Long Coat and Smooth Coat Chihuahuas: The Genetics, and the Coat Myths

By Rev. T. Stringer, Lead Researcher, SIPGS

Chihuahuas come in two coats: smooth and long.

That is it. Two.

You will hear about double coats, triple coats, thick smooths, fluffy smooths, and half coats.

None of these is a separate coat type.

Here is what the genetics and the breed standard actually say.

One gene decides smooth or long

Coat length is controlled by the FGF5 gene, at the L locus.

FGF5 tells the hair when to stop growing.

When both copies are broken, the hair keeps growing longer. That is a long coat.

Genotype	Coat	What it means
Sh/Sh	Smooth	Cannot make long coat puppies with a Sh/Sh mate
Sh/Lh	Smooth	Carries long coat and can pass it on
Lh/Lh	Long	Passes long coat to every puppy

Smooth is dominant.

Long coat is recessive. A puppy needs a long coat copy from both parents.

Labs may label these differently, such as S/L or N/L. They mean the same thing.

Breeding odds

Parents	Long coat puppies
Sh/Sh × anything	0%
Sh/Lh × Sh/Lh	about 25%
Sh/Lh × Lh/Lh	about 50%
Lh/Lh × Lh/Lh	100%

Two smooth parents can have a long coat puppy, when both carry Lh.

Two long coat parents always have long coat puppies.

Most smooth Chihuahuas carry long coat

In SIPGS, 79 Chihuahuas have a coat length DNA result.

Of the smooth coats among them, most carry long coat:

Result	Dogs
Lh/Lh (long)	34
Sh/Lh (smooth, carries long)	32
Sh/Sh (smooth, no long)	13

So about:

71% of our tested smooth coats carry long coat

That is 32 of 45 smooth dogs.

That is why long coat puppies turn up from smooth to smooth breedings all the time.

Ch. A & A's The Dapper Don "Royce" - Long coat from two smooth-coated parents

Long coat puppies from smooth parents are not "lesser" long coats

A long coat puppy from two smooth parents is Lh/Lh.

A long coat puppy from two long coat parents is also Lh/Lh.

They are the same at the coat length gene.

The coat does not "remember" which parents it came from.

Smooth to long breedings do not ruin coats

A common worry is that breeding smooth to long gives poor coats in both.

The genetics do not support that.

• The long coat puppies are Lh/Lh, the same as any long coat.

• The smooth puppies are Sh/Lh, a normal smooth coat that carries long.

• No puppy gets a "half long" coat.

What makes one long coat fuller than another is not the smooth parent.

It is the many other genes that shape coat density and undercoat, plus age, season, hormones, and care.

Tom - Produced from smooth-coated and long-coated parents

An early SIPGS finding

We looked at the DNA right around FGF5.

We compared Chihuahua long coat copies with about 190 breeds.

The closest match was the Pekingese.

• Long coat copies matched the Pekingese about 8.6 times more than smooth copies did.

• About 1 in 4 long coat copies shared a long stretch (7 cM or more) with a Pekingese.

• We found the same match in a separate group of Chihuahuas from a 2015 study.

What it means:

Many breeds share old versions of the long coat gene.

One Chihuahua version matches one the Pekingese also carries.

What it does not mean:

It does not show a recent cross.

It does not prove where the long coat came from.

This is early work.

Only 13 Pekingese were in the comparison.

We need more dogs, and coat scores, to confirm it.

A 1920s experiment, explained by DNA

Ida H. Garrett was one of the early authorities on the breed.

She bred both long coats and smooths.

Kauffman's 1952 book quotes her:

"For years at various times I crossed the real Chihuahua with the Yorkshire, the Pomeranian and the Poodle and then crossed the offspring back again to the Chihuahua. In none of these crosses or interbreedings have I ever been able to reproduce the genuine longhaired Chihuahua."

Today we know why.

The Chihuahua long coat is FGF5 long coat (Lh/Lh) with no other coat genes added.

A Poodle cross brings curl (KRT71) and furnishings (RSPO2).

A Yorkie cross brings furnishings and a different texture.

Furnishings show with one copy.

So a cross could make a long coat.

It could not make a Chihuahua long coat.

Garrett used her result to show the long coat Chihuahua is purebred.

The DNA agrees.

Litter from smooth-to-long pairing showing both smooth and long-coated puppies

The myths: coat words that are not coat types

"Double coat"

A double coat just means the dog has an undercoat: short, soft hair under the longer outer hair.

It is not a third variety, and it is not a separate gene in Chihuahuas.

Both smooth and long coats can have an undercoat or very little of one.

The AKC standard already covers it:

• Smooth: "Heavier coats with undercoats permissible."

• Long: "undercoat preferred."

Researchers have linked one region of the genome to having an undercoat or not, in breeds like Poodles and Maltese.

It has not been studied in Chihuahuas.

Undercoat also changes with season, climate, age, and hormones. Many females drop their coat after a season or a litter.

"Triple coat"

There is no such thing in dogs.

A dog's hair follicles make outer hair and undercoat hair. There is no third layer.

"Triple coat" is a marketing word, not genetics.

"Thick smooth," "heavy smooth," "fluffy smooth," "plush smooth"

These are smooth coats with more undercoat.

Genetically they are still smooth: Sh/Sh or Sh/Lh.

The AKC standard allows heavier smooth coats, so they are correct smooth coats.

They are not a separate variety.

"Half coat" or "medium coat"

There is no medium coat gene in Chihuahuas.

A dog is either smooth (at least one Sh) or long (Lh/Lh).

A young long coat can look "medium" while the coat grows in.

A heavy smooth can look "medium" too.

Neither is a third type.

"Smooth carriers are fluffier"

We know of no published evidence that one Lh copy makes a smooth coat longer.

A smooth that looks fuller usually has more undercoat.

The only way to know if a smooth carries long is a DNA test.

"That long coat has a poor coat because it is young"

This one is often true.

Long coats take a long time to finish.

A long coat puppy at 8 to 12 weeks looks nothing like it will as an adult.

The full coat, with ear fringes, ruff, pants, and plume tail, can take two years or more.

What the standard actually asks for

Smooth Coat

• Soft texture, close and glossy.

• Heavier coats with undercoats permissible.

• Ruff on the neck preferred. Hair on the tail preferred furry.

Long Coat

• Soft texture, flat or slightly wavy.

• Undercoat preferred.

• Fringed ears, plume tail, feathering on feet and legs, pants, and a large ruff.

• Disqualification: a long coat so thin it "resembles bareness."

Both: groomed only to look neat.

Other coat genes on a DNA report

Embark and other tests may show more coat genes.

In Chihuahuas, most of them do not vary much.

• Curly coat (KRT71): rare in Chihuahuas. In SIPGS, 2 of 131 dogs carry it. One copy can make a long coat wavy, which the standard allows.

• Furnishings (RSPO2): the beard and eyebrows of wire breeds. Not part of the Chihuahua.

• Shedding (MC5R): a weak predictor. All Chihuahuas shed.

• Hairless (FOXI3): not found in Chihuahuas.

The gene that matters for coat type in Chihuahuas is FGF5.

Key takeaways

• Chihuahuas have two coats: smooth and long.

• FGF5 decides it. Long coat is recessive (Lh/Lh).

• About 71% of our tested smooth coats carry long coat.

• Long coats from smooth parents are the same as long coats from long parents.

• "Double coat" means undercoat. Both varieties can have one.

• "Triple coat," "half coat," and "fluffy smooth" are not real coat types.

• Long coats can take two years or more to finish.

• Test your smooth dogs if you want to know whether they carry long.

Sources

• AKC Official Standard of the Chihuahua (1990).

• Kauffman, R.E. (1952). The Chihuahua. Judy Publishing Company, Chicago.

• Housley DJE, Venta PJ. (2006). The long and the short of it: evidence that FGF5 is a major determinant of canine 'hair'-itability. Animal Genetics, 37(4):309-315.

• Cadieu E, et al. (2009). Coat variation in the domestic dog is governed by variants in three genes. Science, 326(5949):150-153.

• Dierks C, et al. (2013). Allelic heterogeneity of FGF5 mutations causes the long-hair phenotype in dogs. Animal Genetics, 44(4):425-431.

• Whitaker DT, Ostrander EA. (2019). Hair of the dog: identification of a cis-regulatory module predicted to influence canine coat composition. Genes, 10(5):323.

• Shannon LM, et al. (2015). Genetic structure in village dogs reveals a Central Asian domestication origin. PNAS, 112(44):13639-13644.

• UC Davis Veterinary Genetics Laboratory. Coat Length (FGF5) test information.

• Stringer, T.A. (ongoing). Stringer Integrated Pedigree-Genomic Study (SIPGS).

© 2026 A & A's Chihuahuas and Rev. T. Stringer. All rights reserved.

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