Heterozygous vs Homozygous, Explained With Dogs
Homozygous means two copies of the same gene version; heterozygous means two different ones. Here's what that looks like in dog coats, what it means for puppies, and how it connects to inbreeding.
By PuppyGenes · How we research and write our guides
A dog is homozygous for a gene when it carries two copies of the same version, one from each parent, and heterozygous when it carries two different versions. A doodle with two copies of the furnishings variant is homozygous for it, and a doodle with one copy is heterozygous. Both have beards and eyebrows, but they pass the gene on differently.
The short version
- Homozygous: two copies of the same version of a gene.
- Heterozygous: two different versions, one from each parent.
- For puppies: a homozygous dog always passes on the same version; a heterozygous dog can pass on either one.
- Inbreeding raises homozygosity: related parents are more likely to pass on matching copies from a shared ancestor.
Examples from dog coats
| Genotype | Furnishings (RSPO2) | Curl (KRT71) |
|---|---|---|
| Homozygous for the variant | Furnished | Usually curly |
| Heterozygous | Furnished | Usually wavy |
| Homozygous for the usual version | No furnishings | Straight |
The table shows why the terms matter. For furnishings, a heterozygous dog looks the same as a homozygous one. For curl, it looks different: wavy rather than curly. Our guides to furnishings and curly coats explain the research behind both genes.
What it means for puppies
A homozygous dog can only pass on the version it has two copies of, so a dog with two furnishings copies has only furnished puppies, whatever its partner carries. A heterozygous dog passes each of its versions to about half its puppies, on average.
That's why doodle generations differ. A first-generation doodle with a Poodle parent and a Golden Retriever parent is usually heterozygous for both genes: it gets one curl copy and one furnishings copy from the Poodle and none from the retriever. Our guide to doodle generations follows those copies into the next generation.
Homozygosity and inbreeding
When a dog's parents are related, it's more likely to inherit two identical copies of a stretch of DNA that both trace back to the same ancestor. Measures of inbreeding, such as the coefficient of inbreeding, estimate how much of a dog's genome is homozygous for that reason. Our guide to COI in dogs explains the idea, and our guide to the genetic diversity percentile explains how a DNA report puts it in context.
Frequently asked questions
- What does heterozygous mean?
- Heterozygous means a dog carries two different versions of a gene, one from each parent. A dog with one furnishings copy and one non-furnished copy is heterozygous for furnishings.
- What does homozygous mean?
- Homozygous means a dog carries two copies of the same version of a gene. A Poodle with two furnishings copies is homozygous for furnishings.
- Can a heterozygous dog show a recessive trait?
- Usually not. A dog with one long-hair FGF5 copy and one short-hair copy usually has a short coat but can pass the long-hair copy on. The exception is a dog with two different long-hair variants: researchers call it compound heterozygous, and it still grows a long coat.
- Is it better for a dog to be heterozygous?
- For a single gene, neither is better; the terms only describe which versions a dog carries. Across the whole genome, long stretches of matching copies usually point to more inbreeding, which is why diversity results compare a dog with other dogs.
From general to personal
See how this applies to your dog specifically.
Our at-home Complete Dog DNA Test adds your dog's own results to these tips across six report sections: breed ancestry, coat colour, physical appearance, temperament and size predictions, genetic diversity and oral microbiome. The report does not include genetic disease or health screening.