Family Structure of Animal Population | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction

Even in supposedly randomly breeding populations, individuals do not have equal probabilities of being genetically similar to one another, creating a family structure. The concept of a family involves genetic likeness among members, extending beyond immediate relatives. The biological basis for treating a group as a family is the average genetic similarity measured by the coefficient of relationship.

Inbreeding and its Effects

Definition and Purpose:

• Inbreeding, a non-random mating condition, significantly impacts family structures.
• Increases relatedness to 1 times that of the foundation population, aiding family selection.

Effect on Standard Deviation:

• Increases the standard deviation, enabling a larger selection differential in mass selection.

Impact on Heritability:

• Raises heritability, allowing a larger fraction of the selection differential to be gained in offspring.
• Aids family selection, especially for traits with slight hereditary influence.

Enhancing Coefficient of Relationship (r):

• Inbreeding increases r beyond natural limits, crucial for effective family selection.
• Over a few generations, even mild inbreeding significantly raises r.

Relation to Maternal Environmental Influences:

• Allows high values of r without necessarily having high values of the characteristic, crucial when maternal environmental influences are strong.

Inbreeding and Family Distinctness

Correlation Between Dominance Deviations:

• In a family-related through both parents, there is a correlation between dominance deviations.
• Contributes to the C term in each family.

Impact of Intense Inbreeding on Variance:

• As inbreeding intensifies, heterozygotes decrease, causing an increase in dominance deviation, making the B term larger.
• Correlations between epistatic deviations contribute to the C term.

Shift in Variances:

• Regular fall-sib inbreeding shifts genetic variance from within families (1-1) to between families (r).
• The effect is distinctly larger B and slightly larger C terms.

In the context of selecting between related families, the effective relationship within the same family is approximately 1-ry, 

where r_z represents the internal relationship within the family, and y denotes the relationship between the two families. This implies that when distinct families originate from common and closely related foundational stock, the level of inbreeding needs to be increased for selection between families to achieve the desired effectiveness. When choosing among individuals from related families, less emphasis should be placed on family averages compared to selecting among individuals from unrelated families. The common sense behind this becomes evident when considering extreme cases. The family average becomes unhelpful in distinguishing between fits, as it remains the same for both. For instance, when selecting between two half-siblings, the family averages are influenced in part by the common parent, making them less informative in determining which of the two has a higher breeding value.