cleistogamous flowers and during lateral conjugation (e.g. in Spirogyra). Inbreeding increases homozygosity and hence decreases genetic variability. Thus when a population heterozygous for a particular gene, e.g. Aa, is selfed, 50% of the next generation will be homozygous for that gene (i.e. either AA or aa). Since the frequency of homozygous recessive lethals and semilethals increases by inbreeding, this can lead to a decline in vigour in species that are normally outbreeding. This is called inbreeding depression. Some species have developed systems (outbreeding mechanisms) to inhibit excessive inbreeding. Nevertheless, some degree of inbreeding can be an advantage, for example to a rapidly speciating population where it prevents the population being 'swamped' by foreign genes. Compare outbreeding. |
