Determining gene sequence in a three-point mapping cross.
Of the 3 possible arrangements, arrangement III (with w in the middle) yields the observed DCO (least frequent) phenotypes: yellow, echinus flies and white flies.
Mapping gene distances.
The map distance between two genes is the sum of all SCO and DCO frequencies between them.
The distance between y and w is the sum of the frequencies of offsprings (3)/(4) and (7)/(8): 1.50% + 0.06% = 1.56 mu.
The distance between w and ec is the sum of offsprings (5)/(6) and (7)/(8): 4.00% + 0.06% = 4.06 mu.