With many yeasts, molds, algae, and fungi the four products of meiosis remain together in a sac known as an ascus. The collection of the four meiotic products is known as a tetrad, and in cases where one further mitotic division has taken place, octads. In certain cases, these four (or eight) haploid cells are ordered, with the result of the first meiotic division occupying the first two (or four) and the second two (or four) spots. These are known as ordered tetrads (octads). The study of unordered and ordered tetrads and octads has thrown much light on the mechanism of meiosis, and of recombination, see below. For the moment, we will mention the segregation of a single pair of alleles A and a in an organism having ordered tetrads (or octads).
First and second division segregation. Wth ordered tetrads (octads) there are six distinguishable arrangements at a locus at which A and a segregate, and these are listed below together with some counts observed by Lindegren (1932).
Position of spore in ascus
(Note: we have amalgamated a few counts, as they are not believed to be exceptions to the rule we are describing.)
These data are explained by the hypothesis that the first four and last four spores are the results of the first meiotic division, and cases I and II are said to illustrate first-division segregation (FDS), while the remaining four cases exhibit second-division segregation (SDS). Note that the four second-division frequencies are (roughly) equal, supporting the hypothesis that spindle-to-centromere attachment at meiosis II is random.
Second-division segregation is postulated to occur when there is one of more exchange between the locus and its centromere. When there is exactly one exchange, this is easy to see (check). When there are two or more exchanges, some lead to SDS and others to FDS, see Ex 2 below. Thus the frequency of SDS is related to the disctibution of exchanges between a locus and its centromere, an observation which has been used to map loci in relation to their centromeres.
Second-division segregation is postulated to occur when there is one of more exchange between the locus and its centromere. When there is exactly one exchange, this is easy to see (check). When there are two or more exchanges, some lead to SDS and others to FDS, see Ex 4 below. This frequency of SDS is related to the disctibution of exchanges between a locus and its centromere, an observation which has been used to map loci in relation to their centromeres.