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.