tetrad
When two matching chromosomes pair up in meiosis, each has already copied itself into two joined strands, so the paired structure contains four strands in all. That bundle of four is a tetrad: two homologous chromosomes, each made of two sister chromatids, lined up together.
Tetrads form during prophase of meiosis I, after synapsis brings the homologous partners side by side. Within a tetrad, non-sister chromatids can cross over and exchange segments, and the crossover points appear as chiasmata holding the four strands together. The tetrad is therefore the stage on which recombination plays out.
Counting and analysing tetrads is a classic tool in organisms like fungi, where all four products of a single meiosis stay together in one sac and can be examined directly. This tetrad analysis lets geneticists see the outcome of each meiosis individually, making it a powerful way to study recombination and gene linkage.
The word tetrad is also used for the four haploid cells produced at the end of meiosis. In this entry it means the paired-homolog (four-chromatid) structure of meiosis I, which is also called a bivalent to avoid confusion.