tetrad

A tetrad refers to a group of four. In biology, it can describe a group of four cells produced by a mother cell's division or, more specifically in genetics, a group of four synapsed chromatids visible during the pachytene stage of meiotic prophase.1

In microbiology, tetrads are a specific arrangement of coccus bacteria, which are spherical. When cocci group into fours in a square formation, they are identified as tetrads. This grouping is a result of variations in the bacteria's reproduction process. A tetrasome is a similar concept, referring to an association of four homologous chromosomes in meiotic prophase.23