Pedigree Method – Procedure, Applications, Advantages, Disadvantages

What is Pedigree Method?

• The pedigree method is a systematic approach utilized in plant breeding to handle segregating populations derived from crosses. This method stands as one of the three principal techniques—alongside bulk and backcross methods—employed for developing pureline varieties from such populations.
• The pedigree method begins with the selection of individual plants from the segregating generations, such as those from F1 or F2 populations. The fundamental objective of this approach is to isolate and propagate purelines, which are genetically uniform lines resulting from the selection of individuals that exhibit consistent desirable traits.
• Central to the pedigree method is the maintenance of a detailed pedigree record. This record meticulously documents the lineage and genetic relationships of each plant selected throughout the breeding process. This documentation is crucial as it provides insight into the inheritance patterns and allows breeders to trace the origin of desirable traits.
• Selection continues through successive generations until the progenies no longer exhibit segregation. At this stage, the genetic variation within the progenies has been sufficiently reduced, and the focus shifts to selecting among these progenies. By this point, the aim is to identify and propagate those individuals that exhibit the desired traits with a high degree of uniformity, ensuring that the resulting pureline is both stable and consistent.
• Therefore, the pedigree method is essential for developing pureline varieties from segregating populations, offering a structured approach to achieve genetic purity and stability in plant breeding.

Definition of Pedigree Method

The pedigree method is a plant breeding technique where individual plants from segregating generations are selected and documented through detailed pedigree records. This method aims to develop genetically uniform pureline varieties by maintaining and selecting plants until progenies exhibit no further genetic variation.

Procedure for the Pedigree Method

The pedigree method is a structured approach used in plant breeding to develop pureline varieties from segregating generations. The procedure involves several distinct stages, each aimed at progressively refining and stabilizing desirable traits. The following outlines the general procedure for the pedigree method:

1. Hybridization:
   • Selection of Parents: The process begins with selecting appropriate parent plants for crossing. This choice is crucial as it determines the genetic diversity of the resulting offspring.
   • Crossing: The selected parents are crossed to produce F1 seeds. Both simple and complex crosses can be utilized, depending on the breeding objectives.
2. F1 Generation:
   • Seed Production: Approximately 15-30 F1 plants are grown to produce enough seed for the next generation. This seed is crucial for establishing the F2 population.
   • Population Size: The number of F1 plants should be sufficient to generate a large F2 population, typically 2,000-10,000 plants.
3. F2 Generation:
   • Planting and Selection: F2 plants are spaced to facilitate selection. Around 100-500 plants are selected based on desirable traits. The F2 population should be 10-100 times the number of plants to be selected.
   • Characteristics for Selection: Selection in F2 focuses on traits that are simply inherited, such as plant height, seed color, and disease resistance. Traits influenced by the environment or heterozygosity, such as yield, are less reliable.
4. F3 Generation:
   • Individual Progeny Evaluation: F3 progenies are space-planted, with each progeny consisting of about 30 plants. Selection focuses on superior progenies while eliminating those with undesirable traits, such as disease susceptibility.
   • Progeny Comparison: Selection is based on the performance of individual progenies. Progenies from superior F2 plants are preferred.
5. F4 Generation:
   • Further Selection: F4 progenies are evaluated, and plants are selected from superior progenies. Progenies with defects are rejected. Selection at this stage emphasizes improving the uniformity and quality of the progenies.
6. F5 Generation:
   • Plot Planting: Progenies are planted in multi-row plots to facilitate comparison. This stage helps in assessing the consistency and performance of the progenies across different plots.
   • Evaluation and Reduction: Progenies are visually evaluated, and inferior lines are discarded. The remaining progenies undergo preliminary yield trials to assess their performance.
7. F6 Generation:
   • Advanced Trials: Progenies are subjected to replicated yield trials at multiple locations. These trials assess yield, disease resistance, lodging resistance, and other critical traits.
   • Selection for Release: Lines that outperform current commercial varieties in key characteristics are selected for potential release.
8. Final Stage (F7 and Beyond):
   • Seed Multiplication: If a progeny shows superior traits and is likely to be released as a new variety, its seed is multiplied in the final year of trials. The breeder supplies the seed to the National Seeds Corporation for further production of foundation seed.
   • Distribution: The newly developed variety is then distributed to farmers for commercial cultivation.

Basis of Selection in the Pedigree Method

The basis of selection in the pedigree method involves evaluating a large number of plants to identify those with desirable traits. The process is inherently complex due to the genetic variability and environmental factors affecting plant characteristics. Here is a detailed explanation of the selection criteria and methods used:

1. Visual Evaluation:
   • Morphological Traits: Selection often relies on quick visual assessment of easily observable characteristics. These include plant height, leaf size, shape, head type, grain color, and the presence of awns. Traits such as days to flowering and maturity, as well as disease resistance, are also evaluated visually.
   • Effectiveness: Selection for these morphological traits is generally effective because they are typically governed by one or a few major genes and tend to have moderate to high heritability.
2. Disease Resistance:
   • Observation Challenges: Although disease resistance is an observable trait, uniform disease outbreaks are rare. Disease intensity varies annually, which complicates the selection process.
   • Artificial Inoculation: To overcome variability, breeders may induce uniform disease outbreaks through artificial inoculations, creating conditions for consistent evaluation.
3. Lodging Resistance:
   • Environmental Influence: Lodging resistance is affected by environmental conditions, which may not be present every year.
   • Assessment Methods: Breeders often select for lodging resistance by evaluating the breaking strength of straw and the force required to uproot plants. These physical characteristics are positively correlated with lodging resistance.
4. Cold Tolerance:
   • Artificial Testing: Cold tolerance may be assessed by exposing plants to artificial low temperatures if natural conditions do not reach critical levels.
   • Evaluation: This method ensures that the plants’ ability to withstand cold temperatures is accurately tested.
5. Quality Characters:
   • Diverse Requirements: Quality characteristics vary by crop and its intended use. For instance, wheat quality is assessed based on milling, baking, and chapati-making qualities, whereas barley is evaluated for malting quality.
   • Testing Procedures: Quality tests can be complex and time-consuming. Therefore, they are typically performed in later generations (e.g., F4 or F5) when the number of progenies is manageable. Simple and inexpensive tests, such as determining brix in sugarcane or cyanogenic glucoside levels in Sudan grass, are preferred for earlier stages.
6. Yield Ability:
   • Challenges in Early Generations: Yield is difficult to select for in early segregating generations (e.g., F1 to F3) due to its low heritability and the influence of environmental factors, heterozygosity, and genotype-environment interactions.
   • Current Practices: Selection for yield is often deferred until later generations when progenies are more stable and environmental effects are more predictable.

Merits of the Pedigree Method

The pedigree method offers several advantages in plant breeding, making it a widely used technique for developing new varieties. Here are the key merits:

1. Enhanced Breeder Skill Utilization:
   • Skill and Judgement: The pedigree method provides ample opportunity for breeders to apply their expertise and judgement. This is particularly beneficial in early segregating generations, where careful selection can significantly impact the outcome.
2. Suitability for Easily Identifiable Traits:
   • Simple Inheritance: The method is well-suited for improving traits that are easily identifiable and governed by simple inheritance patterns. This includes morphological characteristics that can be observed without complex testing.
3. Recovery of Transgressive Segregants:
   • Quantitative Traits: The pedigree method allows for the recovery of transgressive segregants—plants exhibiting traits beyond the range of the parent population. This is advantageous for improving quantitative traits like yield and other performance-related characteristics.
4. Efficiency in Time:
   • Faster Development: Compared to the bulk method, the pedigree method typically requires less time to develop a new variety. This is due to the more structured and focused selection process, which accelerates the development of stable lines.
5. Insight into Inheritance Patterns:
   • Pedigree Records: The method facilitates the analysis of inheritance patterns for qualitative traits. Detailed pedigree records provide valuable information about how traits are passed from one generation to the next, aiding in the understanding of genetic mechanisms.
6. Early Elimination of Defective Plants:
   • Early Selection: The pedigree method allows for the early removal of plants and progenies with visible defects or weaknesses. This early culling helps in maintaining high standards throughout the breeding program and reduces the number of inferior plants in subsequent generations.

Demerits of the Pedigree Method

While the pedigree method is a valuable tool in plant breeding, it has several limitations that can affect its effectiveness. Here are the primary drawbacks:

1. Time-Consuming Record Keeping:
   • Pedigree Records: Maintaining accurate and detailed pedigree records is a labor-intensive process. This requirement can be particularly burdensome in large breeding programs where extensive documentation is necessary, potentially limiting the feasibility of the method.
2. Labor-Intensive Selection Process:
   • Selection Effort: The method involves evaluating a large number of progenies in each generation, which is both laborious and time-consuming. Managing numerous crosses and handling many progenies simultaneously can be challenging and may restrict the number of crosses a breeder can effectively manage.
3. Dependence on Breeder Skill:
   • Skill Requirement: The success of the pedigree method is heavily dependent on the skill and experience of the breeder. Effective selection relies on the breeder’s ability to assess plants accurately, as there is no opportunity for natural selection to act on the populations.
4. Ineffectiveness in Early Yield Selection:
   • Yield Evaluation: Selection for yield in early generations (such as F1 and F2) is often ineffective. This is because yield traits are influenced by multiple genes and environmental factors, making early selection less reliable.
5. Risk of Losing Valuable Genotypes:
   • Progeny Retention: If not managed carefully, there is a risk of losing valuable genotypes in the early segregating generations. This loss occurs if too few progenies are retained or if selection pressure is not appropriately applied, potentially eliminating promising genetic material.

Applications of the Pedigree Method

The pedigree method is widely applied in plant breeding, particularly for self-pollinated crops. Its applications are broad and versatile, encompassing various objectives in crop improvement. Below are key applications of the pedigree method:

• Selection from Segregating Generations:
  • The pedigree method is primarily used to select individuals from segregating generations derived from crosses. This process ensures the development of genetically uniform pureline varieties.
• Correction of Specific Weaknesses:
  • This method is effective for addressing specific deficiencies in established varieties. By selecting and breeding plants that correct these weaknesses, breeders can enhance the overall performance of a variety.
• Combination Breeding:
  • The pedigree method is employed in combination breeding to integrate desirable traits from different varieties. This approach helps in developing new varieties with improved characteristics.
• Development of Superior Recombinant Types:
  • It is utilized to identify and select new superior recombinant types that exhibit enhanced traits compared to the original parents. This includes traits such as disease resistance, plant height, and maturity time.
• Transgressive Segregation:
  • The method often aims to recover transgressive segregants, which are individuals exhibiting extreme traits beyond those of their parents. This is beneficial for developing varieties with exceptional characteristics.
• Improvement of Yield and Quality:
  • Besides correcting specific weaknesses, the pedigree method is also focused on improving yield and quality. Breeders generally expect that the varieties developed through this method will demonstrate enhanced performance in these areas.
• Enhancement of Specific Characteristics:
  • The pedigree method is suitable for enhancing specific characteristics such as disease resistance, plant height, and maturity time. It allows breeders to fine-tune these traits to meet desired standards.

Achievements of the Pedigree Method

The pedigree method has played a pivotal role in plant breeding by facilitating the development of numerous improved crop varieties. This method’s effectiveness is evident across a wide range of crops, including cereals, pulses, oilseeds, and vegetables. Below are key achievements resulting from the application of the pedigree method:

1. Development of Improved Varieties:
   • Wheat (Triticum aestivum): Several high-yielding and disease-resistant wheat varieties have been developed using the pedigree method. For instance:
     • K 65: A tall variety suited for rainfed conditions, derived from the cross C 591 × NP 773.
     • K68: Known for its amber-colored grains and excellent chapati-making quality, developed from NP 73 × K 13.
     • WL 71: Selected from (S 308 × Chris) × Kalyan, a high-yielding dwarf variety, though susceptible to Karnal bunt.
     • Malviya 12: Exhibits amber, hard grains and performs well under low fertility and restricted irrigation conditions, developed from NP 876 × NP 6B.
2. Advancements in Rice Breeding:
   • Taichung Native 1 and IR 8: These varieties have significantly influenced the development of high-yielding rice varieties.
     • Jaya and Padma: Resulting from the cross Taichung Native 1 × T 141, with Padma being notable for its shorter duration and finer grains compared to Jaya.
     • Additional Varieties: Bala, Cauveri, Karuna, Krishna, Ratna, and Sabarmati are other notable varieties developed using this method.
3. Enhancements in Cotton Breeding:
   • Laxmi: Developed from the cross Gadag 1 × CC 2, this variety outperforms Gadag 1 in terms of ginning outturn, fiber properties, earliness, and resistance to red leaf blight. Gadag 1, originally susceptible to red leaf blight, was improved through this cross.
4. Improvements in Tomato Varieties:
   • Pusa Early Dwarf: This variety, developed from the cross Meeruti × Red Cloud, features a short stature and early maturation. It yields about 33% more in the first pick and 25% more total fruit yield compared to Pusa Ruby, with medium-sized and slightly flattish fruits.