Resemblances Between Relatives | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Resemblances Between Relatives

Genetic Resemblance:

• The resemblance between relatives is a fundamental genetic phenomenon exhibited by measurable traits.
• The degree of resemblance in genetic traits can be determined through simple measurements in populations.
• Resemblance helps estimate additive genetic variance, crucial for determining breeding methods for enhancement.

Partitioning Phenotypic Variance:

• Phenotypic variance can be divided into causal components denoted by symbol V.
• Observational components of phenotypic variance (symbol σ²) are estimated from phenotypic values.
• Components are determined by grouping individuals into families, such as full siblings.

Analysis of Variance:

• Variance is partitioned into between-group and within-group components.
• Between-group variance represents differences among group means, while within-group variance reflects individual deviations from group means.

Intraclass Correlation Coefficient:

• It quantifies resemblance by comparing between-group and within-group variances.
• Expressed as the proportion of between-group variance to total variance.

Covariance of Relatives:

• Genetic and environmental factors contribute to covariance among relatives.
• Common relationships studied include offspring with parents, half siblings, full siblings, and twins.

Regression of Relatives:

• Correlation and regression coefficients are used to quantify resemblances among different relatives.
• Examples include correlations between offspring and parents, mid-parents, half siblings, full siblings, and twins.

Genetic and Environmental Influences on Resemblance:

• Genetic and environmental factors contribute to the resemblance among relatives.
• When family members share a common environment, such as in human families or litters of animals like pigs or mice, they exhibit similarities.
• The shared environment introduces an environmental variance that impacts the differences between family means but not within families.
• This shared environmental component, known as common environment (V), influences the covariance of related individuals.
• The remaining environmental variance (Vg) stems from factors that are unrelated to genetic relatedness.
• Together, common environment and other non-shared environmental factors form the total environmental variance (Vg).

Effective Population Size

• When the breeding structure deviates from the idealized population, the dispersion process can still be assessed through gene frequency variance or inbreeding rate.
• To handle deviations, the concept of effective population size (N) is introduced, representing the number of individuals that would yield the same variance or inbreeding rate as the ideal population.
• For instance, if the inbreeding rate (AF) is calculated for a specific breeding structure, it is related to the population size (N) as AF = 1/2N.
• The effective size (N_e) is derived from AF as N_e = 1/2AF, providing a measure of population size under non-ideal breeding conditions.
• Effective population size can be estimated based on the actual number of breeding individuals, with approximate relationships available for common deviations from idealized breeding structures.
• It's crucial to differentiate between the actual number of breeding individuals and the census count, especially when different age groups are involved.
• By determining the effective population size for any breeding structure, one can then calculate the rate of inbreeding using the formula AF = 1/2N.

Dominance and Epistatic Deviation Overview

• Genes interact when a gene's response changes in the presence of another gene.
• Interactions lead to genetic variation as the same gene may produce different responses in progeny.
• Dominance and epistasis contribute significantly to variation in traits controlled by one or a few loci.

Importance of Gene Interactions:

• Genetic variance studies have shown that only a small portion of quantitative traits' variation is due to gene interactions.
• Selection for gene interactions involves choosing gene combinations with added genetic values, requiring complex breeding programs.

Types of Gene Interactions

Dominance: Occurs when one gene masks the effect of another at the same locus.

• Example: In cattle, a gene for horn growth is dominant in the hh combination but recessive to the allelic gene H, resulting in heterozygotes not developing horns.

Epistasis: Involves a gene at one locus influencing another gene's effect at a different locus.

• Example: Gene c in many species causes albinism by blocking pigment formation, affecting the expression of other genes related to color and patterns.

Copsequences of gene interactions

• The outcome of gene interactions results in the creation of genotypes that are not easily passed on to offspring.
• For quantitative traits, this leads to confusion and a decrease in the effectiveness of selection, similar to environmental variability.
• This situation can cause the selection of parents who do not pass on their superior traits to their offspring.

Genotype x environmental correlation and genotype-environmental interaction:

• Two complications arise when dividing variance into genotypic and environmental components.
• Typically, these can be overlooked without significantly impacting variance partitioning conclusions, but understanding the implications of neglect is important.

Correlation

• It is usually assumed that environmental deviations and genotypic values are independent of each other.
• Correlation between genotype and environment is often negligible in experimental populations due to randomization of environments.
• However, correlations can exist in certain situations, such as in milk-yield in dairy cattle or human intelligence.

Interaction

• An assumption is sometimes made that a specific environmental difference affects all genotypes equally, but this is not always the case.
• An interaction between genotypes and environments may occur, leading to varied effects on different genotypes under different conditions.
• This interaction component contributes to additional variation in phenotypic values.

In summary, gene interactions can lead to challenges in transmitting genotypes to offspring, while correlations between genotypes and environments and interactions between genotypes and environments can introduce complexities in understanding and predicting traits. It is crucial to consider these factors when analyzing genetic and environmental influences on phenotypic outcomes.

Genetically Uniform Groups in Experiments:

• In experiments involving genetically uniform groups, each individual shares the same genotype, resulting in any observed variation being primarily due to environmental factors.
• The variability within such groups stems from how each genotype reacts to distinct environmental conditions, with differences not rooted in genetic diversity but in environmental influences.
• It is crucial to account for genotype-environment interactions, as some genotypes may exhibit greater sensitivity to environmental changes than others, impacting the overall variance.

Environmental Variance and Genotype Sensitivity:

• The environmental variance, which contributes to the overall variability within a genetically uniform group, is a result of how a specific genotype responds to environmental stimuli.
• While the environmental variance is influenced by the genotype's characteristics, the source of variation remains environmental rather than genetic in nature.
• Therefore, any variance attributed to genotype-environment interactions should be considered part of the environmental variance estimate, not a genetic factor.

Logical and Experimental Considerations:

• It is both logical and essential in experimental settings to include any variance arising from genotype-environment interactions within the broader environmental variance estimation.
• By treating genotype-environment interactions as part of the environmental variance, researchers ensure a comprehensive assessment of the total variability present in the study.

Genotype-Environment Interaction

Importance of Genotype-Environment Interaction:

• When individuals from a specific population are raised under different conditions, understanding how their genetic makeup interacts with their environment becomes crucial.

Specific Environments and Treatments:

• Specific environments such as different farms, seasons, or locations where individuals are reared are shared conditions that act as treatments.

Studying Genotype-Environment Interaction:

• When individuals are raised in specific environments, we can delve deeper into how their genetic traits interact with these conditions.
• Analyzing the interaction between genotypes and environments involves replicating genotypes and rearing multiple individuals of each genotype in various specific environments.

Outcome of Interaction Analysis:

• An analysis of variance in a two-way classification of genotypes x environments helps determine the variance between genotypes, specific environments, and the impact of genotype-environment interaction.
• If there is minimal interaction, the best genotype in one environment will perform well in all environments. However, significant interaction indicates the need for specific genotypes for particular environments.

Multiple Measurements: Repeatability

Concept of Repeatability

• When multiple measurements of a characteristic are taken from each individual, the phenotypic variance can be divided into variance within individuals and variance between individuals.
• This division results in a ratio of variance components known as repeatability, which serves several purposes:
  (i) Demonstrates the benefit of repeating measurements.
  (ii) Establishes upper limits for variance ratios.
  (iii) Allows for predicting future performance based on past records.

Practical Uses of Repeatability

• Repeatability analysis doesn't directly relate to genetic theory as it focuses on environmental, not genetic, variance.
• However, it holds significance for genetic analysis and breeding programs.
  (i) Helps in understanding environmental variance.
  (ii) Assists in predicting performance and guiding breeding decisions.

Types of Repeated Measurements

• Temporal Repetition: Character measurements taken at different time points (e.g., milk yield in successive lactations, litter size in successive pregnancies).
• Spatial Repetition: Character measurements taken at different locations or on repeated parts of an organism (e.g., fruit characteristics in plants, bristle count in Drosophila).

Analysis of Variance Components: 

• Within-Individual Component: Reflects temporary environmental differences within the same individual.
• Between-Individual Component: Indicates permanent differences between individuals, influenced by both environmental and genetic factors.
• Environmental Influence: Temporary environmental effects are separated from genetic and permanent environmental influences.