Abstract
By dividing the total no. of viable breaks by the no. of mutations occurring at breakage points at a single typical locus, an estimate of the no. of loci present can be obtained. Postulating the white locus of D. melanogaster to be a typical one and that all viable breaks involving this locus result in the white phenotype, the no. of loci on the X chromosome is calculated to be approx. 726. The assumptions involved in this calculation are presented together with the evidence affecting their validity.