Steph has a PhD in Entomology and teaches college biology and ecology.
What is Heritability?
Say you are the champion weightlifter at school. You take your date home to meet your parents (who are both almost as big and beefy as you are).
'Wow,' says your date. 'Are you so big because you work out, or did you get that from your parents?'
Intuitively, you already know the answer- both.
But suppose you and your date want to quantify your answer. That is when you might use heritability.
Heritability is how much of the variation seen in a certain trait within a population can be attributed to genetic variation, as opposed to environment. A high heritability means that for all the variation of a certain trait in the population, a large portion is caused by genetic differences. A low heritability means that for all the variation of this trait, more of it is caused by the environment. For example, a recent study showed that several major mental illnesses, including schizophrenia, bipolar disorder, and ADHD, were 17-28% heritable, meaning that genetic variation accounted for about 17-28% of the risk of illness.
Heritability is not the proportion of a trait that is caused by genetic factors. Heritability can change dependent on the environment- even heritability for a single trait within a species. For instance, in a society where most people have adequate medical care and enough to eat, you may expect to be tall if your parents were tall, and short if your parents were short. But if your parents experienced disease and famine as children and you did not, you might be tall even if your parents were short. When environmental variation increases, heritability decreases.
The geneticist Richard Lewontin illustrated this point with the following example. Imagine you plant two trays full of seeds. You put one tray in good lighting, with adequate water and fertilizer. You give the other tray poor soil with no fertilizer, barely enough water, and poor lighting. The heritability of traits you see within each tray is close to 100%; the differences you see between each individual plant within a tray are probably due to genetics. But when you compare the two trays, it is clear that environment played a large role in how your plants look.
How to Calculate Heritability
Broad-sense heritability is represented as H2 and indicates all the genetic factors that go into phenotypic variance in a population. It can be calculated as:
H2 = Var(G) / Var(P)
where Var(G) is the variance in genotype and Var(P) is the variance in phenotype in a population. In other words, Var(G) is the genetic differences in your allele of interest (say, one that contributes to plant height) while Var(P) is the difference in phenotype (how tall a plant is).
Narrow-sense heritability is the variance due to the additive effect of alleles. Because you are diploid, you get one allele from each parent, and it is how those alleles interact- not what they do individually- that determines your phenotype. Narrow-sense heritability is represented as h2.
It can be calculated as
h2 = Var(A) / Var(P)
where Var(A) is the additive or average effects of the allele.
Why do we care about narrow-sense heritability? Natural (or artificial) selection can only act on phenotypes, not alleles. Often, we talk about narrow-sense heritability with respect to artificial selection- when humans select for desired traits. For instance, if I'm breeding cows to give more milk, I care more about how much milk my cows give than I care about the presence of a certain allele at a certain locus.
If milk production has a low heritability in my herd, my breeding program might not be very successful. There might just not be enough genetic variation for me to improve milk production in my herd, or it might be the case that there are other non-genetic factors that are causing low milk production. If milk production has high heritability, breeding may be able to solve my problem.
Controversy in Heritability
Heritability in humans can also be controversial. Why? Think about topics in human psychology, behavior, and sociology that are also controversial, such as IQ. The legitimacy of IQ has already been questioned for various reasons, and it can be a touchy subject to some people. Also, since you and your brothers and sisters (if you have any) were most likely raised by the same people, environment can be a confounding factor that is hard to tease out. For instance, high-IQ parents may be more likely to provide high-IQ genes. But they also may be more likely to provide intellectually stimulating environments for their children, which can also lead to higher IQs.
Many sources cite the heritability of IQ as 70%. But if you remember Dr. Lewontin's plants, you will understand that this can still be true even if a child's environment has a huge impact on his or her IQ. And like Dr. Lewontin's plants, you might be bigger and stronger than average even if you never worked out- but it is the combination of genes and environment that made you a champion weightlifter.
Heritability is the measure of how much of the variation in a given trait is due to genetic variation. It doesn't just mean how much of a trait is due to genetics. Just because something is genetically determined doesn't mean it doesn't vary with the environment.
Broad-sense heritability indicates all the genetic factors that go into phenotypic variance in a population. Narrow-sense heritability is the variance due to the added effects of alleles, which is important since you have two copies of most of your genes.
Heritability is used when breeding agricultural crops and livestock, and when discussing evolution, but it is also used in discussions about human behaviors- and when it is, it should be discussed with care, because it can get touchy.
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