A Map of the Fourth Chromosome of Drosophila melanogaster, Based on Crossing over in Triploid Females

Under ordinary conditions there is so little crossing over in the fourth chromosome of Drosophila melanogaster that the usual method of constructing a map is not practicable. Deduction from the behavior of translocations has been utilized, but as will be shown here, has led to an incorrect result. Bridges and Brehme (1944) give the seriation bt (bent), sv (shaven), ci (cubitus interruptus), gvl (grooveless), ey (eyeless), with 0.2 per cent crossing over for the whole series. This crossover value is certainly too high; it may be doubted if as many as five crossovers have ever been detected from diploid females. The results presented below show also that the above sequence is altogether incorrect, the true order being ci, gvl, bt, ey, sv, (with a possibility that the positions of ci and gvl should be reversed).