Heterozygote advantage
In this instance, the state of the organism's environment will provide selection, with a net effect either favoring or working in opposition to the gene, until an environmentally determined equilibrium is reached.
Previous research, comparing measures of dominance, overdominance and epistasis (mostly in plants), found that the majority of cases of heterozygote advantage were due to complementation (or dominance), the masking of deleterious recessive alleles by wild-type alleles, as discussed in the articles Heterosis and Complementation (genetics), but there were also findings of overdominance, especially in rice.
[3] More recent research, however, has established that there is also an epigenetic contribution to heterozygote advantage, primarily as determined in plants,[5][6] though also reported in mice.
[8][9] The first experimental confirmation of heterozygote advantage was with Drosophila melanogaster, a fruit fly that has been a model organism for genetic research.
[10] If weakness were the only effect of the mutant allele, so it conveyed only disadvantages, natural selection would weed out this version of the gene until it became extinct from the population.
This mutation, which at first glance appeared to be harmful, conferred enough of an advantage to heterozygotes to make it beneficial, so that it remained at dynamic equilibrium in the gene pool.
In experimental populations, the ebony allele was more prevalent and therefore advantageous when flies were raised at low, dry temperatures, but less so in warm, moist environments.
When untreated, a person with SCA may suffer from painful periodic bouts, often causing damage to internal organs, strokes, or anemia.
Because the genetic disorder is incompletely recessive, a person with only one SCA allele and one unaffected allele will have a "mixed" phenotype: The sufferer will not experience the ill effects of the disease, yet will still possess a sickle cell trait, whereby some of the red blood cells undergo benign effects of SCA, but nothing severe enough to be harmful.
Cystic fibrosis (CF) is an autosomal recessive hereditary monogenic disease of the lungs, sweat glands and digestive system.
The disorder is caused by the malfunction of the CFTR protein, which controls intermembrane transport of chloride ions, which is vital to maintaining equilibrium of water in the body.
The presence of a single CF mutation may influence survival of people affected by diseases involving loss of body fluid, typically due to diarrhea.
[18] The most recent hypothesis, published in the Journal of Theoretical Biology, proposed having a single CF mutation granted respiratory advantage for early Europeans migrating north into the dusty wasteland left by the Last Glacial Maximum.
Triosephosphate isomerase (TPI) is a central enzyme of glycolysis, the main pathway for cells to obtain energy by metabolizing sugars.
These findings constitute evidence that heterozygosity provides an advantage among carriers of different supertype HLA-DRB1 alleles against HCV infection progression to end-stage liver disease in a large-scale, long-term study population.
[20] Multiple studies have shown, in double-blind experiments, females prefer the scent of males who are heterozygous at all three MHC loci.
[23] The latter claim has been tested in an experiment, which showed outbreeding mice to exhibit MHC heterozygosity enhanced their health and survival rates against multiple-strain infections.
A variant of the gene containing a deletion (GCTGT—>A) renders a shorter mRNA transcript that escapes degradation by microRNA, thus increasing expression of BAFF, which consequently up-regulates the humoral immune response.
