Dominance (genetics)
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In genetics, dominance is defined as the interactions between alleles at the same locus on homologous chromosomes and the associated phenotype.[1] In the case of complete dominance, one allele in a heterozygote individual completely overrides or masks the phenotypic contribution of the other allele. The overriding allele is referred to as dominant and the masked one recessive.[2] Complete dominance, also referred to as Mendelian inheritance, follow Mendel's laws of segregation. The first law states that each allele in a pair of genes is separated at random and have an equal probability of being transferred to the next generation, while the second law states that the distribution of allele variants is done independently of each other. However, this is not always the case as not all genes segregate independently and violations of this law are often referred to as "non-Mendelian inheritance".[3]
The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (
Letters and
Dominance is not inherent to an allele or its traits (phenotype). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, and co-dominant with a fourth. Additionally, one allele may be dominant for one trait but not others[1].
Background
Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.
In 1928, British population geneticist
Types of dominance
Complete dominance (Mendelian)
In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks are considered dominant to the other allele, and the masked allele is considered recessive.[2]
When we only look at one trait determined by one pair of genes, we call it monohybrid inheritance. If the crossing is done between parents (P-generation, F0-generation) who are homozygote dominant and homozygote recessive, the offspring (F1-generation) will always have the heterozygote genotype and always present the phenotype associated with the dominant gene.
However, if the F1-generation is further crossed with the F1-generation (heterozygote crossed with heterozygote) the offspring (F2-generation) will present the phenotype associated with the dominant gene ¾ times. Note that although heterozygote monohybrid crossing can result in two phenotype variants, it can result in three genotype variants - homozygote dominant, heterozygote and homozygote recessive, respectively.[9]
In dihybrid inheritance we look at the inheritance of two pairs of genes simultaneous. Assuming here that the two pairs of genes are located at non-homologous chromosomes, such that they are not coupled genes (see genetic linkage) but instead inherited independently. Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively. The offspring (F1-generation) will always heterozygous and present the phenotype associated with the dominant allele variant.
However, when crossing the F1-generation there are four possible phenotypic possibilities and the phenotypical ratio for the F2-generation will always be 9:3:3:1.[10]
Incomplete dominance (non-Mendelian)
Incomplete dominance (also called partial dominance, semi-dominance, intermediate inheritance, or occasionally incorrectly co-dominance in reptile genetics
When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White).[12]
Co-dominance (non-Mendelian)
Co-dominance occurs when the contributions of both alleles are visible in the phenotype and neither allele masks another.
For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus. The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to a membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification. Thus the IA and IB alleles are each dominant to i (IAIA and IAi individuals both have type A blood, and IBIB and IBi individuals both have type B blood), but IAIB individuals have both modifications on their blood cells and thus have type AB blood, so the IA and IB alleles are said to be co-dominant.[12]
Another example occurs at the locus for the
Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate – in reality, such cases should not be said to exhibit dominance at all.[12]
Relationship to other genetic concepts
Dominance can be influenced by various genetic interactions and it is essential to evaluate them when determining phenotypic outcomes. Multiple alleles, epistasis and pleiotropic genes are some factors that might influence the phenotypic outcome.[13]
Multiple alleles
Although any individual of a diploid organism has at most two different alleles at a given locus, most genes exist in a large number of allelic versions in the population as a whole. This is called polymorphism, and is caused by mutations. Polymorphism can have an effect on the dominance relationship and phenotype, which is observed in the ABO blood group system. The gene responsible for human blood type have three alleles; A, B, and O, and their interactions result in different blood types based on the level of dominance the alleles expresses towards each other.[13][14]
Pleiotropic genes
Pleiotropic genes are genes where one single gene affects two or more characters (phenotype). This means that a gene can have a dominant effect on one trait, but a more recessive effect on another trait.[15]
Epistasis
Epistasis is interactions between multiple alleles at different loci. Easily said, several genes for one phenotype. The dominance relationship between alleles involved in epistatic interactions can influence the observed phenotypic ratios in offspring.[16]
See also
- Ambidirectional dominance
- List of Mendelian traits in humans
- Mitochondrial DNA
- Punnett square
- Summation theorems (biochemistry)
- Chimerism
References
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- ^ Mayo, O. and Bürger, R. 1997. The evolution of dominance: A theory whose time has passed? Archived 2016-03-04 at the Wayback Machine "Biological Reviews", Volume 72, Issue 1, pp. 97–110
- ^ Bourguet, D. 1999. The evolution of dominance Archived 2016-08-29 at the Wayback Machine Heredity, Volume 83, Number 1, pp. 1–4
- ^ Bagheri, H.C. 2006. Unresolved boundaries of evolutionary theory and the question of how inheritance systems evolve: 75 years of debate on the evolution of dominance Archived 2019-07-02 at the Wayback Machine "Journal of Experimental Zoology Part B: Molecular and Developmental Evolution", Volume 306B, Issue 4, pp. 329–359
- PMID 35852997.
- ISBN 9781324033394.
- ^ Bulinski, Steven (2016-01-05). "A Crash Course in Reptile Genetics". Reptiles. Living World Media. Archived from the original on 2020-02-04. Retrieved 2023-02-03.
The term co-dominant is often used interchangeably with incomplete dominant, but the two terms have different meanings.
- ^ S2CID 239528980.
- ^ S2CID 10695794.
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- PMID 25428896.
- PMID 18852697.
- "On-line notes for Biology 2250 – Principles of Genetics". Memorial University of Newfoundland.
- Online Mendelian Inheritance in Man (OMIM): Hemoglobin—Beta Locus; HBB - 141900 — Sickle-Cell Anemia
- Online Mendelian Inheritance in Man (OMIM): ABO Glycosyltransferase - 110300 — ABO blood groups