tetrad

A unit of four blocks in the game Tetris. They come in seven distinct shapes, namely line, L, r, S, Z, T, and square. They can be moved left and right, rotated (clockwise and/or counter-clockwise depending on the particular implementation of the game), and dropped.

While in the tetrad, the sister chromatids "cross-over", to exchange genetic information. This accounts for some of the genetic variation we see in children when compared with their parents.

This event happens only during the first prophase of meiosis.

Tet"rad (?), n. [L. tetras, -adis, Gr. , : cf. F. t'etrade.]

1.

The number four; a collection of four things; a quaternion.

2. Chem.

A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad.

© Webster 1913.