Chocolate Chihuahuas: How the Brown Gene Really Works

Nov 7, 2022

6 min read

Updated: 3 days ago

Chocolate and white long coat Chihuahua puppy with a brown nose

By Rev. T. Stringer, Lead Researcher, SIPGS

Chocolate is one of the richest looks in the breed.

Brown nose. Warm eyes. A coat that glows in the sun.

It is also one of the most misunderstood.

Here is how it works, gene by gene.

Chocolate is a pigment, not a coat color

This is the most important idea in this post.

A coat color is what you see on the whole dog: fawn, sable, black and tan, brindle, spotted on white.

Chocolate is not one of those.

Chocolate is a pigment type.

It decides what color the dark pigment will be, everywhere on the dog.

That is why a chocolate dog can be:

solid chocolate
chocolate and tan
chocolate sable
chocolate brindle
chocolate merle
chocolate spotted on white
even a fawn or cream with almost no brown hair at all

The pattern genes decide where the dark pigment goes.

The B locus decides whether that dark pigment is black or brown.

What makes a dog chocolate

Dogs make two kinds of pigment:

Eumelanin: the dark pigment, black or brown.
Phaeomelanin: the light pigment, from cream to deep fawn.

Chocolate only changes the dark pigment.

The gene is called TYRP1, at the B locus.

TYRP1 makes an enzyme that helps finish black pigment.

When both copies of the gene are broken, the enzyme does not work.

The dark pigment comes out brown instead of black.

Chocolate is not a dilute of black

Many people think chocolate is a faded or watered-down black.

It is not.

Dilute (the D locus) takes the pigment a dog already makes and spreads it thinner. Black becomes blue.
Chocolate (the B locus) changes the pigment itself. The dog makes brown instead of black.

Chocolate is a full-strength pigment.

A deep chocolate is as rich as a deep black. It is just a different pigment.

Dilute can then act on chocolate too. That is how you get lavender (see below).

Chocolate is recessive

A dog needs two broken copies to be chocolate.

Genotype	What you see
B/B	Black pigment. Cannot make chocolate puppies with a B/B mate.
B/b	Black pigment. Carries one chocolate copy and can pass it on.
b/b	Chocolate pigment.

Breeding odds

Parents	Chocolate puppies
B/B × anything	0%
B/b × B/b	about 25%
B/b × b/b	about 50%
b/b × b/b	100%

Two black-pigmented parents can have a chocolate puppy.

That happens when both carry one chocolate copy.

It did not "skip a generation." It was there all along, hidden.

What chocolate changes on the dog

When a dog is b/b:

Every black hair becomes brown. That includes saddles, sable tips, brindle stripes, merle patches, and tan-point markings.
Fawn and cream hair does not change.
The nose, lips, eye rims, and paw pads turn brown.
The eyes are lighter: amber, hazel, or light brown.

The hidden chocolate Chihuahua: fawn and cream dogs

Some Chihuahuas have no black or brown hair at all.

Clear fawns and creams can still be chocolate underneath.

Look at the nose, eye rims, and paw pads:

Black nose, rims, and pads: the dog makes black pigment.
Brown nose, rims, and pads: the dog is chocolate (b/b).

A cream dog with a brown nose is a chocolate dog.

It will pass chocolate on like any other b/b dog.

This is the clearest proof that chocolate is a pigment, not a coat color.

The coat is cream. The pigment is chocolate.

Snow nose is not chocolate

Some fawn and cream Chihuahuas have a black nose that fades to pinkish brown.

It often happens in winter or as the dog ages.

This is called snow nose or winter nose.

It is not chocolate.

How to tell the difference:

Snow nose: only the nose fades. Eye rims and paw pads stay black. The nose often darkens again in summer.
Chocolate: the nose, eye rims, and paw pads are all brown, from birth, all year.

Lavender: chocolate plus dilute

Lavender needs two separate genes:

b/b at the B locus makes the dark pigment brown.
d/d at the D locus thins it out.

Together they make a pale, silvery brown called lavender.

Genotype	Color
b/b with D/D or D/d	Chocolate
b/b with d/d	Lavender

Chocolate is old, not new

When chocolate was first mapped in 2002, researchers found 3 broken versions of TYRP1:

bs
bd
bc

Any two together make a chocolate dog.

All of them look the same.

These same 3 variants turn up in many breeds that are not related to each other.

A mutation shared by many unrelated breeds is most likely older than the breeds themselves.

So chocolate did not show up recently in Chihuahuas.

It came in with the dogs the breeds were built from.

Not every brown variant is on every test

Since 2002, researchers have found at least 3 more brown variants:

one in Australian Shepherds (2017)
one in Lancashire Heelers (2019)
one in Siberian Huskies, called bh (2021)

Many DNA panels still test only the first 3.

So a dog with a brown nose can test B/B.

That does not mean the test is broken.

It may mean the dog carries a variant the panel did not check.

Breeder tip: if the nose and the test disagree, trust the nose.

Then ask your lab which B locus variants it tests.

How to read a B locus test result

Most DNA reports list the B locus with two letters, one from each parent.

Result	What it means	Can pass chocolate?
B/B	Black pigment. No tested chocolate variants found.	No (for the variants tested)
B/b	Black pigment. Carries one chocolate variant.	Yes, to about half of puppies
b/b	Chocolate pigment.	Yes, to every puppy

Some reports also name the exact variant, like bs, bd, or bc.

A dog can have two different ones, like bs/bd.

It is still chocolate. The shade looks the same.

Three things to check on every report:

Does the result match the nose? If not, see the tip above.
Which variants were tested? Look for the list in the test details.
Check the D locus too. A b/b dog that is also d/d is lavender, not chocolate.

Same gene, different names

TYRP1 brown is the same gene in every breed.

Each breed just calls it something different:

Chocolate: Chihuahuas, Labrador Retrievers
Liver: Pointers, Dalmatians, Springer Spaniels
Red: Dobermans, and the "red" in Australian Shepherds. A red merle Aussie is a brown merle.
Isabella: lavender Dobermans (brown plus dilute)

So if you read about liver, red, or Isabella in another breed, the science is the same.

Look-alikes

Sun bronzing: a black long coat can turn rusty from sun.

Check the nose. A black nose means black pigment.

Dark fawn or sable: some deep fawns look brown in photos.

Again, the nose tells you.

Is chocolate healthy?

Yes, as far as we know.

No disease has been linked to the TYRP1 brown gene in dogs.

That is different from dilute (blue and lavender).

Dilute is linked to color dilution alopecia, a hair-loss condition, in some dogs.

A chocolate dog that is not dilute does not carry that risk from its color.

Key takeaways

Chocolate is a pigment, not a coat color.
It is not a dilute of black. It is a full-strength brown pigment.
It comes from the TYRP1 gene at the B locus, and it is recessive (b/b).
Brown nose, rims, and pads = chocolate, even on a cream or fawn dog.
Snow nose fades only the nose. Chocolate is brown everywhere, all year.
Lavender = chocolate (b/b) + dilute (d/d).
The main chocolate variants are shared across many breeds, so chocolate is old.
At least 6 brown variants are known, and many tests check only 3.
Liver, red, and Isabella in other breeds are the same gene.

Sources

Schmutz SM, Berryere TG, Goldfinch AD. (2002). TYRP1 and MC1R genotypes and their effects on coat color in dogs. Mammalian Genome, 13(7):380-387.
OMIA 001249-9615: Coat colour, brown, TYRP1-related in dogs. Online Mendelian Inheritance in Animals, University of Sydney. Lists all known brown variants, including the Australian Shepherd variant (Hrckova Turnova et al., 2017), the Lancashire Heeler variant (Wright et al., 2019), and the Siberian Husky bh variant (Van Buren et al., 2021).