Question

Two genes, A and B, are 64 cM apart on a chromosome. How can we measure the genetic distance between A and B?

1. In a test cross between a double heterozygote and a homozygous recessive individual, we should observe 64% recombinant progeny

2. If gene C lies between A and B, we can measure the recombination rate between A-C and C-B.

3. Both of the above will work

4. There is no way to measure genetic distance greater than 50 cM.

I know answer 2 works, but I was wondering if answer 1 also works, and why.

Answer 1

Recombination frequency is used as a measure of genetic distance (It does not exactly correspond to physical distance...!).
1 cM = 1 mu = 1% recombination frequency.
If recombination frequency is greater than 50%, the genes are said to be unlinked i.e. located on two different chromosomes or at a distant location on the same chromosome.
If recombination frequency is less than 50%, the genes are said to be linked i.e. closely located on the same chromosome.

64% recombination frequency does not really mean anything.
If the genes are unlinked, they will exhibit recombination frequency anywhere above 50%
The value to be obtained may change with the experiment or the environment.

So, the double heterozygote test cross results may vary each time.
So, identifying the genetic distance with close markers is a better idea.