Figure 1.

Tetrad analysis. Tetrad analysis allows us to determine the type and frequency of different kinds of exchange events. An E₀ tetrad did not undergo crossing over and thus results in four noncrossover chromatids (NCC). An E₁ tetrad experienced one crossover and results in two chromatids that each exhibit a single crossover (SCC) and two NCCs. An E₂ tetrad experienced two crossovers; E₂ tetrads result in some combination of NCCs, SCCs, or double-crossover chromatids (DCCs), depending on which chromatids were involved in the two crossover events. Although four chromatids are associated with an exchange (or nonexchange) event, only one chromatid will eventually make its way into the Drosophila oocyte nucleus; the remaining three are discarded. Because it is not currently possible in Drosophila to recover full tetrads, the frequency of E₀, E₁, E₂, … E_n. tetrads must be calculated based on the number and type of exchange chromatids actually observed. This calculation is derived from classic Weinstein algebraic tetrad analysis [Weinstein 1918; Merriam and Frost 1964; adapted from Zwick et al. (1999)].