Genetics – Rossi River Miniature Bull Terriers

General Rules of Color Inheritance

Color Inheritance

General rules of color inheritance in bull terriers

While there are always exceptions, the general rules of color inheritance in miniature bull terriers are as follows:

Two white parents will produce an all white litter. Puppies may display small colored patches on the head or ears, but less than 50% of the body will be colored.
Two solid parents will produce all solid puppies. The term "solid" refers to a colored coat with minimal white markings. All puppies will be colored on approximately 75% (or more) of the body.
Two colored & white parents will produce 1 white, 1 solid color, and 2 colored & white puppies, in a litter of 4. This is the typical 1:2:1 ratio of Mendelian inheritance.
One white parent and one solid parent will result in an all colored & white litter. On average, puppies will possess a 50/50 colored and white pattern.
One solid color parent and one colored & white parent will produce 2 colored & white pups and 2 solid colored pups, in a litter of 4.

*Note that these are rules of thumb, and color ratios may vary. Larger litters tend to adhere to these guidelines more closely.

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Coat Color Overview

Color Color Overview

Coat colors in bull terriers

When you consider the various allelic combinations, there are 13 possible phenotypes (observable coat colors). (1) white with red spot, (2) white with black/tan spot, (3) white with brown brindle spot, (4) white with black brindle spot, (5) solid black/tan, (6) solid red, (7) solid brown brindle, (8) solid black brindle, (9) solid white, (10) red and white, (11) black brindle and white, (12) brown brindle and white, and (13) black/tan and white.

Based on simple Mendelian genetics (dominant vs. recessive genes) it's possible to predict offspring coloration if the parents' genotypes are known. Genotype is just a fancy word for what genes a dog carries on its chromosome. This can be determined with a little knowledge of how genetic inheritance works but usually requires an advanced scientific background. In general, the genotype of a dog can be worked out if the colors of the siblings, parents, and grandparents are known.

There are two main types of miniature bull terriers: white and colored. Dogs, in general, have around 14 different genes at various loci (locations on the chromosome) that code for coat color. Only 5 of these genes are believed to be involved in bull terrier coloration. These genes are found at the A-locus, K-locus, E-locus, S-locus, and B-locus. Multiple alleles (different versions of a gene) are possible at each location, hence the range of colors exhibited by the miniature bull terrier.

*White bull terriers may or may not express color (as eye or body patches) because the white gene is epistatic and ultimately masks pigment.

A-series locus

Determines the base coat. Depending on the combination of alleles present, the dog will exhibit a fawn coat, a dark red coat, or a black and tan coat. There are various shades of fawn, red, and tan ranging from light to dark. Brindling is determined by another gene and is not a base coat color. There may or may not be white markings; however, white markings are not controlled by the A-series locus.

Fawn coat

Red and white coat

Black and tan coat

K-series locus

The brindle gene. This determines whether the dog will have a brindled coat pattern. Brown brindle and black brindle are the only two patterns associated with miniature bull terriers. Brown brindles have a uniform pattern across the entire coat, sometimes displaying slighty darker coloration around the face area. Black brindles possess a black coat and usually exhibit brindling on the legs and head area. There may be minimal striping observed on the body. Reverse brindling (tiger striped) is not exhibited in this breed.

Brown brindle coat

Black brindle coat

S-series locus

The white gene. This gene determines whether or not a dog has white markings or a solid white coat. Miniature bull terriers are not considered true white dogs. The solid white coloration in the breed is caused by the masking of another gene that actually codes for color. White terriers may have small colored markings on the head area (i.e. a colored ear, or the sought-after "pirate" with an eye spot).

Solid white coat

White coat with spot

E-series locus

Controls the presence of black masking on the face. Gene manifests as a black mask on the muzzle known as "smut". This is mostly observed in red or fawn dogs due to the light coat coloration. Brindle dogs sometimes carry and exhibit this gene but it may not be apparent due to the dark brindle coloration of the coat.

Solid coat with smut mask

Colored & white with smut mask

B-series locus

The B-series locus (TYRP1 gene) determines whether a dog has black or brown eumelanin. While most bull terriers have as BB genotype, meaning that they are genetically coded to express black pigment, the b allele is carried in some lineages. A bull terrier with a bb genotype will possess a liver/chocolate coat. Liver is a naturally-occuring color in bull terriers, but it is not a preferred color. It was once believed that liver color was linked to certain health issues, such as LAD, but this was a misconception. The dilute coloration of LAD is not linked to the B-series locus, but rather the melanophilin (MLPH) gene responsible for distribution of melanosomes and melanocytes. Noting this important distinction, it should also be said that liver-colored dogs can suffer from 'color dilution alopecia'-- a genetic condition that affects blue or chocolate dogs of any breed.

The PMEL gene

This gene codes for the merle coat pattern. Merle is not found in bull terriers. However, many backyard breeders have crossbred bull terriers with other breeds (Catahoulas, shepherds, etc.) to produce mixed-breed puppies with merle coats. Those puppies are not purebred, even if they are marketed with AKC paperwork. Also, most merle "bull terriers" are poorly bred because they are bred for color rather than health and temperament.

Fad colors and exotics

Over the past few years, profit breeders have crossbred bull terriers to create a myriad of emergent, non-standard colors like merle, lilac, blue tricolor, and others. There are even a handful of fluffy bull terriers popping up. More recently, certain individuals began using bull terriers in attempts to make a new breed of dog altogether. Although they are marketed as 'rare' or 'exotic', fad-colored bull terriers are not purebred, nor is their health prioritized by their breeders. Many times, their health, temperament, and breed traits are sacrificed for profit. Not only is that detrimental to breed integrity, but unethical breeding practices also undermine decades of careful breeding by responsible stewards of this wonderful breed.

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