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Hips and Dysplasia and Heritability, Oh My!

Nothing Has Spawned as Much Confusion and Misinformation Surrounding Hip Dysplasia as the Misinterpretation of Heritability.

‘Heritability’ is a statistical concept used in quantitative genetics and selective breeding. Used properly it tells a breeder how to choose breeding animals that are likely to produce improvements in important traits. On the other hand, misinterpreting heritability leads to faulty conclusions about inherited traits and poor breeding decisions. Ergo the pervasive myth that environment can create Hip Dysplasia all by itself, without a genetic predisposition.

The primary problem is that heritability is perceived to define environment as a “cause” of Hip Dysplasia. It doesn’t.

Why Do We Need to Talk About Heritability?

Confusion Over Hip Dysplasia and HeritabilityHeritability may sound like an obscure subject, but it’s important to discuss it in our Hip Dysplasia series because the failure to understand its purpose is currently a major barrier to progress.

Heritability is frequently, and incorrectly, presented in a way that makes it seem like environment is a significant cause of dysplasia, and as a supposed explanation for why breeders haven’t made more progress. This misinterpretation leads people down the rabbit hole of worrying about environment, when the only way to reduce dysplasia is to weed out the genes.

This article will, as briefly as possible, explain why heritability doesn’t support the myth of “environmental” Hip Dysplasia, along with the basics of using it properly in a breeding program. We also provide links to sources of more in-depth information that, in addition to what we present here, could serve as a reliable resource for anyone who is confronted with misinformation on heritability.

For Puppy Buyers:
Dogs don’t develop dysplasia unless they inherit a genetic blueprint for it, and the myth that they do has roots in the misinterpretation of heritability. You don’t really need to understand heritability beyond knowing that anyone who says there are environmental cases of Hip Dysplasia is wrong. However, if you are interested in learning more, and don’t mind a little technical explanation, read on.

For Breeders:
Understanding heritability is an invaluable asset for a breeding program. Applying that knowledge to the selection of breeding dogs is the only reliably effective way to reduce the number of dysplasia genes our dogs carry, and in the fewest possible generations. Plus, understanding the genetic principles behind heritability helps us breed better dogs overall.

What Exactly Is Heritability?

Heritability is used to predict how readily a trait that is controlled by multiple genes (polygenic) will respond to selection pressure. It does this by estimating the amount of observable (phenotypic) variation there is in a population that can be attributed to differences in additive genes between individuals (more on this mouthful in a bit).

Most Traits Are Polygenic

Canine Hip Dysplasia is a complex, polygenically inherited disease. In polygenic inheritance the combination of many genes from each parent produces a continuous range of outcomes in the offspring. For instance, puppies in a litter will have a range of heights, tail lengths, head shapes, etc. In addition, the expression of polygenic traits, the way they actually turn out (their phenotype), is almost always influenced by the environment. All of this makes it difficult for a breeder to predict the outcome of a mating.

This is where heritability comes in. Effective breeding strategies for polygenic traits are based on heritability.

Heritability helps a breeder avoid situations like this one:

A dog whose hips were certified normal by the Orthopedic Foundation for Animals produces dysplasia in several of its offspring. As far as the owner was aware, this is the first hint that this dog inherited dysplasia genes.

A breeder can’t tell by looking at this dog, or at its OFA certificate, that it carries dysplasia genes. As frustrating as this is, it’s actually totally normal for polygenic traits. Offspring don’t end up with exactly the same set of genes as their parents, and the environment can effect the expression of some of the genes they do carry.

However, a breeder who understands heritability would not have been surprised by the dysplastic offspring. If a proper selection strategy had been implemented, this dog’s genetic potential to throw dysplasia would be known because there would be dysplasia in its relatives. Maybe it has a couple of dysplastic siblings or a dysplastic parent, a grandparent, or maybe it’s in the uncles and aunts. That knowledge would tell the breeder to expect this dog to carry dysplasia genes.

Heritability Is Hard to Understand, and Even Harder To Explain

For starters, ‘heritability’ is a terribly confusing term. It sounds like it’s asking whether a trait is ‘inherited’ but, by definition, if a trait has a heritability estimate it is known to be inherited.

A good approach is to put ‘inherited’ out of your mind when considering heritability and substitute: The ‘predictability’ that animals will pass their own phenotype to their offspring.

Most explanations of heritability that will pop up on an Internet search are overly simplified, and they are often contradictory. Many refer to how it is used in human studies, which can be different than the way it is used in animals. You have to dig into selective animal breeding to understand how to interpret heritability estimates regarding Canine Hip Dysplasia.

We can’t cover heritability beyond the very basics in one post, so here are a couple of recommended primers that don’t leave out too many details. Be forewarned though. Heritability is counter-intuitive and even though these are basic primers it may take a number of read-throughs to grasp some of the concepts if you are not already familiar with them.

A little more in-depth primer to follow up with:

A Key to Understanding Heritability: It’s the Variation!

The heritability of a trait is estimated through statistical analysis of phenotypic variation, which refers to the observable differences between the animals in the group that’s studied.

The total phenotypic variation has multiple components:

  • Additive genes– the genes that respond to selective pressure so therefore determine an animal’s breeding value
  • Non-additive genetic factors called Dominance and Epistasis
  • Environment
  • Environment-gene interactions

These components are used in two different ways to estimate heritability. In some human and plant studies the genetic components are all lumped together, called ‘broad-sense’ heritability.

Selective breeding however, uses ‘narrow-sense’ heritability, which refers specifically to the additive genes component. Non-additive genes are unpredictable statistically and don’t respond easily to selective pressure, so they are lumped with the non-genetic portions. This is an important distinction. Unless noted otherwise, all heritability estimates for animals refer to differences in additive genes only. They do not refer to all of the genetic differences in the population.

The Key Concept:
Estimating the portion of phenotypic variation that’s from additive genes predicts, on average, the likelihood that animals will pass their phenotypes to their offspring, which in turn gives guidance on the best selection strategy to affect positive change in the population.

Heritability Estimates

Heritability is estimated as a number between 0 and 1, often described as a percentage. ‘0’ would mean none of the variation in the trait can be attributed to differences in additive genes, ‘1’ means additive genes account for all of the variation.

As an example, a heritability estimate of .35 for OFA hip scores would mean that additive genes account for (in a statistical sense) 35% of the variation in hip phenotypes within the population studied. The other 65% would be accounted for by differences in non-additive genes and the environmental components in unknown proportions.

This estimate does not translate to: Dysplasia is 35% due to genes. First off, we don’t know what portion of the phenotypic variation is from non-additive genetic differences. More importantly though, heritability IS NOT an estimate of the AMOUNT of genetic or environmental influence on a trait. It is an estimate of the relative proportions of variation, which is independent of, and says nothing about, the genetic nature or the “cause” of a trait.

  • Heritability does not measure the actual influence of genes on the development of a trait, or the degree to which a trait is inherited.
  • Heritability does not determine how often a trait is due to genes or to the environment.
  • A heritability estimate does apply to one particular group/population, and only at the time of the study. The estimates are not static- they will change depending on when they are calculated, even within the same population.

The classic example of human hair color will help to illustrate this:

Human hair color can have a wide range of heritability depending on the population studied. For instance, black color has one estimate of .96 in the Netherlands, a population that has diversity in hair color. However, in an ethnic population where everyone’s hair is black there would be few genetic differences, so heritability would be near zero.

Low heritability in the ethnic population does not imply less genetic influence on hair color. Black hair is still certainly determined by genes. It doesn’t imply a large amount of influence from the environment either. What it says is that additive genes account for a tiny portion of the total phenotypic variation present in that particular population. In other words, there is very little additive genetic difference in this population so it would be difficult to change hair color through selective breeding.

What does our .35 estimate tell us then? That there is variation from additive genes in the population of dogs studied, and plenty enough of it to successfully exert selective pressure to improve the hip scores. That’s it.

The Value of Heritability

Heritability has one extremely valuable application in selective breeding:
It tells a breeder the most effective strategy to use when selecting breeding animals.

Although any estimate is a snapshot specific to a particular group of animals, there still tend to be general trends in the estimates for important traits, and recommendations for effective selection methods are based on those trends.

Breeding Strategies

In a nutshell, a dog breeder needs to know the following:

High Heritability:
If heritability estimates for a trait are generally high, over .40, progress can usually be made by selecting individual animals that are superior for that trait. The individual animal’s phenotype is a good reflection of its genetic potential.

Moderate Heritability:
If heritability is in the .15 to .40 range, the individual animal’s genetic potential is less clearly reflected in its phenotype. Progress can still be made just by picking superior individuals, but it will be slower and there is more risk of accidentally going backwards. The genetic potential of the individual can be clarified by considering the phenotypes of its relatives (offspring, siblings, parents, aunts and uncles, grandparents, etc.), which will make more rapid and steadily forward progress possible.

Hip scores have typically been found to have moderate heritability, although it can be high or low depending on the group of dogs studied. The OFA’s recommendations for breeders are based on moderate heritability:
The Use of Health Databases and Selective Breeding pg 5-6

Low Heritability:
If heritability is very low, .15 or lower, progress from selection pressure will be slow (or in some cases not possible). Traits that have generally low heritability estimates are typically related to things like fertility and fitness, which are mainly controlled by non-additive genes. These types of traits usually respond better to out-crossing.

Heritability will be low for any trait whenever there is little variation from additive genes between the animals in a population. As an example of how this happens, when a breeder makes overall improvements in hip quality among his dogs, the heritability of hip scores in that kennel will conversely go down. Low heritability is a natural consequence of selective breeding- selecting for more uniform phenotypes reduces differences in additive genes and therefore lowers heritability.

Heritability can be increased by bringing in new genes through out-crossing to increase variation from additive genes. This might be the only course of action for a kennel that has rampant Hip Dysplasia and there is little genetic difference between the dogs. On the other hand, when heritability has become low due to selectively eliminating dysplasia from a breeding program, new genes may bring with them the risk of increasing its incidence. Increasing heritability isn’t automatically a good thing.

Breeding for Success

The point of all of this is that our focus needs to be on heritability. What it tells us and what it doesn’t. It tells us nothing about environment. The history of animal breeding illustrates that focusing on heritability is the only way to effectively select for a polygenic trait.

Breeders who don’t understand heritability, or who operate under the illusion that some cases of Hip Dysplasia are not genetic, cannot make well-founded breeding choices. They are unlikely to make significant progress and may end up just perpetuating more of the disease.

However, breeders who choose to employ effective selection methods, informed by heritability, have had spectacular success reducing dysplasia. By following well-known breeding strategies the incidence can be reduced to the 2% range, or even lower. One of the best examples is The Seeing Eye, Inc breeding program, which we will discuss in a later post.

Recommended Reading

Coming Up

Our next article in this series will discuss environment in more depth, using the Purina Life Span Study of 48 Labrador Retrievers as an example. The Purina study is the most famous experiment to demonstrate that if a dog has inherited a genetic blueprint to develop Hip Dysplasia, environmental influences can modify the phenotypic expression of those genes. We will also discuss how the findings of studies like this relate to the heritability of hip phenotypes.

Founders series on Canine Hip Dysplasia. Our comprehensive guide to HD and the effective steps breeders and buyers can take to control it.

Upcoming Series on Hip Dysplasia
Old Age Arthritis is Not Normal, It Comes From Hip Dysplasia
Chronic Pain From Canine Hip Dysplasia: Can You Miss It?
There Is No Such Thing As “Environmental” Hip Dysplasia
Hips and Dysplasia and Heritability, Oh My!

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