A linear sequence of four haploid meiotic cells (or pairs of each of four haploid cells produced by a postmeiotic division) within a fungal ascus. This physical arrangement allows identification of chromatids participating in crossover events. Drawing A illustrates that, in a tetrad heterozygous for alleles controlling ascospore pigmentation, single crossovers between these genes and the centromere will generate spores showing 2-2-2-2 and 2-4-2 segregation patterns. Drawing B illustrates that in Neurospora such patterns are observed, together with noncrossover asci showing 4-4 distributions.
Subjects: Genetics and Genomics.