Narrow-Sense vs Broad-Sense Heritability in Dogs

Heritability is one of the most widely quoted and most widely misunderstood ideas in dog breeding. People hear a number like 0.3 or 0.4 and immediately translate it into "this dog is 40 percent genetic." That is not what heritability means. Heritability is a population statistic about variance, not a personal destiny score for one dog. Documented

What It Means

Heritability asks how much of the observed variation in a trait, inside a defined population under defined environmental conditions, is attributable to genetic differences.

That definition has three important parts that often get lost:

• it is about variation in a population
• it depends on the environment in that population
• it says nothing simple about one individual's composition

The broad idea is straightforward. If dogs in a population differ in hip scores, fearfulness, or body size, some of that variation comes from genes and some from environmental factors. Heritability estimates how much of the between-dog variation is genetic.

But there are two major versions of the concept.

Narrow-sense heritability, usually written h2, includes only additive genetic variance. Additive effects are the ones that stack in a way selection can reliably act on from one generation to the next. This is the version breeders care about most because it helps predict response to selection.

Broad-sense heritability, usually written H2, includes all genetic variance:

• additive effects
• dominance effects
• epistatic interactions between loci

That makes broad-sense heritability a larger umbrella. It captures more of the total genetic contribution, but not all of those components are equally useful for selection decisions. A dominance effect or epistatic interaction can matter biologically without translating cleanly into predictable generational progress in a breeding program.

That is why narrow-sense heritability is the more operational number for breeders. If a breeder wants to shift a population mean over time, additive variance is the engine that matters most.

This is also why two heritability numbers for "the same trait" may not actually contradict each other. One study may be reporting h2. Another may be reporting H2. One may be using pedigree data. Another may use genomic data. One may study a tightly managed working-dog population. Another may rely on owner questionnaires in pet homes. Those are not interchangeable contexts.

The classic interpretive error is to turn heritability into an individual pie chart. A heritability of 0.4 does not mean 40 percent of one dog's temperament is genetic and 60 percent is environmental. It means that, in the population studied, 40 percent of the observed variation between dogs was attributable to additive genetic differences.

Change the population and the number can change.

Change the environment and the number can change.

Change the measurement tool and the number can change.

That flexibility is not a weakness of the concept. It is the point. Heritability is a description of a trait in a particular population at a particular time, not a universal constant.

Dog-breeding examples make this easier to see. Hip dysplasia heritability estimates often land in a low-to-moderate range, commonly around 0.2 to 0.4 depending on breed, scoring system, and dataset. That means there is meaningful additive genetic variance available for selection, but also substantial environmental and non-additive variance in the picture.

Temperament traits usually show the same pattern. Fearfulness, boldness, sociability, and trainability often come back as low-to-moderate heritability traits, but the exact estimates vary sharply with breed, instrument, age, and environment. In Golden Retrievers, some measured behavioral dimensions show clear non-zero heritability, while still leaving large room for developmental effects, maternal effects, and later environment.

This is where breeders sometimes misread the literature in opposite directions.

One mistake is genetic fatalism: "Temperament is heritable, so the puppy is basically born fixed." Wrong.

The opposite mistake is environmental romanticism: "Because raising matters, genetics do not matter much." Also wrong.

Heritability sits between those extremes. It says additive genetic differences are real and selectable, but they operate inside an environment that can amplify, suppress, or reshape expression.

What This Cannot Predict

Heritability cannot tell you how much of one dog's behavior, hips, or health is "caused by genes."

It cannot tell you whether one specific puppy will develop a problem.

It cannot be compared across studies as if every 0.25 means the same thing.

And it cannot rescue vague breeder claims. If someone quotes a heritability number without naming the trait definition, the population, or the environment, the number is being stripped of the very context that gives it meaning.

This matters especially in temperament writing. A breeder may cite a heritability estimate for boldness, fear, or trainability and then speak as if the litter has therefore been mapped. It has not. The estimate describes the population studied, not the destiny of every puppy in the whelping box.

Why It Matters for Your Dog

Families do not need to become quantitative geneticists, but they do need the basic filter this page provides.

When a breeder says a trait is heritable, the right takeaway is:

• selection can matter
• progress is usually gradual
• environment still matters

That is especially relevant for temperament. JB's breeding decisions care deeply about social stability, composure, and the kind of dog that can live naturally inside the Five Pillars. But those are not single-switch traits. A breeding program can bias the odds in the right direction over time. It cannot produce a litter whose outcomes are mechanically guaranteed.

The same reasoning helps families read orthopedic claims more soberly. If hip scores have meaningful heritability, then careful breeder selection matters. But nutrition, growth rate, body condition, and injury history still shape the final expression. Good genetics reduce risk. They do not erase management.

That is the right middle ground: selection matters, raising matters, and heritability is the population-level bridge between those truths.

The Evidence

Sources

• Falconer, D. S., & Mackay, T. F. C. Introduction to Quantitative Genetics.
• Source_JB--Temperament_Heritability_and_Behavioral_Genetics.md.
• Source_JB--Canine_Genetic_Diversity_and_Population_Health.md.
• Canine hip and behavior heritability studies summarized in the JB source layer.