tetrad

1. A collection of four things having something in common such as a deformity with four features, e.g., Fallot tetralogy. SYN: tetralogy. 2. In chemistry, a quadrivalent element. 3. In heredity, a bivalent chromosome that divides into four chromatids during meiosis. [G. tetras (t.-), the number four]

- Fallot t. SYN: tetralogy of Fallot.

- narcoleptic t. the clinical syndrome of narcolepsy, cataplexy, sleep paralysis, and hypnagogic hallucinations.

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tet·rad 'te-.trad n a group or arrangement of four: as

a) a tetravalent element, atom, or radical

b) a group of four cells arranged usu. in the form of a tetrahedron and produced by the successive divisions of a mother cell <a \tetrad of spores>

c) a group of four synapsed chromatids that become visibly evident in the pachytene stage of meiotic prophase and are produced by the longitudinal splitting of each of two paired homologous chromosomes

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n. (in genetics)

1. the four cells resulting from meiosis after the second telophase.

2. the four chromatids of a pair of homologous chromosomes (see bivalent) in the first stage of meiosis.

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tet·rad (tetґrad) [Gr. tetra- four] a group of four similar or related entities, as (1) any element or radical having a valence, or combining power, of four; (2) a group of four homologous chromatids formed in the pachytene state of the first meiotic prophase; (3) a square of cells produced by the division into two planes of certain cocci (Sarcina).