In a test cross of a trihybrid individual ($AaBbCc$) where genes $A$ and $B$ are linked with 20 map units distance and gene $C$ is on a different chromosome, what is the frequency of the double crossover gamete $ABC$?

Solution

1. Double crossovers occur between three linked genes.
2. Genes $A$ and $B$ are linked, but gene $C$ is on a different chromosome.
3. It is impossible to have a double crossover involving only two linked genes.
4. Therefore, the frequency of double crossover gametes is $0$.
5. Hence the answer is (D).