What does "heritable" mean, in the context of genetics? If you've casually followed genetics stories over the last few years, you've surely seen the term, and probably assumed that it refers to the extent to which a particular trait is genetically inherited.
For instance, here are a few recent uses in the New York Times:
"Studies of twins show that homosexuality, especially among men, is quite heritable, meaning there is a genetic component to it." (4/10/07)
"Since personality is heritable, this would be a mechanism for Yanomamo nature to evolve and become fiercer than usual." (3/12/06)
"A genetic analysis using tissue samples from 185 dolphins, 13 of them spongers, showed that it was highly unlikely that sponging was a heritable trait." (6/7/05)
So why does noted biologist and geneticist Tim Tully tell author Matt Ridley: "I can prove in a court of law that heritability has nothing to do with biology"?
And how is it that the heritability of the number-of-fingers on the human hand is close to zero?
And how can height be 90% heritable and yet subject to extraordinary influence from nutrition?
And how can I.Q. be 50-60% heritable in some well-designed studies and almost 0% in other, equally valid studies?
Welcome to the truly bizarre world of heritability and its misuses. If you're a non-scientist like me, you will be stunned to learn the word's actual definition, and its implications. In fact, a close look at how the term is misused by journalists and scientists alike goes a long way to helping us understand why the public still has such a profound misunderstanding of genetics and human development.
One thing is clear enough, to me anyway: the terms "heritable" and "heritability" should never, ever be used in a general interest publication without extensive context.
So -- here now the definition, from Merriam-Webster:
heritability: the proportion of observed variation in a particular trait (as height) that can be attributed to inherited genetic factors in contrast to environmental ones.
Let's break that down:
-- a heritability estimate comes from a statistical analysis of a given population, not the results of a biological investigation (that, of course, doesn't mean it's invalid; but it does convey some important limitations); further, heritability can only be estimated, never actually known -- in fact, the "proportion" heritability seeks to estimate is ultimately unknowable, for reasons I will explain below;
"of observed variation"
-- heritability cannot actually look directly at what causes a trait (partly because every complex trait has multiple causes); it can only look at the amount of variation in that trait in a particular population. As Matt Ridley explains, heritability studies come from "measuring how similar identical twins are, how different fraternals are, and how both identicals and fraternals turn out if separately adopted into different families."
-- since the observed variation is going to differ from population to population, a heritability estimate is only relevant to the particular population studied. "It is strictly a property of a particular population," University of Arizona's Bruce Walsh explains. "Different populations, even if closely related, can have very different heritabilities."
And, adds Matt Ridley, heritability is "meaningless for any individual person." This is critical to keep in mind.
"in a particular trait that can be attributed to inherited genetic factors in contrast to
-- What traits are we talking about? Not the basic Mendelian stuff like eye and skin color, obviously, but the really complex stuff like sexuality, aspects of intelligence, personality factors, etc. How are those traits formed, to the best of our understanding? From an extremely complex combination of, and interaction between, genes and the environment. The actual equation used by behavioral geneticists is: V(p) = V(a) + V(d) + V(i) + V(e) + V(g X e) + COV(G,E). In layman's terms, there are four basic variables.
1. One or more separate genes, some of them interacting with one another (taking height, for example: hypothetically, you might inherit four different genes that help determine your height, two of whom are completely independent actors, and the other two which also have independent influence AND which will interact with one another to effectively produce a unique fifth genetic influence).
2. Environment (your in-utero and post-utero nutrition will have a direct and significant effect upon your adult height)
3. Gene-environment interactions (some of your nutritional experience will have a specific interaction with your particular genes -- with a *different* outcome than the interaction my genes might have with the exact same nutrition)
4. Gene-environment covariation (these are genetic influences on environment; for example, let's say from early on you are a particularly active baby, and as a result you get placed in a daycare group with other particularly active babies; that group happens to get a slightly different snack chosen by a different teacher, and that affects your nutrition, which effects your height. In effect, you have some non-height genes having had unexpected impact on your environment, which in turn is going to have an effect on your height. This indirect but very real phenomenon is called "covariation).
It actually gets more complex than that, but that's a fair summary. What this means, of course, is that for any such traits the nature vs. nurture paradigm simply DOES NOT EXIST. It's a false choice, like saying a meal either comes from the ingredients or the cook. Instead, the development process is continuously and inseparably nature-and-nurture.
If nature and nurture are inextricably intertwined, how can we determine what portion of a certain trait is due to genetics? We can't. We can only come up with crude averages from population studies. As a matter of biology, such a proportion does not exist. Which is why the Human Genome Project website calls heritability "a statistical construct," and warns: "heritability statements provide no basis for predictions about the expression of the trait in question in any given individual."
Heritability is not a useless measure, by any means. It has wide applications in genetics and agriculture. But in the context of conveying scientific understanding to a general audience, it is inherently misleading.