Genetic drift 

Genetic drift is an evolutionary force, in which allelic frequency change over generation Just by chance (sampling error). 

The deviations from the expected ratios are random, the direction of change is unpredictable.

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#Section 2: General Biology

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T-cell education

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Lineage commitment

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T-Cell Activation and Differentiation

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Three signals are required for activation of a naïve T cell

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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.