Marker-Assisted Selection in crop breeding
Agricultural Biotechnology primarily focuses on developing better varieties of crop. This involves selection of plants with the desired characteristics. Earlier Conventional plant breeding was based on phenotypic selection of the plants. This involved cross breeding two parent plants and then waiting for the off -spring sapling to grow and exhibit it's traits. This was expensive and time consuming as it was more of a trial and error method.

The discovery that different traits of a plant are coded and controlled by its genes opened a new array of inventions in agricultural biotechnology. Certain characteristics for example, Color of the flower is controlled by a single gene while certain other traits like Crop yield which is a multifaceted trait are controlled by more than one gene. Advancements in the field of Crop Biotechnology have made use of this fact and developed certain techniques which have increased the pace of experimentation. One such technique that has helped to speed up the pace of these experiments is Marker-Assisted Selection (MAS).

Marker Assisted Selection
Molecular markers are specific short sequences of nucleotides constituting a small fragment of DNA which is positioned on the same chromosome near the gene which controls a particular trait. By the virtue of their proximity to each other on a single chromosome, markers and the specific genes are collectively inherited by every generation of the plant- a phenomenon named genetic linkage. This makes these molecular markers effective tools in determining the presence or absence of the desired genes.
Thus Marker assisted selection or marker aided selection (MAS) is a technique in which a marker is used for indirect selection of genetic determinant or determinants of a desired trait which can be disease resistance, abiotic stress tolerance etc. The presence of specific markers implies that the gene of interest is also on the same chromosome and absence of the marker implies absence of the genes controlling the desired traits.


Genetic linkage Map:
Plant geneticists can construct a genetic linkage map of the markers based on two important aspects:
1. The specificity of the DNA sequence of the marker
2. The proximity of the marker to the desired genes on the same chromosome.

How is Genetic linkage map different from the earlier maps?
Earlier, genetic maps were constructed on the basis of the traits expressed in the offspring when two parent plants were crossed. And every time two plants were crossed only a few characteristics were expressed. Hence this method was tedious and time consuming as several crosses were required to construct even a basic genetic map. With the advent of Genetic linkage map plant geneticists are now able to easily choose the plant which has the desired traits.

Construction of Genetic linkage Map based on Markers
A small bit of plant tissue from a newly germinated seedling is sufficient for genetic analysis. This saves the trouble of waiting for the sapling to mature and exhibit the absence or presence of the desired trait.
The plant tissue is then subjected to molecular analysis which reveals the absence or presence of the specific marker in question. This way, scientists can either choose the seedling or discard it and proceed to study another one.

Applications of MAS:
MAS technique is now applied in many a plant breeding experiments. One successful and ongoing application Of MAS is at present is introgression of genes coding for bacterial blight resistance, in Rice plants.
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These Molecular markers can also help in establishing the genetic profile of a particular variety of a crop. The genetic profile of a plant variety is analyzed with the help of Random primers. The information is then computerized and thus a library of genetic profiles of all the analyzed varieties is created. This library can then be used to study the extent of relatedness between the lines. Usually two highly unrelated parents are chosen to get a high quality hybrid.

Limitations of MAS
Though MAs has proved to be a very good developmental tool in plant breeding, it has its own limitations:
1.) In comparison to genetic engineering of plants, MAS is useful as a tool only in experiments where the characteristics of the plants are known.
2) It is not an efficient tool in clonally propagated crops which include bananas, sweet potato etc.

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