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.

TLS Online TPP Program

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#Section 2: General Biology

T-cell education

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#Section 2: General Biology

Lineage commitment

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#Section 2: General Biology

T-Cell Activation and Differentiation

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#Section 2: General Biology

Three signals are required for activation of a naïve T cell

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#Section 2: General Biology

Positive Costimulatory Receptors: ICOS

ICOS binds to another member of the growing B7 family, ICOS-ligand (ICOS-L), which is also expressed on a subset of activated APCs.

ICOS plays a key role in maintaining the activity of already differentiated effector and memory T cells.

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#Id: 10303

#Section 2: General Biology

T-cell costimulatory