Let us start by defining what heritability is. A heritability is defined for each trait in a given population. In general, heritability indicates the proportion of the phenotypic variation that is explained by the genes that an animal has, as denoted by h_B^2=(genetic variation)/(phenotypic variation). It can be interpreted as the relative contribution of breeding values (unobserved) to the phenotypic values (observed) for a trait in a population. This definition is known as heritability in the broad sense since it reflects all the genetic effects. The heritability is always positive, ranging from zero to one, where zero means the genetic influence is null and 1 indicate that all variation is due to the genetic effects. As a rule, traits with a h2 ≤ 0.2 are considered lowly heritable, traits with a h2 between 0.2 and 0.4 are considered moderately heritable, and traits with a h2 > 0.4 are considered highly heritable. Unfortunately, the heritability of all reproductive and survival characteristics is generally less than 0.2, so their genetic improvement is slow and difficult. Traits in pigs with such heritabilities are for instance the number of total born, born alive, mummies or stillborn piglets. That is why Choice is committed to continually improve these characteristics and thus deliver to our customers the best maternal animals on the market.
However, in animal genetics, the classically used definition for heritability is in the narrow sense. It indicates what proportion of the phenotypic variation is explained by the additive effects of the genes, is denote by h_^2=(additive genetic variation)/(phenotypic variation). The heritability is still always positive, ranging from zero to one, where zero means there is no additive genetic effect and 1 indicate that all variation is due to the additive gene effects. The same order of values as for heritability in the broad sense remain to classify lowly, moderately and highly heritable traits.
At this point, the reader may wonder why animal breeders usually use the definition of heritability in the narrow sense. When an egg or sperm cell is produced, it carries only one copy of each gene of the animal’s genome. Genes are not inherited in pairs from each parent, so the effects like heterosis or hybrid vigor that we see on crossbred animals are not transmitted from parent to offspring.
Unless otherwise indicated, in this article we will always be talking about heritability in the narrow sense, and just as reminder, heritability is a measure of how close the phenotypes and the breeding values are in a particular population and trait.
Why is it important to know the heritability?
The importance of knowing the value of h2 lies in the fact that it is a guide to know how performance visible at phenotypic level should be included in a breeding program. For example, if a trait has h2 = 0, then we know that the variation observed in that particular population is not due to genetic factors but to non-genetic factors, which may include nutrition, animal management, health, , facilities, sex, age, and many others. It is also an indication (and very important) of the possible response expected in a selection program. The higher the h2, the greater the response to the selection program. More details will be given when we talk about the key equation of genetic improvement in another Choice Weezyou article.
Some misconceptions about heritability.
There are some misconceptions around h2. Many people think that the higher the h2 of a trait, the higher the estimated breeding values will be. Let us remember that the breeding values depend on the unit of measurement and not on the h2. For example, if we measure offtest weight (at the end of the fattening period) in kilograms, the values will be much lower than if we measure it in grams though in both cases, they will have exactly the same value of h2.
A heritability is only applicable to the trait within the population in which it was calculated. This implies two very important things:
- h2 estimated in different populations may differ and so are not comparable across lines
- and for this reason, breeding values from two different populations should not be compared either.
For example, it is wrong to apply the h2 values of a Landrace population to a Large White population even if it is located in the same farm, as well as incorrect to compare their estimated breeding values.
Heritability is not a fixed value for each trait; it varies according to the population where it is estimated, the population being defined by the time, space and breed. For example, if the h2 of litter size at birth (total born) is 0.15, this does not imply that it will be this value forever in the line. We should remember that h2 is a relationship between additive genetic variation and phenotypic variation and both change with time and the amount of information that is available to estimate h2. For that reason, the values of h2 must be re-estimated frequently to account for the changes in both the environment and the animals that were born in a particular period, as well as the new phenotypic information that is collected on the farms. Choice has a team of geneticists dedicated to this complex task.
The value of the estimate of h2 does not indicate anything with respect to which specific genes are contributing to the phenotypic variation of the productive characteristic. In the genetics world, this is referred to as the “black box”. Despite this, major genetic progress has been achieved by using quantitative genetic principles.
Finally, as an example, suppose that a trait has an h2 of 0.2. What does this value tell us? On average about 20% of the variation that can be observed between individuals in a herd, for example the number of piglets born alive, can be attributed to individual additive effects of genes. It does not mean that 20% of the differences among sows in the number of piglets born alive is due to genetic effects and 80% due to the environment where she has to perform. We must remember that most of economically important traits have a h2 less than 0.5. Traits related to reproduction or survivability have a h2 of 0.2 or less, and this means that all non-genetics effects have a huge impact on the phenotypes of these traits and on the genetic program used to improve them. Therefore, the success of any genetic program as well as the production system the animals are used in starts with providing a good environment (nutrition, health, facilities, good management, etc.) for the animals so they can express their genetic potential.