Introduction

Overdominance is a rare condition in genetics where the

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...

of the

heterozygote Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

lies outside the phenotypical range of both

homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

parents. Overdominance can also be described as

heterozygote advantage A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of lo ...

regulated by a single

genomic Genomics is an interdisciplinary field of biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, three-dim ...

locus, wherein

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

individuals have a higher fitness than

individuals. However, not all cases of the heterozygote advantage are considered overdominance, as they may be regulated by multiple genomic regions. Overdominance has been hypothesized as an underlying cause for

heterosis Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions o ...

(increased fitness of hybrid offspring). Sickle cell disease overdominance and high fitness

Examples

Sickle cell anemia

An example of overdominance in humans is that of the

sickle cell anemia Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red blo ...

. This condition is determined by a single polymorphism. Possessors of the deleterious allele have lower life expectancy, with

homozygotes Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. ...

rarely reaching 50 years of age. However, this allele also yields some resistance to

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or deat ...

. Thus in regions where

exerts or has exerted a strong selective pressure,

has been selected for its conferred partial resistance to the disease. While

will have either no protection from

or a dramatic propensity to

heterozygotes Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

have fewer physiological effects and a partial resistance to

Salmonoid major histocompatibility complex

Major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are cal ...

(MHC) genes exhibit extensive variation, generally attributed to the notion of heterozygous individuals identifying a wider range of peptides than homozygous individuals. In

arctic char The Arctic char or Arctic charr (''Salvelinus alpinus'') is a cold-water fish in the family Salmonidae, native to alpine lakes and arctic and subarctic coastal waters. Its distribution is Circumpolar North. It spawns in freshwater and populat ...

population in

Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bot ...

, fish heterozygous for MHC alleles had fewer cysts, grew larger, and had a better chance at survival, all indicating a higher fitness of the heterozygotes.

''Gymnadenia rhellicani'' colour polymorphism

In ''

Gymnadenia rhellicani ''Gymnadenia rhellicani'' (common name: dark vanilla orchid or black vanilla orchid) is a European species of orchid. Description ''Gymnadenia rhellicani'' grows high, with a dense, globose to cylindrical inflorescence of red-brown to chocolate ...

'', flower pigmentation is controlled by changes to

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s 612 and 663 in GrMYB1, which plays a role in

anthocyanin Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical com ...

pigment production. Red flowers, heterozygous with black and white alleles, maintain a reproductive fitness advantage over white and black varieties presumably because they attract both bee and fly pollinator populations. Since the emergence of the white allele, the frequency of the red phenotype has been increasing in wild populations in multiple regions of the alps.

Polar overdominance

Polar overdominance is a type of overdominance where either only the paternal or maternal allele is being synthesized in the offspring. An example of this was illustrated by a famous

ram Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch ...

named Solid Gold and his offspring. This ram was known for its callipyge phenotype (pronounced muscular features and hindquarters) caused by a mutated allele, but only 15% of its offspring received these same traits. Solid Gold’s offspring only expressed the same callipyge phenotype if they inherited the mutated allele from Solid Gold and a wildtype allele from their mother, which would result in a C^pat/N^mat genotype. Offspring with genotypes such as: C^pat/C^mat, N^pat/N^mat, and N^pat/C^mat did not express the callipyge phenotype.

Gillespie model

Population Geneticist John H. Gillespie established the following model:Gillespie 2004 Where h is the heterozygote effect and s is the recessive allele effect. Thus given a value for s (i.e.: 0

References

* {{cite book , last = Gillespie , first = John , authorlink = John_H._Gillespie , title = Population Genetics: A Concise Guide, Second Edition , publisher =

Johns Hopkins University Press The Johns Hopkins University Press (also referred to as JHU Press or JHUP) is the publishing division of Johns Hopkins University. It was founded in 1878 and is the oldest continuously running university press in the United States. The press publ ...

, year = 2004 , isbn = 0-8018-8008-4 Classical genetics Genetics concepts