Three point analysis

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Different positions on a chromosome.

The three-point analysis is a method of determining the relative position (explanation follows) of three genes that are on the same chromosome to each other. Three-point analysis is a form of gene mapping. These results can be used to create gene maps.

principle

Course of recombination over time.

You must exchange frequencies ( English recombination frequency , see Crossing Over ) who between the three genes.

An example: Three genes A, B and C Exchange frequencies: AC = 7% BC = 2%

Possible positions of the genes to one another:

1. A - - - - - - - - C - - B <7%> <2%>

2. A - - - - - B - - C <2%> <7%>

In order to be able to make a clear statement, the third exchange frequency is required: AB = 9%

So the first option is correct (7% + 2% = 9%). The distance between genes is measured in cM (see centiMorgan ) and corresponds to the exchange frequency (between A and B = 9 cM).

Only with this procedure can the sequence of the genes on a chromosome, based on their exchange frequencies, be clearly determined. However, it is not possible to determine the absolute position, since other genes can also lie between the genes examined. With a large number of three-point analyzes of various genes, it is possible to create gene maps and thus find out the absolute position of the genes. However, the method shows a few pitfalls: It seems that it only provides usable exchange values ​​for short allelic distances. The experimentally obtained exchange value of two more distant genes is usually smaller than the sum of the exchange values ​​of all genes in between. This phenomenon can be explained by the fact that several crossovers can take place between genes that are further away. For example, if a second crossing-over is assumed, the decoupling caused by the first exchange is canceled again. Only in the case of an odd number of crossingovers between two linked genes are they actually separated.

Individual evidence

  1. ^ Derivation of mapping function , from Introduction to Genetic Analysis. Griffiths, AJF; Miller, JH; Suzuki, DT; Lewontin, RC; Gelbart, WM New York: WH Freeman & Co .; 1999