Mapping the centromere. If no crossover event occurs between a gene and the centromere, the pattern of ascospores within an ascus is (aaaa++++) due to first-division segregation, since the two alleles are separated during the first meiotic division. Crossover would produce recombinant patterns due to second-division segregation.

To calculate the distance between the gene and the centromere: d = ((1/2 recombinant asci) / total asci) * 100.

The distance (d in mu) counts half the number of recombinant asci. since crossing over in each occurs in only two of the four chromatids during meiosis.