In a test cross of a femal drosophila heterozygous for three linked, recessive genes, the following phenotypes of progeny were obtained.
a+ b+ d 413
a b d+ 423
a+ b d+ 12
a b+ d 12
a+ b d 19
a b+ d+ 20
a+ b+ d+ 0
a b d 1
Construct a map of the genes.