What is "tetrad"

Tetrad \Tet"rad\, n. [L. tetras, -adis, Gr. ?, ?: cf. F. t['e]trade.]

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.

"the number four, collection of four things," 1650s, from Greek tetras (comb. form tetrad-) "group of four, number four" (see tetra-).

n. 1 A group of four things. 2 (context cartography English) A unit of land area of two by two (that is, four) square kilometres. 3 (context chemistry English) A tetravalent atom or radical. 4 (context biology English) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process. 5 (context biology English) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.

n. the cardinal number that is the sum of three and one [syn: four, 4, IV, quatern, quaternion, quaternary, quaternity, quartet, quadruplet, foursome, Little Joe]

A tetrad is a "group of four". It may refer to:

• Tetrad (chromosomal formation)
• Tetrad (genetics), the four spores of a yeast
• Tetrad (general relativity), a mathematical frame of reference in general relativity
• Tetrad (index notation), a concept in differential geometry
• Tetrad (music), a set of four notes
• Tetrad (symbol) or tetractys
• Tetrad (unit of area)
• Tetrad, a group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis
• Tetrad (astronomy), an occurrence of 4 total lunar eclipses in a row with intervals of 6 lunations ( semester).
• Tetrad (computing), a 4-bit group

A tetrad is an area 2 km x 2 km square. The term has a particular use in connection with the British Ordnance Survey national grid, and then refers to any of the 25 such squares which make up a standard hectad.

Tetrads are sometimes used by biologists for reporting the distribution of species to maintain a degree of confidentiality about their data, though the system is not in universal use.

The tetrads are labelled from A to Z (omitting O) according to the "DINTY" system as shown in the grid below, which takes its name from the letters of the second line.

E

J

P

U

Z

D

I

N

T

Y

C

H

M

S

X

B

G

L

R

W

A

F

K

Q

V

A tetrad is a set of four notes in music theory. When these four notes form a tertian chord they are more specifically called a seventh chord, after the diatonic interval from the root of the chord to its fourth note (in root position close voicing). Four-note chords are often formed of intervals other than thirds in 20th- and 21st-century music, however, where they are more generally referred to as tetrads (see, for example, , , and ). Allen Forte in his The Structure of Atonal Music never uses the term "tetrad", but occasionally employs the word tetrachord to mean any collection of four pitch classes . In 20th-century music theory, such sets of four pitch classes are usually called "tetrachords" (; ).

The tetrad is the four spores of a yeast, other Ascomycota or Chlamydomonas produced after meiosis. After parent haploids mate, they produce diploids. Under appropriate environmental conditions, diploids sporulate and undergo meiosis. The meiotic products, spores, remain packaged in the parental cell body to produce the tetrad. If the two parents have a mutation in two different genes, the tetrad can segregate these genes as the parental ditype (PD), the non-parental ditype (NPD) or as the tetratype (TT).

In astronomy, a tetrad is a set of four total lunar eclipses within two years.