tetrad

A tetrad is a unit of four related items or individuals. The term is used across disciplines to denote a four-part grouping, rather than a single object, and it appears in biology, genetics, botany, microbiology, astronomy, and social sciences.

In cell biology and genetics, a tetrad refers to the four chromatids that compose two homologous chromosomes during meiosis I after synapsis. Each chromosome has been replicated, so the tetrad consists of two homologous chromosomes, each with two sister chromatids, and crossing over can occur between non-sister chromatids.

In plant reproduction, a pollen tetrad is a group of four microspores that remain attached after meiosis. Some plants release pollen as tetrads, while others separate the four microspores into individual grains.

In microbiology, tetrad describes cocci arranged in a square, a common cellular grouping seen in several bacterial genera, such as Micrococcus. The arrangement contrasts with other patterns like chains or clusters.

In genetics, tetrad analysis is a method for studying linkage and recombination by examining the four products of a single meiosis, a technique notably used in fungi such as Neurospora crassa to infer genetic maps and crossover events.

In astronomy, a tetrad refers to a sequence of four total lunar eclipses occurring within about two years. Such tetrads are relatively rare and are of interest to both scientists and skywatchers.

In sociology and anthropology, a tetrad can denote a social group of four individuals, used to analyze patterns of interaction and network structure. The term thus serves as a generic descriptor across fields for any set of four related elements.