7.7: Quantitative Genetics and Heritability

Image credit: Ingo Joseph from Pexels: https://www.pexels.com/photo/brown-cattle-16009/

Traits can be classified based on their mode of inheritance and expression. Qualitative traits are controlled by one or a few genes with large effects and fall into discrete categories that are easily classified (e.g., coat color, polled versus horned). Quantitative traits are controlled by many genes, each with small effects, and vary continuously across a range of values (e.g., milk yield, weaning weight, feed efficiency). Most economically important traits in livestock are quantitative.

The observable characteristics of an animal—its phenotype—result from both genetic and environmental influences. The genetic component itself can be partitioned into additive effects (transmitted reliably from parent to offspring), dominance effects (interactions between alleles at the same locus), and epistatic effects (interactions between alleles at different loci).

Heritability (h²) measures the proportion of phenotypic variation in a trait that is due to additive genetic effects and can be passed from parent to offspring. It indicates how readily a trait responds to selection. Heritability values range from 0 to 1, with higher values indicating greater genetic influence:

• Low heritability (0–0.2): Reproductive traits such as fertility, conception rate, and litter size. These traits are heavily influenced by environment and management, making genetic progress slower. However, they show high levels of heterosis in crossbreeding programs.
• Moderate heritability (0.2–0.4): Growth traits such as weaning weight, yearling weight, and rate of gain. These traits respond well to selection and show moderate heterosis.
• High heritability (0.4–0.6): Carcass traits such as ribeye area, marbling, and backfat thickness, as well as fleece traits in sheep. These traits respond rapidly to selection but show little heterosis.

Genetic correlation occurs when the same genes or many of the same genes control two different traits. Positive correlations mean that breeding values for both traits tend to move in the same direction (e.g., improving rate of gain often improves feed efficiency). Negative correlations mean that increasing one trait tends to decrease the other (e.g., selecting for increased yearling weight may increase calving difficulty). Understanding these relationships is essential for balanced breeding programs that avoid unintended consequences.

Photo credit: Elina Sazonova: https://www.pexels.com/photo/cow-and-calf-1838564/

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