Analysis of Tetrads from the yeast Saccaromyces cerevisiae

When normally haploid yeast cells of two different mating types encounter each other, they fuse to form a diploid zygote (this constitutes a cross), which immediately undergoes meiosis to regenerate four individual haploid spores – a tetrad (the equivalent of an F1 generation). Since each meiotic tetrad is contained in a sac-like structure called an ascus, it can be recovered as an intact group. The spores are not arranged in any particular order in the ascus, so they are called random or unordered spores, rather than ordered spores.