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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/07/27 20:09:45」(JST)

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Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside 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.^{[citation needed]}

Gillespie model

Population Geneticist John H. Gillespie established the following model:^[1]

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 (i.e.: 0<s<1), h can yield the following information:

h=0	A₁ dominant, A₂ recessive
h=1	A₂ dominant, A₁ recessive
0<h<1	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 .

Notes

^ Gillespie 2004

References

Gillespie, John (2004). Population Genetics: A Concise Guide, Second Edition. Johns Hopkins University Press. ISBN 0-8018-8008-4.

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English Journal

[Characterization of developmental changes in energy metabolism of fiber flax in heterosis].

Titok VV, Iurenkova SI, Titok MV, Khotyleva LV.
Genetika.Genetika.2005 May;41(5):668-75.
The contents of adenine nucleotides and nicotinamide coenzymes, along with morphological traits, were studied in fiber flax cultivars and F1 hybrids at various ontogenetic stages. The energy parameters of heterotic hybrids proved to be significantly higher than in the parental cultivars, suggesting
PMID 15977819

Superdominance among immunodominant H-2Kb-restricted epitopes and reversal by dendritic cell-mediated antigen delivery.

Sandberg JK1, Grufman P, Wolpert EZ, Franksson L, Chambers BJ, Kärre K.
Journal of immunology (Baltimore, Md. : 1950).J Immunol.1998 Apr 1;160(7):3163-9.
To examine possible interference patterns between immunodominant CTL Ags, we analyzed the response to mixtures of five well-characterized H-2Kb-restricted epitopes, each of which had earlier been described as immunodominant within its antigenic system. Clear patterns of dominance were observed betwe
PMID 9531271

[The HLA system and its contribution to the genetics of rheumatic diseases].

Benevolenskaia LI, Miakotkin VA, Iakovleva DB, Folomeeva OM.
Genetika.Genetika.1986 May;22(5):861-7.
The results of the study of histocompatibility antigens at loci A, B and Dr in patients with RA and SLE, and their first degree relatives are presented. HLA antigens B12. B18, B27, Dr2 and Dr4 were associated with RA. The antigens HLA A11, B7, B35, Dr2 and Dr3 were associated with SLE. The influence
PMID 3488243