Dominance versus overdominance
Encyclopedia
Dominance versus overdominance is a scientific controversy in the field of genetics
that has persisted for more than a century. These two alternative hypotheses were first stated in 1908.
is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be homozygous for recessive
alleles that are mildly harmful (or produce a trait that is undesirable from the standpoint of the breeder). Heterosis
or hybrid vigor, on the other hand, is the tendency of outbred strains to exceed both inbred parents in fitness.
Selective breeding of plants and animals, including hybridization, began long before there was an understanding of underlying scientific principles. In the early 20th century, after Mendel's laws came to be understood and accepted, geneticists undertook to explain the superior vigor of many plant hybrids. Two competing hypotheses, which are not mutually exclusive, were developed:
Dominance and overdominance have different consequences for the gene expression profile
of the individuals. If over-dominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents. On the other hand, if dominance is the cause, fewer genes should be under-expressed in the heterozygous offspring compared to the parents. Furthermore, for any given gene, the expression should be comparable to the one observed in the fitter of the two parents.
, who in his younger days believed that overdominance was a major contributor to hybrid vigor undertook a retrospective review of the developing science. According to Crow, the demonstration of several cases of heterozygote advantage
in Drosophila and other organisms first caused great enthusiasm for the overdominance theory among scientists studying plant hybridization. But overdominance implies that yields on an inbred strain should decrease as inbred strains are selected for the performance of their hybrid crosses, as the proportion of harmful recessives in the inbred population rises. Over the years, experimentation in plant genetics has proven that the reverse occurs, that yields increase in both the inbred strains and the hybrids, suggesting that dominance alone may be adequate to explain the superior yield of hybrids. Only a few conclusive cases of overdominance have been reported in all of genetics. Since the 1980s, as experimental evidence has mounted, the dominance theory has made a comeback.
Crow writes, "The current view ... is that the dominance hypothesis is the major explanation of inbreeding decline and the high yield of hybrids. There is little statistical evidence for contributions from overdominance and epistasis
. But whether the best hybrids are getting an extra boost from overdominance or favorable epistatic contributions remains an open question."
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
that has persisted for more than a century. These two alternative hypotheses were first stated in 1908.
Genetic basis of heterosis
When a population is small or inbred, it tends to lose genetic diversity. Inbreeding depressionInbreeding depression
Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...
is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be homozygous for recessive
Recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...
alleles that are mildly harmful (or produce a trait that is undesirable from the standpoint of the breeder). Heterosis
Heterosis
Heterosis, or hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic....
or hybrid vigor, on the other hand, is the tendency of outbred strains to exceed both inbred parents in fitness.
Selective breeding of plants and animals, including hybridization, began long before there was an understanding of underlying scientific principles. In the early 20th century, after Mendel's laws came to be understood and accepted, geneticists undertook to explain the superior vigor of many plant hybrids. Two competing hypotheses, which are not mutually exclusive, were developed:
- Dominance hypothesis. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable recessive alleles from one parent by dominant alleles from the other. It attributes the poor performance of inbred strains to loss of genetic diversity, with the strains becoming purely homozygous at many loci. The dominance hypothesis was first expressed in 1908 by the geneticist Charles DavenportCharles DavenportCharles Benedict Davenport was a prominent American eugenicist and biologist. He was one of the leaders of the American eugenics movement, which was directly involved in the sterilization of around 60,000 "unfit" Americans and strongly influenced the Holocaust in Europe.- Biography :Davenport was...
. - Overdominance hypothesis. Certain combinations of alleles that can be obtained by crossing two inbred strains are advantageous in the heterozygoteHeterozygote advantageA heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
. The overdominance hypothesis attributes to heterozygote advantage the survival of many alleles that are recessive and harmful in homozygotes. It attributes the poor performance of inbred strains to a high percentage of these harmful recessives. The overdominance hypothesis was developed independently by Edward M. East (1908) and George ShullGeorge Harrison ShullGeorge Harrison Shull was an eminent American plant geneticist. He was born in Clark Co., Ohio, graduated from Antioch College in 1901 and from the University of Chicago in 1904, served as botanical expert to the Bureau of Plant Industry in 1903-04, and thenceforth was a botanical investigator of...
(1908).
Dominance and overdominance have different consequences for the gene expression profile
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
of the individuals. If over-dominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents. On the other hand, if dominance is the cause, fewer genes should be under-expressed in the heterozygous offspring compared to the parents. Furthermore, for any given gene, the expression should be comparable to the one observed in the fitter of the two parents.
Historical retrospective
Population geneticist James CrowJames F. Crow
James F. Crow is Professor Emeritus of Genetics at the University of Wisconsin–Madison.Some of his most significant peer-reviewed contributions were coauthored with Motoo Kimura. His major contribution to the field, however, is arguably his teaching...
, who in his younger days believed that overdominance was a major contributor to hybrid vigor undertook a retrospective review of the developing science. According to Crow, the demonstration of several cases of heterozygote advantage
Heterozygote advantage
A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
in Drosophila and other organisms first caused great enthusiasm for the overdominance theory among scientists studying plant hybridization. But overdominance implies that yields on an inbred strain should decrease as inbred strains are selected for the performance of their hybrid crosses, as the proportion of harmful recessives in the inbred population rises. Over the years, experimentation in plant genetics has proven that the reverse occurs, that yields increase in both the inbred strains and the hybrids, suggesting that dominance alone may be adequate to explain the superior yield of hybrids. Only a few conclusive cases of overdominance have been reported in all of genetics. Since the 1980s, as experimental evidence has mounted, the dominance theory has made a comeback.
Crow writes, "The current view ... is that the dominance hypothesis is the major explanation of inbreeding decline and the high yield of hybrids. There is little statistical evidence for contributions from overdominance and epistasis
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
. But whether the best hybrids are getting an extra boost from overdominance or favorable epistatic contributions remains an open question."