Practice question

When second division segregation occurs, only two of four chromatids involved in single crossover become recombinant with respect centromere, two remain parental genotype. Recombination frequency therefore half of SDS asci proportion because half chromatids show recombination per tetrad. Map distance in centimorgans equals half percent SDS, calculated as percent SDS divided by two or SDS frequency ×50. Example illustrates 60 percent SDS corresponds 30 map units. Calculation assumes no chromatid interference and multiple crossovers rare. For very distal genes multiple exchanges may cause SDS plateau below theoretical 66 percent due to even crossovers.