Three-point attempt

The three-point experiment is one of the simplest genetic mapping methods. Its principle is based on a hybridization experiment and gene linkage.

The three-point test is of practical use in a situation where we are trying to more closely determine the position of a certain gene (or locus ) on a specific chromosome. If we know the position of two genes (markers) on the same chromosome, we can determine the order of all three genes on the respective chromosome from the results of the hybridization experiment (hence the three-point experiment).


 * Example

We have two known genes A and B and we are trying to find the relative position of gene X to these known genes.


 * 1) We therefore cross a recessive homozygote (aabbxx) with an F1 hybrid (AaBbXx).
 * 2) We will evaluate the frequency of individual genotypes in the offspring:
 * 3) * ABX /abx or abx /abx = 75%
 * 4) * aBX /abx or Abx /abx = 15%
 * 5) * AbX /abx or aBx /abx = 9%
 * 6) * ABx /abx or abX /abx = 1%
 * 7) The least represented group (ABx or abX) represents the group of so-called double recombinants, i.e. individuals in which two recombinations occurred at the same time (which is the least likely phenomenon). It is clear that due to two recombinations (between the middle and both outer genes), the middle gene will be separated from the two outer genes. Thanks to this reasoning, we can find out which gene lies in the middle (or between the extreme genes).
 * 8) In this case, the X gene lies in the middle, i.e. the final order of the genes on the chromosome is A−X−B or B−X−A (however, only one option is correct, since we know the position of both markers on the chromosome, it is not a problem to determine which variant is correct).

Related articles

 * Gene linkage