Quantitative Genetics Key Concepts

Strength of Association Between Variables

The strength of the association between two variables is quantified by the correlation coefficient, which ranges from -1 to 1, indicating the direction and strength of the relationship.
- A correlation coefficient close to 1 implies a strong positive relationship.
- A value close to -1 indicates a strong negative relationship.
- Example: In a study of height and weight, a high correlation coefficient suggests that as height increases, weight tends to increase as well.

Quantitative traits are typically characterized by a continuous distribution, meaning they can take on a range of values rather than discrete categories.
These traits are influenced by multiple genes (polygenic), which contribute to the overall phenotype.
Environmental factors also play a significant role in the expression of quantitative traits, leading to variability in phenotypes even among genetically similar individuals.
- Example: Height in humans is a quantitative trait influenced by both genetic makeup and environmental factors such as nutrition.

A heritability value close to 1 indicates that nearly all phenotypic variation in a trait is due to genetic factors, suggesting a strong genetic basis for the trait.
Heritability does not indicate the degree to which a trait is genetically determined but rather the proportion of variance in a trait attributable to genetic differences in a population.
- Example: In agricultural studies, high heritability values can guide selective breeding programs by indicating traits that can be reliably passed on to offspring.

The total variance of a population for a quantitative trait is the sum of genetic variance and environmental variance, which helps in understanding the overall variability in the trait.
Genetic variance can be further divided into additive variance (effects of individual alleles) and dominance variance (interactions between alleles).
Environmental variance includes all non-genetic factors that can affect the phenotype, such as climate, soil quality, and management practices in agriculture.

After several generations of selective breeding, a plateau may be reached where further selection does not lead to significant improvements; this is known as the selection limit.
The selection limit occurs due to factors such as reduced genetic variation and the influence of environmental factors that cannot be controlled through breeding alone.
- Example: In livestock breeding, after achieving desired traits, further selection may yield diminishing returns due to genetic constraints.