There are a number of misconceptions about the heritability. We will discuss a few below.
Misconception 1.” A heritability of 0.40 indicates that 40% of the trait is determined by genetics”
This is a very common misconception and especially originates from a misunderstanding of the definition. A heritability of 0.40 indicates that 40% of all the phenotypic variation for that trait is due to variation in genotypes for that trait. This has a very different meaning from the definition that in each animal 40% of the expression of the trait is due to genes and the rest due to other influences.
Misconception 2. “A low heritability means that traits are not determined by genes”
A heritability that is larger than 0 always indicates that genes have an effect on the expression of the phenotype. The heritability is determined by the proportion of genetic variance relative to the phenotypic variance. A low heritability thus can indicate that the genetic variance is low. For example, number of fingers on a hand is very much genetically determined, but because by far most people have five fingers on each hand, the genetic variance is very low.
Misconception 3. “ A low heritability means that genetic differences are small”
A low heritability not automatically indicates that the genetic variance is small. It can also mean that the error variance is large. And this can be caused by large influence of the environment, but also by inaccurate phenotype recording. For example: resistance to a certain infection will depend on the genetic potential to withstand that infection. The problem is how to measure that potential. If you go into the field and measure once whether a sheep is infected with nematodes, for example, then you will find the ones that are infected at that time. But in the other sheep you cannot distinguish between the ones that either have not been infected yet, already recovered, or are resistant to nematode infections. In other words: there is a lot of inaccuracy in your observations. Because you cannot assign the correct phenotype to each animal, this will result in a relatively large error variance, and thus in a low heritability. If you would improve the recording of nematode infection, for example by going into the field more often and/or improve the measurement methodology, then you obtain more accurate recording of the potential of the sheep to withstand nematode infection, and thus a more accurate estimate of genetic and environmental variance for this trait. The heritability can still remain low if there is not much genetic variation present, but at least it no longer is due to inaccurate phenotypes.
Misconception 4. “ A heritability is a fixed value”
The heritability reflects the relative weight of the genetic variance component in the phenotypic variance in a specific population and based on observations that were taken on a specific moment in time. The size of the heritability depends on genetic variance in a population, but also on the influence of the environment and on the accuracy of the observations (see misconception 3). The genetic variance in one population may be (somewhat) different from that in another population. Especially if that other population is of a different breed. But also within a population the heritability can change in time. For example if the new set of phenotypic observations were collected using a more accurate recording method. Or if the housing system has changed since the last recording, so that the influence of the environment has changed. It, therefore, is wise to re-estimate the heritability at regular basis.
To summarise: The heritability indicates what proportion of the phenotypic variance is determined by the additive genetic variance, for a specific population in a specific environment. The specific population dictates the additive genetic variance, the specific environment influences the size of the environmental variance, as is the accuracy of recording of the phenotype so that differences between animals are revealed.