Fig. 10. The four ways in which "second division segregation" can occur when diploids heterozygous for A and TRP1 are sporulated. Because TRP1 is extremely close to its centromere, usually no crossovers occur between it and its centromere. If A is very far from its centromere, then all 4 Ts are formed, and each is formed at the same frequency as PDs and NPDs, so PD:NPD:T = 1:1:4. However, if A is sufficiently close to its centromere that the frequency of crossovers is reduced, then fewer Ts are formed, and PD:NPD:T = 1:1:<4. The closer A is to its centromere, the lower the frequency of Ts. Thus the frequency of Ts, in this case, provides a measure of the distance between A and its centromere.