balanced polymorphism

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balanced polymorphism

[′bal·ənst ¦päl·i′mȯr‚fiz·əm]
(genetics)
Maintenance in a population of two or more alleles in equilibrium at frequencies too high to be explained, particularly for the rarer of them, by mutation; commonly due to the selective advantage of a heterozygote over both homozygotes.
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Balancing selection may have enabled humans and chimps to retain multiple lines of defense that can be called on when a pathogen evolves new weapons.
It is assumed that at any given location there will be balancing selection on egg depth that reflects the costs and benefits of an egg being placed shallow or deep.
Put another way, there may be balancing selection for the same ovipositor length in all first generation bivoltine crickets, while diversifying selection among populations with differing season lengths is the norm for crickets producing overwintering eggs.
If either mutation had occurred long ago and been maintained by balancing selection, DNA differences would have accumulated around different copies of each allele.
Because the balancing selection on female total length results from opposing selection pressures on longevity and daily fecundity, our results may be interpreted as evidence of a cost of reproduction (Roff 1992; Stearns 1992).
When these fitness estimates are combined to estimate lifetime fecundity, females appear to be under balancing selection for total length.
If survival selects for smaller male body size, then males may be under balancing selection for total length.
melanogaster where allozyme studies had already revealed the presence of a protein polymorphism subject to balancing selection.
For the two models with strong balancing selection - overdominance and SAS-CFF with B = 5 - the rate is remarkably insensitive to [Alpha].
The nonlinearity is the result of our concern with the fixation of sites, not alleles, and because there are many alleles maintained in the population by balancing selection.
Recall that the TIM model is not a model of balancing selection and thus has many fewer segregating alleles than does the overdominance model.

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