Three signals are required for activation of a naïve T cell

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Overdominance the heterozygote has higher fitness than the  fitnesses of the two homozygotes

Underdominance, in which the heterozygote has lower fitness than both homozygotes.

Directional selection leads to fixation of the favored allele and elimination of the other allele, as  long as no other evolutionary forces act on the population.

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Artificial selection- Genetically improving animals and plants by breeding together the best individuals in their fields and flocks.

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Types of selection

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The allelic frequency at equilibrium (qˆ) depends on the relative fitnesses (usually expressed as selection coefficients) of the two homozygotes:

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Hitchhiking

An important consequence of linkage disequilibrium is hitchhiking. This happens when an allele at  one locus spreads by natural selection acting on a second locus that is in linkage disequilibrium with  the first.