Genetics

Genetics studies heredity and variation in organisms.

A genetic variant is a heritable difference in a DNA sequence relative to a chosen reference. Genetic variants can be used as instrumental variables in Mendelian randomization when the required causal assumptions are credible.

Quantitative genetics studies how genetic and environmental variation contribute to continuously varying traits.

Heritability is the fraction of phenotypic variance in a specified population and environment attributed to genetic variation. It is a population parameter, not the genetic fraction of one individual's trait.

A twin study compares trait resemblance between twins with different degrees of genetic relatedness to separate genetic and environmental variance components.

Monozygotic twins develop from one zygote and are modeled as sharing essentially all additive genetic variation.

Dizygotic twins develop from two zygotes and, absent assortative mating and other complications, share one half of additive genetic variation on average.

The ACE model decomposes trait variation into additive genetic effects $A$, common environmental effects $C$, and unique environmental effects $E$.

Falconer's formula estimates narrow-sense heritability from monozygotic and dizygotic twin correlations:It relies on equal relevant environments and a dizygotic additive-genetic correlation of one half.

Assortative mating is nonrandom mating in which partners are more or less similar than random pairs for a trait or correlated characteristic. Positive assortment can increase genetic similarity among siblings.