Assume That a and B Are Two Linked Genes on an Autosome

Multiple Choice

Assume that A and B are two linked genes on an autosome in Drosophila. A testcross is made where AB/ab flies are crossed to ab/ab flies and the progeny are counted and shown below. However, it is known that the Aa/bb genotype is lethal before the flies hatch and does not appear among the testcross progeny counted. What is the most precise map distance between the two genes that can be calculated from these data? Aa Bb = 235
Aa bb = 225
Aa Bb = 20

A) 4.2 m.u.
B) 4.0 m.u.
C) 16.4 m.u.
D) 8.0 m.u.
E) 50 m.u.