In Drosophila melanogaster there is a dominant allele for the gray body color and a dominant allele of another gene for normal wings. The recessive alleles of these two genes result in black body color and vestigial wings respectively. Flies homozygous for gray body and normal wings were crossed with flies that had black bodies and vestigial wings. The F1 progeny were then crossed with homozygous recessives, with the following results:
236 flies with gray body and normal wings, 253 with black body and vestigial wings, 50 with gray body and vestigial wings and 61 with black body and normal wings.
If these two genes are linked, how many units apart are they on the chromosome?