Suppose that the genetic map distance from A to B is 8 map units, from B to C is 10 map units and from A to C is 2 map unit. A true-breeding wild type parent was crossed with a true breeding mutant parent. A three-point testcross has been performed using an F1 heterozygote, and 1000 progeny were obtained. How many of the test-cross progeny do you expect to be double crossover?

This question includes the following options to choose from:

A) 2     B) 12     C) 8     D) 9     E) 10  

3 point testcrosses focus on analyzing the inheritance blueprints of alleles by crossing triple heterozygotes with triple recessive homozygotes, allowing therefore to find out the distance between the alleles, and also their location order on the chromosome.

And it all comes down to the frequency of recombination, in this case such frequency between C&A is 0.02, in the same fashion that it's 0.08 between A&B; thereby the answer is A). 2