additive genetic variance

Additive genetic variance is the portion of a trait's variation produced by alleles whose effects simply add up, each one contributing the same fixed amount no matter which other alleles it happens to share the genome with. Imagine every favourable allele as a small weight dropped on a scale: the reading is just the sum of the weights, with no surprises from how they combine.

This piece of the variation is special because it is the part that passes faithfully from parents to offspring. Since a parent transmits single alleles rather than its whole genotype, only effects that hold steady regardless of partner alleles will reliably show up in the next generation. That is exactly the additive component, which is why it sits at the heart of how populations resemble their ancestors and respond to selection.

Additive variance is what you divide by total phenotypic variance to get narrow-sense heritability, and the larger it is relative to the whole, the faster a trait answers to breeding. It stands apart from non-additive variance — dominance and epistasis — where the effect of an allele depends on its company. Those interaction effects can be real and large yet contribute little to parent-offspring resemblance because the winning combinations are broken up each generation.