Gene Linkage

Abstract

The proximity of a pair of genes located on the same chromosome affects their independent distribution because recombination between them is less frequent than between distant genes, with the result that the alleles of these closely spaced genes are transmitted together to the offspring and therefore their segregation becomes rare.

The closeness of these genes is called linkage. The closer the genes on a chromosome, the more closely linked they are and the more obviously they affect the independent distribution proposed by Mendel. Linkage detection is done in segregating generations, either backcross or F2, using a χ2 test in which the null hypothesis is that genes are independently distributed. Once the presence of a linkage is detected, its magnitude can be estimated by calculating the recombination frequency, which is an indirect measure of the distance that separates the genes under study. The calculation of distances between genes is used for the construction of linkage maps, which are graphical representations of the order in which the genes are on the chromosome and the distances between them. The linkage maps are of great importance in marker-assisted selection since they can provide information about the suitability of a specific marker for selection purposes.

Knowledge Integration Questions

Suppose that you are interested in starting a selection program assisted by molecular markers for resistance to disease in soybean (Glycine max) caused by Cercospora sojina. For this purpose, two RAPDs have been detected linked to the gene that confers resistance. Suppose that a cross is made from a plant resistant (R allele), which possesses the bands representing the two RAPD (M1 and M2 alleles), with a susceptible plant (r allele) and not possessing the RAPD bands linked to resistance (m1 and m2 alleles). The F1 obtained (RrM1m1M2m2) was backcrossed to the parent r r m1 m1 m2 m2, resulting in the following offspring:

• 385R r M1 m₁ m₁ M₂ m₂ 12R r m₁ m₁ m₂ m₂ 60R r M₁ m₁ m₂ m₂ 35R r m₁ m₁ M₂ m₂ 399r r m₁ m₁ m₂ m₂ 8r r M₁ m₁ M₂ m₂ 62r r m₁ m₁ M₂ m₂ 39r r M₁ m₁ m₂ m₂

Based on these results:

• (a) Construct a linkage map for the three genes listed.

The parental combinations are the most frequent: RrM1m1M2m2 and rr m1m1m2m2m2.

Double crossing over are the least frequent: rrM1m1M2m2 and,R r m1m1m2m2m2

The order of the genes to obtain the double crossing over is:

Generating the M₁rM₂ and m₁Rm₂ gametes that upon joining the gametes of the recurrent parent

(r m₁rm₂) will form the 20 individuals that turned out to be the least frequent.

The regions will be as follows:

The recombination in each region will be:

$$ {\displaystyle \begin{array}{l}{r}_{\mathrm{I}}\kern0.75em =\kern0.75em \left[\left(39+35\right)+\left(12+8\right)\right]\kern0.5em /1000\\ {}\kern1.25em =\kern0.75em 0.094\\ {}\kern1.25em =\kern0.75em 9.40\ \mathrm{MU}\end{array}} $$

$$ {\displaystyle \begin{array}{l}{r}_{\mathrm{II}}\kern0.625em =\kern0.75em \left[\left(60+62\right)+\left(12+8\right)\right]\kern0.5em /1000\\ {}\kern1.25em =\kern0.75em 0.142\\ {}\kern1.25em =\kern0.75em 14.20\ \mathrm{MU}\end{array}} $$

Based on these results, the linkage map will be:

For marker-assisted selection, which RAPD do you consider most suitable? Why?

RAPD 1 is the most suitable for molecular marker-assisted selection since it is more closely linked to the trait of interest, which in this case is the resistance. Because the recombination fraction is 0.094, it can be said that out of every 100 individuals who are selected by having the RAPD band, 9 will have recombination, i.e., they will not carry the resistance allele, so that the selection efficiency will be 91% (100% – 9%). If RAPD 2 were to be taken, the recombination fraction is 0.142, which indicates that out of every 100 individuals selected for the presence of this marker, 14 will be the product of the recombination, i.e., they will not carry the resistance allele, so that the efficiency is estimated at 86%.