Breeding for Threshold Traits

Introduction

Certain biological traits of paramount significance exhibit a discontinuous variation, deviating from simple Mendelian inheritance patterns. Examples include susceptibility to diseases, where phenotypic classes are affected or unaffected, and litter size in larger mammals, which typically bear one offspring but may occasionally produce more.

Discontinuous Anatomical Differences

Variations in Morphological Characteristics:

• Anatomical features like the number of vertebrae often exhibit non-continuous differences.
• These traits initially appear to fall outside the scope of quantitative genetics but are later found to be inherited similarly to continuously varying characters.

Similarity in Variance of Liability:

• Genetic analysis reveals that the variance in liability is consistent between the two groups, even though their means differ by 0.8 standard deviations of ability.
• Here, 'x' represents the normal deviate of the threshold from the mean, and 'i' is the mean deviation of affected individuals from their group mean.

Understanding the Threshold Concept:

• The key to comprehending the inheritance of such traits lies in the notion that the trait possesses an underlying continuity with a threshold, which introduces a discontinuity in its visible expression.
• Figure 1 illustrates this concept, indicating that when the underlying variable is below the threshold, the individual exhibits one form of phenotypic expression, such as normal.

Phenotypic Expression in Relation to Threshold:

• The phenotypic expression varies based on the position relative to the threshold.
• Individuals below the threshold exhibit one form, while those above manifest a different phenotypic expression.

Fig: Two populations or groups with different incidences, p, of a threshold character and consequently different mean liabilities.

Breeding Strategies for Threshold Traits in Genetic Analysis

Certain biological traits of significant importance exhibit a discontinuous variation, challenging traditional Mendelian inheritance patterns. These threshold traits, such as susceptibility to diseases or litter size in mammals, are not inherited in a simple Mendelian manner. Genetic analysis reveals that the underlying continuous variable, termed "liability," influences the visible expression of these traits through a threshold.

Understanding Liability in Threshold Characters

Continuous Variation of Liability:

• Liability, the underlying variable, reflects both genetic and environmental factors.
• It may represent the concentration of a substance or the rate of a developmental process not easily measured as a discrete character.

Realistic Application to Litter Size:

• Litter size in mice or pigs, though seemingly discontinuous, can be treated as a continuous variable due to the presence of numerous classes.
• The physiological causes of twinning in different species may share commonalities, emphasizing the underlying principle of factors influencing reproductive processes.

Two Classes and One Threshold

Phenotypic Classes:

• Two visible classes, normal and affected, with an underlying threshold.
• The visible scale assigns values of 0 for normal and 1 for affected, allowing for the classification of individuals, families, or populations.

Incidence and Prevalence:

• Incidence, the proportion of affected individuals, is converted to genetic liability through the transformation of liability as a normally distributed variable.
• The mean liability is related to incidence through the normal deviate 'X,' representing the deviation of the threshold from the mean.

Three Classes and Two Thresholds

• The genetic analysis of threshold characters becomes more intricate when three phenotypic classes and two thresholds are involved, such as single, twin, or triplet births.

Breeding or Selection for Threshold Characters

Application of Selection:

• Breeding or selection for threshold characters is practically significant, particularly in reducing abnormality incidence and altering responses to treatments.

Selection Differential:

• The response to selection is influenced by the selection differential, which is based on the incidence of the desired phenotypic class.

Manipulating Incidence for Selection Advantage:

• Altering the incidence of certain characters provides a means of increasing the selection differential and enhancing the response to selection.

Manipulating Incidence and Thresholds in Genetic Selection Strategies

Altering the incidence of certain characters plays a pivotal role in genetic selection strategies. This involves shifting the threshold about the mean liability of the population, particularly in threshold traits where the visible expression depends on a specific threshold. This shift in the threshold, achieved by changing the treatment conditions, impacts the expression of the character and influences the efficiency of genetic selection.

Controlling Thresholds for Optimal Progress

Threshold Adjustment for Maximum Progress:

• The speed of progress under selection is optimized by controlling the threshold, ensuring that the incidence aligns closely with the desired proportion for breeding.
• Continuous assessment of progress involves subjecting the population to the original treatment, and maintaining the threshold at its original level.

Treatment-Induced Threshold Changes:

• Altered treatment conditions induce a shift in the threshold, leading to a different liability threshold for character expression.
• The goal is to fine-tune the threshold to achieve optimal selection outcomes.

Continuous Variable in Family Selection:

• In family selection or progeny testing, the criteria for selection are based on a continuous variable-the mean of the liability of the family.
• Optimal discrimination between families is achieved when the sailing variance of liability is minimal.

Genetic Assimilation

Environmental Stimulus and Threshold Appearance:

• If a threshold character emerges due to an environmental stimulus, continued selection for this character may lead to its spontaneous appearance without the need for the stimulus.
• The process follows orthodox selection principles, making the character inheritable.

Dual Thresholds: Spontaneous and Induced:

• Two thresholds exist-a spontaneous one initially outside the population's range, and an induced one within the range of liability covered by the population.
• The induced threshold allows for the selection of individuals towards the tail end of the distribution, influencing the mean genotypic value of the population.

Transition to Spontaneous Incidence:

• As the mean genotypic value shifts, some individuals may eventually cross the spontaneous threshold, becoming spontaneous variants.
• Once the spontaneous incidence reaches a sufficient level, selection can be sustained without the environmental stimulus, further increasing the spontaneous incidence.

Manipulating incidences and thresholds is a powerful approach in genetic selection, allowing for fine-tuning selection outcomes and potentially leading to the genetic assimilation of threshold characters. Understanding these principles enhances the efficiency of breeding programs.