How do you detect double crossovers in a genetic map?

One Response to “How do you detect double crossovers in a genetic map?”

1. yutgoyun says:

   December 23, 2009 at 12:54 am

   Well, first thing, make sure you know what a double cross-over is.

   Unfortunately, I can’t give you specific answers, because I don’t know what your data look like!

   But here’s the idea:
   Let ABC represent genes on the same chromatid, in that order (i.e. linked). Big and small letters are alleles. o is the centromere.
   Parent:
   oABC x oabc (remember, each of these represents a chromatid. The parent is AaBcCc, but the chromatid arrangements are ABC abc.
   During meiosis we get crossover. The resulting haplotypes (haploid gametes) possible are:
   1. ABC or abc = no crossover
   2. ABc or abC = single crossover between B and C. More common than 3 (because C is farther from the centromere).
   3. Abc or aBC = single crossover between A and B. Less common than 2.
   4. AbC or aBc = your double crossover event. The least common. It requires BOTH events 2 and 3 to occur.

   With a testcross (or progeny test -> selfing) you can infer what the genotype is and thus the crossover events.

   To detect double crossovers you need your markers (phenotypic or genotypic) to be rather dense around the site. In this case without all 3 markers you wouldn’t be able to see the double crossover at the B locus.