The Pith: Even traits where most of the variation you see around you is controlled by genes still exhibit a lot of variation within families. That's why there are siblings of very different heights or intellectual aptitudes.
In a post below I played fast and loose with the term correlation and caused some confusion. Correlation is obviously a set of precise statistical terms, but it also has a colloquial connotation. Additionally, I regularly talk about heritability. Heritability is in short the proportion of phenotypic variance which can be explained by genetic variance. In other words, if heritability is ~1 almost all the variation in the trait is due to variation in genes, while if heritability is ~0 almost none of it is. Correlation and heritability of traits across generations are obviously related, but they're not the same.
This post is to clarify a few of these confusions, and sharpen some ...
