159. A variety “X” is a donor for resistance to blast, but it has an undesirable gene for lodging susceptibility that
is tightly linked to the blast resistant gene. The best breeding method which has high probability of breaking
this linkage is:
1. Pedigree method
2. Bulk method
3. Single seed decent method
4. Backcross method

Introduction

In plant breeding, breeders often face the challenge of undesirable genes being tightly linked to desirable traits. For instance, a variety might possess a blast resistance gene but also harbor a gene for lodging susceptibility that is closely linked to it. The presence of this undesirable trait poses a problem when trying to transfer resistance to new varieties. In this article, we will discuss the best breeding methods to break undesirable linkages and improve crop varieties, specifically focusing on methods like the backcross method.

Understanding the Problem: Tightly Linked Genes

In many cases, genes controlling desirable traits, such as disease resistance, are located close to genes controlling undesirable traits (like lodging susceptibility) on the chromosome. When these genes are tightly linked, they tend to be inherited together, making it difficult to isolate the desired trait without also inheriting the undesirable one. This linkage poses a significant challenge in breeding, as the undesirable trait (such as lodging susceptibility) can compromise the value of the variety.

Best Breeding Method to Break Linkage: The Backcross Method

To break the linkage between the resistance gene and the undesirable gene, the backcross method is the most effective breeding approach. This method involves crossing the variety with the donor of the desirable gene back to the original parent variety that lacks the undesirable trait. Through repeated backcrossing, breeders can remove the undesirable gene while maintaining the desired resistance gene, significantly improving the variety’s quality.

Key Steps in the Backcross Method:

1. Crossing the Parent Variety with the Donor Variety: The initial cross is made between the recipient (parent) variety and the donor of the desired resistance gene (blast resistance).

2. Selecting for the Desired Trait: In each generation, breeders select plants that exhibit the desired resistance to blast while minimizing the expression of the undesirable lodging susceptibility gene.

3. Repetition of Backcrosses: The process is repeated over several generations to gradually reduce the presence of the undesirable gene.

Advantages of the Backcross Method:

• High Probability of Breaking Linkage: The backcross method is highly effective at breaking the genetic linkage between desirable and undesirable traits, allowing breeders to isolate specific genes.

• Faster Recovery of the Desired Trait: This method ensures that the desired trait is retained while minimizing the undesirable one.

• Precision: By focusing on specific genetic loci, the backcross method allows for greater precision in transferring traits.

Alternative Breeding Methods

While the backcross method is ideal for breaking linkages, other breeding methods may also be used depending on the situation:

1. Pedigree Method: This involves tracking the inheritance of traits through generations but is not as effective in breaking linkages as the backcross method.

2. Bulk Method: The bulk method involves growing large populations and selecting for the best traits, but it is less efficient for breaking linkages compared to the backcross method.

3. Single Seed Descent Method: This method accelerates the development of homozygous lines but does not specifically target the breaking of linkages.

Conclusion

The backcross method is the most effective approach for breaking linkages between desirable and undesirable genes in plant breeding, especially when dealing with tightly linked genes like blast resistance and lodging susceptibility. By repeatedly crossing the donor variety back to the parent variety, breeders can eliminate the undesirable trait while retaining the desirable one, ultimately creating a better variety with improved resistance and fewer drawbacks.