# Selfing

Selfing or self-fertilization is the union of male and female gametes and/or nuclei from same haploid, diploid, or polyploid organism. It is an extreme degree of inbreeding.[1][2][3][4][5][6][7]

Selfing is widespread – from unicellular organisms – to the most complex hermaphroditic plants and animals (especially invertebrates). In unicellular organisms such as Protozoa, selfing can occur when two individuals (or their cell nuclei) interbreed that were produced from a previous mitotic division of the same individual.

Among hermaphrodite animals there are some that regularly reproduce by self-fertilization. In others, it is a rare event; selfing in such species is more common in adverse environmental conditions, or in the absence of a partner.

## Genetic consequences of selfing

Self-fertilization results in the loss of genetic variation within an individual (offspring), because many of the genetic loci that were heterozygous become homozygous. This can result in the expression of harmful recessive alleles, which can have serious consequences for the individual. The effects are most extreme when self-fertilization occurs in organisms that are usually out-crossing. After several generations, inbreeding depression is likely to purge the deleterious alleles from the population because the individuals carrying them have mostly died or failed to reproduce.

If no other effects interfere, the proportion of heterozygous loci is halved in each successive generation, as shown in the following table.

• Parental ${\displaystyle (P)}$: ${\displaystyle Aa}$ x ${\displaystyle Aa}$ (100%), and in
• ${\displaystyle F}$1 generation gives: ${\displaystyle 1AA}$ : ${\displaystyle 2Aa}$: ${\displaystyle 1aa}$, which means that the frequency of heterozygotes now is 50% of the starting value.
• By the ${\displaystyle F}$10 generation, heterozygotes have almost completely disappeared, and the population is polarized, with almost exclusively homozygous individuals (${\displaystyle AA}$ and ${\displaystyle aa}$)

Illustration model of the decrease in genetic variation in a population of self-fertilized organisms derived from a heterozygous individual, assuming equal fitness

 Generation AA (%) Aa (%) aa (%) P – 100 – F1 25 50 25 F2 37.5 25 37.5 F3 43.75 12.5 43.75 F4 46.875 6.25 46.875 F5 48.4375 3.125 48.4375 F6 49.21875 1.5625 49.21875 F7 49.609375 0.78125 49.609375 F8 49.8046875 0.390625 49.8046875 F9 49.90234375 0.01953125 49.90234375 F10 49.995117187 ≈ 50.0 0.009765626 ≈ 0.0 49.995117187 ≈ 50.0