A recent post by Megan McArdle on her Atlantic blog about the heritability of obesity prompted a discussion on ScienceBlogs about the often confused meaning of heritability. As Razib explains on Gene Expression, "Heritability is the proportion of trait variance within the population explainable by variance of genes." The more an environment is able to effect differences between individual expressions of a trait in a single population, the lower the heritability of that trait. In the case of the US obesity epidemic, generational differences in obesity levels do not mean that obesity is not heritable; only that the entire generational population is subject to similar environmental conditions. Mike the Mad Biologist points out the danger in conflating heritability with genetic control and with irreversibility. "This kind of obesity genetics-as-destiny fatalism gives cover to a lot of people with unhealthy habits," he says. But Jessica Palmer of bioephemera contends that realizing how genetics contribute to one's weight can help people find the best approach to maintaining their personal health, and Peter Lipson shares his own approach on White Coat Underground.