The recombination frequency between linked genes A and B is 40%, between B and C is 20%, between C and D is 10%, between C and A is 20%, and between D and B is 10%. What is the sequence of the genes on the chromosome?

ACDB (or BDCA)

Explanation:

The linked genes A and B have a greater frequency of recombination (40%) and therefore they are found furthest away on the chromosome. Conversely, the smaller distance is between D and B, which exhibit a lower frequency of recombination (10%), while C is localized at the same physical distance of both A and B (20%), so the physical map is ACDB.

Physical map:

A--------C-----D-----B

[   20   ][ 10 ][ 10 ]

the answer is

c. parental