Determining gene sequence in a three-point mapping cross. First, determine the arrangement of alleles by locating the reciprocal NCO (most frequent) phenotypes: we conclude the pr^+ is on the other homolog from the v^+ and bm^+ alleles, as shown in (a). Now find DCO (least frequent) phenotypes among the 3 possibilities in (b), (c), and (d): pr must be in the middle. Mapping gene distances. The map distance between two genes is the sum of all SCO and DCO frequencies between them. The distance between v and pr is the sum of the frequencies of SCO (14.5%) and DCO (7.8%), or 22.3 mu. The distance between pr and bmc is the sum of the frequencies of SCO (35.6%) and DCO (7.8%), or 43.4 mu.