Plant breeding is a crucial aspect of agriculture, aimed at improving crop characteristics like yield, disease resistance, and nutritional value. Several methods are employed to achieve these goals, one of which is backcrossing. Backcrossing is a powerful breeding technique used to transfer one or a few desirable genes from a wild relative or another variety into a highly adapted, elite cultivar, while simultaneously retaining the genetic background of the elite cultivar. It’s particularly useful when introducing specific traits without significantly altering the overall performance of the crop. Understanding Backcrossing Backcrossing is a hybridization technique where a hybrid (F1 generation) is crossed repeatedly with one of its parents (usually the recurrent parent, which is the elite cultivar). The goal is to recover the desirable agronomic traits of the recurrent parent while incorporating the desired gene(s) from the donor parent. The Process of Backcrossing The backcross method involves the following steps: F1 Hybridization: The first step involves crossing the elite cultivar (recurrent parent) with the donor parent possessing the desired gene(s). This produces the F1 hybrid. Backcrossing (BC1): The F1 hybrid is then crossed back to the recurrent parent. This results in the BC1 generation, which contains approximately 50% genes from each parent. Selection & Backcrossing (BC2, BC3…): Individuals with the desired gene(s) are selected from the BC1 generation and crossed again with the recurrent parent (BC2). This process is repeated for several generations (BC3, BC4, etc.). With each backcross, the proportion of genes from the recurrent parent increases, ideally reaching over 95% after 5-6 backcrosses. Selection in Each Generation: Throughout the backcrossing process, selection for the desired gene(s) and against undesirable traits from the donor parent is crucial. Evaluation and Release: After several backcrosses, the resulting lines are evaluated for agronomic performance, yield, and the stability of the desired gene(s). The best lines are then released as new varieties. Applications of Backcrossing Backcrossing finds extensive application in plant breeding: Disease Resistance: Introducing genes for resistance to specific diseases from wild relatives into cultivated varieties. For example, incorporating rust resistance genes into wheat. Insect Resistance: Transferring genes for insect resistance from wild species to improve crop protection. Improving Quality Traits: Introducing genes for improved nutritional content, such as vitamin enrichment or protein content. Adapting to Stress Conditions: Incorporating genes for tolerance to abiotic stresses like drought, salinity, or cold. Advantages of Backcrossing Retention of Elite Background: It effectively retains the superior agronomic traits of the recurrent parent. Efficient Gene Transfer: It’s a relatively efficient method for transferring one or a few genes. Reduced Linkage Drag: Repeated backcrossing helps to eliminate undesirable genes linked to the desired gene from the donor parent. Disadvantages of Backcrossing Time-Consuming: The process can be time-consuming, requiring multiple generations of crossing and selection. Linkage Drag: Despite repeated backcrossing, some undesirable genes from the donor parent may still be carried along due to linkage. Difficulty in Identifying Heterozygous Individuals: Identifying heterozygous individuals carrying the desired gene can be challenging. Potential for Recessive Alleles: Undesirable recessive alleles from the donor parent may be masked in early generations but reappear later. Backcrossing vs. Other Breeding Methods Method Goal Genetic Background Time Required Backcrossing Introduce 1-2 genes into an elite variety Primarily retains recurrent parent’s background Moderate (5-6 generations) Pedigree Selection Improve multiple traits simultaneously Combines genes from both parents Long (6-8 generations) Mass Selection Improve traits governed by additive genes Relies on phenotypic selection Short (3-4 generations) Backcrossing remains a cornerstone of plant breeding, offering a powerful strategy for improving crop varieties by incorporating specific desirable traits while preserving their overall genetic integrity. While it has limitations, careful selection and repeated backcrossing can minimize undesirable effects. Continued advancements in molecular marker-assisted selection are further enhancing the efficiency and precision of backcrossing, making it an even more valuable tool for addressing the challenges of global food security and climate change.

Backcross Method: Genetic Analysis | UPSC Mains BOTANY-PAPER-II 2016

Back cross method

How to Approach

This question requires a detailed explanation of the backcross method in plant breeding. The answer should define the method, explain its purpose, outline the steps involved, discuss its advantages and disadvantages, and provide examples of its application. Structure the answer by first defining backcrossing, then detailing the process, followed by its applications, advantages, disadvantages, and finally, differentiating it from other breeding methods. Use clear language and avoid overly technical jargon.

Understanding Backcrossing

Backcrossing is a hybridization technique where a hybrid (F1 generation) is crossed repeatedly with one of its parents (usually the recurrent parent, which is the elite cultivar). The goal is to recover the desirable agronomic traits of the recurrent parent while incorporating the desired gene(s) from the donor parent.

The Process of Backcrossing

The backcross method involves the following steps:

F1 Hybridization: The first step involves crossing the elite cultivar (recurrent parent) with the donor parent possessing the desired gene(s). This produces the F1 hybrid.
Backcrossing (BC1): The F1 hybrid is then crossed back to the recurrent parent. This results in the BC1 generation, which contains approximately 50% genes from each parent.
Selection & Backcrossing (BC2, BC3…): Individuals with the desired gene(s) are selected from the BC1 generation and crossed again with the recurrent parent (BC2). This process is repeated for several generations (BC3, BC4, etc.). With each backcross, the proportion of genes from the recurrent parent increases, ideally reaching over 95% after 5-6 backcrosses.
Selection in Each Generation: Throughout the backcrossing process, selection for the desired gene(s) and against undesirable traits from the donor parent is crucial.
Evaluation and Release: After several backcrosses, the resulting lines are evaluated for agronomic performance, yield, and the stability of the desired gene(s). The best lines are then released as new varieties.

Applications of Backcrossing

Backcrossing finds extensive application in plant breeding:

Disease Resistance: Introducing genes for resistance to specific diseases from wild relatives into cultivated varieties. For example, incorporating rust resistance genes into wheat.
Insect Resistance: Transferring genes for insect resistance from wild species to improve crop protection.
Improving Quality Traits: Introducing genes for improved nutritional content, such as vitamin enrichment or protein content.
Adapting to Stress Conditions: Incorporating genes for tolerance to abiotic stresses like drought, salinity, or cold.

Advantages of Backcrossing

Retention of Elite Background: It effectively retains the superior agronomic traits of the recurrent parent.
Efficient Gene Transfer: It’s a relatively efficient method for transferring one or a few genes.
Reduced Linkage Drag: Repeated backcrossing helps to eliminate undesirable genes linked to the desired gene from the donor parent.

Disadvantages of Backcrossing

Time-Consuming: The process can be time-consuming, requiring multiple generations of crossing and selection.
Linkage Drag: Despite repeated backcrossing, some undesirable genes from the donor parent may still be carried along due to linkage.
Difficulty in Identifying Heterozygous Individuals: Identifying heterozygous individuals carrying the desired gene can be challenging.
Potential for Recessive Alleles: Undesirable recessive alleles from the donor parent may be masked in early generations but reappear later.

Backcrossing vs. Other Breeding Methods

Method	Goal	Genetic Background	Time Required
Backcrossing	Introduce 1-2 genes into an elite variety	Primarily retains recurrent parent’s background	Moderate (5-6 generations)
Pedigree Selection	Improve multiple traits simultaneously	Combines genes from both parents	Long (6-8 generations)
Mass Selection	Improve traits governed by additive genes	Relies on phenotypic selection	Short (3-4 generations)

Additional Resources

Key Definitions

Recurrent Parent

The elite cultivar that is repeatedly crossed with the hybrid in the backcross method. It provides the majority of the genetic background of the improved variety.

Linkage Drag

The phenomenon where undesirable genes located close to the desired gene on the same chromosome are also transferred during backcrossing, hindering the improvement process.

Key Statistics

Approximately 95-98% of the genome of the improved variety comes from the recurrent parent after 5-6 generations of backcrossing.

Source: Allard, R. W. (1999). Principles of Plant Breeding. John Wiley & Sons.

Studies show that marker-assisted backcrossing can reduce the number of generations required to achieve a desired level of genomic recovery of the recurrent parent by up to 30-40%.

Source: Collard, B. C., et al. (2005). A marker-assisted approach to introgressing bacterial blight resistance into elite rice germplasm. Theoretical and Applied Genetics, 110(7), 1209-1218.

Examples

Tomato Breeding for Disease Resistance

Backcrossing was used to introduce the *Tm-2* gene for resistance to Tomato Mosaic Virus (ToMV) from wild tomato species (*Solanum peruvianum*) into commercially grown tomato varieties, maintaining the desirable fruit quality and yield of the cultivated tomatoes.

Frequently Asked Questions

▶What is the role of molecular markers in backcrossing?

Molecular markers (like SSRs or SNPs) linked to the desired gene can be used to identify individuals carrying the gene in early generations, accelerating the backcrossing process and improving selection efficiency. This is known as Marker-Assisted Backcrossing (MAB).