Overdominance
Encyclopedia
Overdominance is a condition in genetics where the phenotype
of the heterozygote
lies outside of the phenotypical range of both homozygote
parents. Overdominance can also be described as heterozygote advantage
, wherein heterozygous individuals have a higher fitness than homozygous individuals.
An example in humans is sickle cell anemia. This condition is determined by a single polymorphism
. Possessors of the deleterious allele have lower life expectancy, with homozygotes
rarely reaching 50 years of age. However, this allele also yields some resistance to malaria
. Thus in regions where malaria
exerts or has exerted a strong selective pressure, sickle cell anemia has been selected for its conferred partial resistance to the disease. While homozygotes
will have either no protection from malaria
or a dramatic propensity to sickle cell anemia, heterozygotes
enjoy a partial resistance to both.
established the following model :
Where h is the heterozygote effect and s is the recessive allele effect.
Thus given a value for s (ie: 0
h=0
A1 dominant, A2 recessive
h=1
A2 dominant, A1 recessive
0
incomplete dominance
h<0
overdominance
h>1
Underdominance
For the case of sickle cell anemia the situation corresponds to the case h<0 in the Gillespie Model .
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
of the heterozygote
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
lies outside of the phenotypical range of both homozygote
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
parents. Overdominance can also be described as 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...
, wherein heterozygous individuals have a higher fitness than homozygous individuals.
An example in humans is sickle cell anemia. This condition is determined by a single polymorphism
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
. Possessors of the deleterious allele have lower life expectancy, with homozygotes
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
rarely reaching 50 years of age. However, this allele also yields some resistance to malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
. Thus in regions where malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
exerts or has exerted a strong selective pressure, sickle cell anemia has been selected for its conferred partial resistance to the disease. While homozygotes
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
will have either no protection from malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
or a dramatic propensity to sickle cell anemia, heterozygotes
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
enjoy a partial resistance to both.
The Gillespie Model
Population Geneticist John H. GillespieJohn H. Gillespie
John H. Gillespie is an evolutionary biologist interested in theoretical population genetics and molecular evolution. In molecular evolution, he emphasized the importance of advantageous mutations and balancing selection. For that reason, Gillespie is well known for his selectionist stance in the...
established the following model :
| Genotype: | A1A1 | A1A2 | A2A2 |
| Relative fitness: | 1 | 1-hs | 1-s |
Where h is the heterozygote effect and s is the recessive allele effect.
Thus given a value for s (ie: 0
Underdominance
In genetics, underdominance is the opposite of overdominance. It is the selection against the mean of a population distribution, causing disruptive selection and divergent genotypes. Underdominance can also be described as homozygote advantage, wherein homozygous individuals have a higher fitness...
For the case of sickle cell anemia the situation corresponds to the case h<0 in the Gillespie Model .