Heritability · Lobeworks/17

Heritability is the share of the variation of a trait within a population that is explained by genetic differences between its members, and it is the number twin studies report for how far genes account for differences in brains, never how much of one brain genes built.

Heritability. Heritability is the share of the variation of a trait within a population that is explained by genetic differences between its members, and it is the number twin studies report for how far genes account for differences in brains, never how much of one brain genes built.

It is measured most often by comparing twins. Identical twins share nearly all their genes and fraternal twins about half, while both pairs grow up in the same home; if identical pairs resemble each other much more than fraternal pairs on a trait, genes explain much of its variation. Falconer's shortcut puts heritability at twice the gap between the two correlations, h2≈2(rMZ−rDZ)h^2 \approx 2(r_{MZ} - r_{DZ})h2≈2(rMZ​−rDZ​). For the brain, twin MRI studies give some of the highest values known for any human trait: about 0.9 for total brain volume, around 0.8 to 0.9 for total grey and white matter, lower and more variable for single regions and for the surface area and thickness of particular patches of cortex (Peper et al., 2007).

A share of variance is not a share of a brain. Saying brain volume is 90 % heritable means that 90 % of the differences between people in that sample track their genes; it does not mean that 90 % of one person's brain came from genes and 10 % from life, a split that has no meaning for one individual, because genes and environment act together on every cell.

It depends on the population. In a group that shares a narrow range of environments, almost all remaining variation is genetic and heritability looks high; the same trait measured across very different environments gives a lower number.

High heritability does not mean fixed. Myopia is strongly heritable and fully corrected by glasses; a heritable brain measure can still change with learning, illness or age, through synaptic plasticity and synaptic pruning.

Heritability measures where differences come from, not where a brain comes from.

It is a statistic of a population in its environments, and it says nothing about the recipe of one person.

Questions: If brain size is about 90 % heritable, how much of one brain did genes build? The question has no answer, because heritability is not a share of one brain. Twin studies find that about 90 % of the differences in total brain volume between people in a sample track their genetic differences; that says where the variation comes from in that population, in those environments. Every brain is built by genes and environment acting together on every cell, so splitting one person's brain into 90 % from genes and 10 % from life, or an 80 to 20 rule for size and wiring, misreads the statistic.