Question

In a two-point linkage analysis, genes a and b have been found to be 26 m.u. apart on the same chromosome. A third gene, c, has just been discovered and found to be located between a and b. A three-point linkage analysis with a, b, and c indicates that a and b are actually 33 m.u. apart rather than 26 m.u. Why does the three-point analysis give a different map distance for a and b than does the two-point linkage analysis, and which is more accurate

Answer 1

Answer:

Because the two-point analysis underestimates the actual distance between A and B for not taking into consideration all the possible crossing-overs. The most accurate result is the one thrown by the three-point analysis.

Explanation:

When calculating genetic distances, we might notice a difference when using a two-point analysis and when making a three-point analysis. Their results are different. This difference is because distances between genes are not completely additive. They might vary. We can notice this difference in the exposed example, in which the map distance between A and B when using a two-point analysis equals 26 map units. However, when using a three-point analysis, it equals 33 map units. The addition of distances from A to C and from C to B is 33 and not 26.  

Calculating the recombination frequency between the two genes located in the extremes and far apart from each other underestimate the actual genetic distances between them, because there might occur other crossing-overs that were not detected. So, when calculating the genetic distance from A to B, we probably will not consider the occurrence of a double recombinant between them, and we might sub-estimate the actual distance. In between A and B many simple and double-crossing over might occur, and might not be detected by the two-point analysis.

The relationship between the actual map distance (number of crossing overs) and the recombination frequency between two loci is not lineal. The farther apart the two genes are, the worse is the distance estimation.

 The most accurate analysis is the three-point one that is based on the addition of distances between every two genes.