What is heritability? How and why should the heritability of a human trait be assessed before trying to find the genetic loci involved? (975 words)
Jennifer Toes, Catie Daubner, Priyanka Bulsara & Shoshi Weisman
Heritability can be described as the proportion of phenotypic variance in an individual that is attributable to variance in their genotype. Phenotypic traits can be influenced both genetically and environmentally, so variance in traits expressed by an individual can be termed ‘phenotypic variance’. Quantitative traits
(such as height and weight) can be used as a measure of heritability, as they are more easily and accurately measured. If a phenotypic trait is very similar between a parent and their child, then it is said to have high heritability and most likely genetic in origin, whereas a low similarity in traits has low heritability and may have been environmentally influenced (Wray & Visscher 2008).
The heritability for a trait in a population, or h 2 , can be expressed as a numerical value ranging from 0.0 to 1.0 and uses the following equation: h 2 =V
G
/V
P
, where V
G
is the variation caused genetically, and V
P
is the is the total variation in the trait, including environmentally influenced variance. The V
P can be affected by environmental factors, which can be further influenced through evolutionary forces such as genetic drift.
Furthermore, another equation can be utilised for studying the relationship between heritability and selection: R=h 2 S, where R is the response to selection (the variance between a trait in the parent and their offspring), S is the selection coefficient, (the variance between the trait at the population level, and the selected population). Obtaining this value demonstrates how selection for advantageous traits (such as overcoming deleterious mutations) may affect the overall mean trait value of the whole population
(Pritchard 2001).
If heritability is calculated to be 0, then genetic factors can be assumed to not make any contribution to the phenotypic variance, and instead environmental factors are responsible. Conversely, if heritability is calculated to be 1, it means that the phenotypic variance is indeed genetic in origin, and environmental factors can be assumed to have had no influence. As a relative measure, heritability can often be over- or underestimated which is an important factor to take into account when making assumptions about the genetic and environmental contributions to phenotypic variance. The effect(s) of the environment on an individual’s phenotype can be termed ‘environmentability’? It must be emphasised that this and heritability are both population concepts and should not applied when studying individuals alone.
Studying heritability in humans is made more accessible through the comparison of concordance rates of phenotypic traits in both monozygotic (MZ) and dizygotic (DZ) twins, as this gives some indication of the relative contributions of genes and the environment. Monozygotic twins are genetically identical, whereas dizygotic will only be 50% identical. Variance in monozygotic twins can occur via genetic mutation or in some cases through environmental factors, which are most prominent when raised in separate homes.
Classical twin studies provide an opportunity to conduct experimental studies of heritability in a naturally occurring way, and allow us to differentiate the importance and relative influence of genetic and environmental factors on phenotypic traits (Boomsa 2002). It is thought that if two or more individuals are raised in the same home, with the exact same environmental factors, that any differences found in phenotype will be due to genotypic variance between the loci, and heritability will be high.
The ACE model can be used to analyse the relative amount that additive genetic (A), common environmental (C), and unique environmental (E) factors influence the phenotypes expressed in sets of twins. MZ twins are from the same fertilised egg and hence share their genes (A) and a common environment (C). DZ twins are non-identical and therefore share half their genetic material (1/2A) and a common environment (C).

One such example studies the correlative values for the number of moles and smoking in mono-and dizygotic twins. Correlative values between traits can demonstrate whether they are purely genetic in origin, or are purely environmental, or whether it is some combination of the two. This is a particularly good example as the number of moles that one twin has cannot be genetically influenced by the number of moles of other twin, but can be influenced behaviourally by smoking (Lyons 2008).
The proportion of variance affected by genetic factors can then be calculated, showing that heritability of smoking in monozygotic twins is 40%. The remaining variance must therefore be due to shared environment. Once the amount of genetic influence on a trait has been ascertained, it is then possible to look for the genetic loci contributing to the observed phenotype.
It is important that the heritability of a particular trait be estimated before attempting to search for specific genetic loci that control or influence the trait. If a trait has extremely low heritability, then it is most likely caused or heavily influenced by external environmental factors, rather than the genotype of the individual. Therefore, it would be illogical to search for specific genetic loci in the genotype of an individual that controls or influences a specific phenotype if that phenotype is almost entirely caused by environmental factors.
However, if a trait is found to have high heritability, searching for a genetic origin becomes much easier, as it is assumed that the trait is almost entirely caused by the genotype of the individual. This means that the expressed phenotype has not been changed or massively altered through environmental factors.
Whilst the heritability of certain phenotypic traits can change through life stages, it is important that it be assessed to establish a basis for searching for genetic loci. If a trait has very low heritability and is caused by environmental factors, trying to find a genetic cause would be irrelevant. This provides further complications for complex traits that are produced through the interaction of both genetics and environment, yet the heritability of a trait is still vitally important for discovering further information about the interplay of genetic loci and external factors of phenotypic traits.
References:
 Boomsa. D., Busjahn. A., Peltonen. L. Classical twin studies and beyond,
,
3: 872-882 (2002)
 Lyons, M., Hitsman, B., Xian, H., Panizzon, M.S., Jerksey, B.A., Santangelo, S., Grant, M.D.,
Rende, R., Eisen, S., Eaves, L., & Tsuang, M.T. A twin study of smoking, nicotine dependence, and major depression in men.
10(1): 97-108 (2008)
 Pritchard. J. Are Rare Variants Responsible for susceptibility to Complex Diseases?,
,69(1): 124-137 (2001)
 Wray, N. & Visscher, P. Estimating trait heritability.
1(1) (2008)