There is an important paper out on calculating heritability (JAVA applet). Heritability is an important and misunderstood concept. Some people have argued that heritability is fallacious reification, a biostatistical construct which has no real relevance (or reality) outside of its utility in quantitative genetic models. But its entwinement with various concepts within evolution and genetics means it can’t be ignored, love it or hate it.1 Heritability is the proportion of phenotypic variation within a population that is attributable to genotypic variation.
This has two quick implications: first, the number of fingers you have on your right hand is not a heritabile trait, it is a genetic one, as there is little variation within the population, second, 0.9 heritability does not imply that for any given person the trait is “0.9 genetic” or “mostly genetic.” Assertions that a heritable trait is preponderantly “nature” or “nurture” doesn’t make sense and becomes confused by the weighting of necessary and sufficient conditions. As usual language does not map the statistical relationships and concepts very well. There are also other downstream considerations, for example, heritable traits are usually not strong in their fitness implications. The evolutionary reasoning is simple, traits subject to variation due to genetics and exhibiting powerful correlated fitness variation will exhaust their own variation as all the alleles which result in variation are fixed.
In any case, the coolest thing about the paper for me was that it showed the deviation from expectation (0.5) of relatedness for siblings using genomic methods. We’re talking gold nuggets of genes, not just a stream of statistical correlations. Though you share 1/2 of your genes with your parents with no variance, siblings are subject to sampling bias because only half the parental genome is contributed in every “draw” (every sibling). Obviously the “same half” of the genome from a given parent is not going to be contributed to each sibling, ergo, we see varied patterns of physical relationship between sets of full siblings due to the variance in the subset of genes that control appearance.
1 – Narrow sense heritability in the form of additive genetic variance is the raw material for microevolution.