What does it mean for a trait to be heritable?

3 minute read

Tall people tend to have tall parents. The height of the body, called stature, is one of the most obvious phenotypic traits in human populations. Anyone who knows very many families can see that some families tend to have a lot of tall members, and others tend to be relatively short.

Still, there is more to being tall than having tall parents. A child may grow to be taller than either of his parents, sometimes much taller. A very tall father and short mother don’t always have children that are in between their statures: sometimes a daughter may match her mother’s height, or even be shorter. Relatives resemble each other in height, but we cannot say that a woman’s stature is determined by the statures of her parents.

There are two key reasons why most phenotypes are not inherited as simple Mendelian traits. Many traits are affected not by one, but by multiple genes. At present, more than 300 genes are known to influence the variation in stature within populations of European ancestry. Other genes may be responsible for differences within other populations and between very different populations such as the short-statured Biaka Pygmies and tall-statured Dinka of equatorial Africa. The combination of so many genes helps to explain why stature is not a Mendelian trait with only two or three distinct forms. Instead, stature is a \term{continuous} trait, in which a person may be 62 inches tall, 61.5 inches, 61.25 inches, 61.0003 inches, or any measurable value in between.

Another reason for variation is that different people may have experienced different environments. The effects of environmental variation are plain in many fields of corn. The corn plants in a single field generally have very little genetic variation: they are hybrids bred for their high yields with adequate fertilizer, pesticide, and water. But many fields have low spots where water pools, or drier spots on the edge of the field. Here, the plants do not grow as tall or yield as much, because their environment is not ideal for high yield by the planted strain. The variation in plant height visible in such fields is entirely the result of environmental variation, because the plants are genetically uniform.

Environmental variation may come in at many points during an individual’s life, from early embryonic development to childhood nutrition and disease. At all stages, this environmental variation can have a major effect on some phenotypes — sometimes more extensive than variation in genes.

Heritability is the proportion of phenotypic variation that is explained by genetic variation.

\begin{figure} \includegraphics[width=\textwidth]{female_stature_heritability.png} \label{fig:female_stature_heritability} \caption[Female stature heritability]{Relationship of student stature to midparent stature in a sample of 100 female college students. The midparent stature is the average of the mother’s and father’s heights. Taller parents tend to have taller offspring, although some women are taller than the average of their parents, and others are shorter. The slope of the line relating the two reflects the heritability of stature in the sample, which for these students is estimated at 0.89. } \end{figure}

For a phenotype like stature, it can be important to know how much of the variation results from genetic variation, and how much of the variation results from the environment. This is important because genetic changes can affect a phenotype only to the degree that the phenotypic variation is influenced by those genes. Geneticists also want to discover if phenotypic similarities among people are the result of shared alleles, or whether instead they are the effect of shared environments. The concept that relates genetic variation and phenotypic variation is called \term{heritability}, which is the proportion of phenotypic variation that can be explained by genetic variation in the population (Falconer and Mackay 1996). Because it is a proportion, heritability varies between 0 and 1.

Phenotypic traits with heritabilities near zero are very weakly affected by genetic variation, while heritabilities near 1 indicate a very strong genetic influence. Very different traits can have similar heritabilities. For example, near-sighted, or myopic, parents are substantially more likely to have children with myopia. In Western populations, genetic variation accounts for more than 80 percent of the variation in juvenile-onset myopia (Gilmartin 2004). Likewise, variation in the many genes that influence stature accounts for approximately 80 percent of the phenotypic variation in stature, at least in Western societies with relatively stable childhood nutrition (Silventoinen 2003). Some human traits have heritabilities very near 1. One example is the mass of the brain, which has a heritability of approximately 0.94 (Bartley et al. 1997).