LINKAGE DRAG BY ANKIT RAJ.pptx

Linkage drag refers to the (usually undesirable) effects of genes linked to the genes or QTL we are trying to introgressive.

1. LINKAGE DRAG When two varieties of a specific crop plant, the „wild type‟ and „inbred line‟ are crossed to develop a novel F1 hybrid, the desirable traits from the „wild type‟ are acquired by the F1 generation, however the process may also result in the acquisition of some undesired traits. This phenomenon is known as ‘linkage drag’. Linkage drag refers to the (usually undesirable) effects of genes linked to the genes or QTL we are trying to introgressive . By Ankit Raj

2.   When you have two block in a single chromosome, one chromosome block carrying good stuffs (i.e. genes controlling good or desirable characters) and other block carrying bad ones (i.e. genes controlling bad or undesirable characters) (Fig. 1.) then you will have both blocks inherited as one (i.e. the good block will be inherited to the progeny along with the bad one because they are both exist in a single chromosome [both blocks are linked]). Part of chromosome # 1 (blue line) with two blocks of chromosome parts having genes controlling desirable characters (green block)  and those controlling undesirable characters (red block). Because the two block reside in a single chromosome (chr#1), they will be inherited as one entity (always together as if they are one block). Fig.1

3. In this case, the genes controlling desirable characters from the good block and ones controlling undesirable characters from the bad block are always presence together in the progeny. In such case, any progeny carries desirable characters will always carry the linked undesirable ones while any progeny does not carry desirable characters neither will carry undesirable characters. When you only want to have the good block (Fig. 1: the green) and do not want the bad block (Fig. 1: red) but you cannot get what you want because the two blocks are linked – than you are talking about linkage-drag (The linkage between the red- and the green block causes the red block always drags along with the green block in the progeny’s genome).

4. Is there different degree or level of linkage drag? Fig. 2. A recombination event may results in breaking of the linkage drag between the green and the red blocks. Two recombinant chromosomes are obtained as the result of the recombination event.

5. Fig. 3. The frequency of recombination event that will separate two blocks (green and red blocks) of the same chromosomes is determined by the distance between the blocks (the green and the red blocks).  The larger the distance between blocks, the more often the recombination event separating the blocks may take place and vice versa.  Recombination event may not be possible if the two blocks overlap.

6. Breaking linkage drag Linkage drag refers to the (usually undesirable) effects of genes linked to the genes or QTL we are trying to Introgression. If a desirable QTL for trait X lies close to an undesirable gene affecting trait Y, you will want to “break” the linkage drag – that is, separate the good QTL from the bad. The solution to this problem lies in backcrossing or in genetic modification using horizontal gene transfer into inbred lines.

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