Biometrical Models and Covariance Between Relatives | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Understanding Biometrical Models and Resemblance Between Relatives

• Resemblance between relatives is a fundamental genetic phenomenon seen in metric characters.
• Degree of resemblance can be measured through simple population-based techniques.
• Additive variance, specifically heritability, plays a crucial role in determining breeding methods for improvement.

• Phenotypic variance is divided into components based on grouping individuals into families.
• Observational components of phenotypic variance (denoted by 'o') are estimated directly from phenotypic values.
• Resemblance between relatives is determined by within-group and between-group components of variance.

• It quantifies the degree of resemblance as the between-group component relative to the total variance.
• Expressed as the covariance between members of the same group.

• Involves computing covariance from sum of cross-products in pairs of observations.
• Regression coefficient of offspring on parent is used to express the degree of resemblance.

Implications of Covariance

• Covariance helps in estimating casual components of variance contributing to resemblance between relatives.
• Understanding the causal components behind covariance aids in genetic analysis and breeding strategies.

Genetic Covariance

• Offspring and One Parent: The genetic covariance between offspring and one parent is half the additive variance. This covariance is calculated based on the genotypic values of individuals and their offspring, where the offspring's mean genotypic value is half the breeding value of the parent.
• Half Siblings: Half-siblings share one common parent and have different other parents. The covariance of half-siblings is a quarter of the additive variance, as it is the variance of the means of half-sibling groups.
• Offspring and Mid-Parent: The covariance between the mean of the offspring and the mean of both parents (mid-parents) is equal to half the additive variance. This covariance is derived from the genotypic values of the offspring and the mean of the parents.
• Full Siblings: The genetic covariance of full siblings is half the additive genetic variance plus a quarter of the dominance variance. It is the variance of the means of full-sibling families.

• Environmental Covariance: Environmental factors contribute to the covariance of relatives by making them resemble each other. This environmental variance affects the variance between family means.
• Phenotypic Resemblance: Phenotypic resemblance is the sum of covariances from genetic and environmental factors. The covariance between relatives, except for full siblings, is typically a quarter or half of the additive genetic variance.
  (i) The resemblance between siblings is expressed as a correlation coefficient, dividing the covariance by the total phenotypic variance.
  (ii) The regression of offspring on parents is a measure of resemblance, calculated by dividing the covariance by the variance of parents.
  (iii) The correlation between half-siblings is equal to the square root of the phenotypic variance.
  (iv) The correlation between full siblings is twice the correlation between half-siblings in the absence of dominance and common environment effects.
• Estimation of Heritability: Measurements of resemblance between relatives help estimate the proportion of additive genetic variance, known as heritability (V/V).