Coat Length

Dogs have 78 pairs of chromosomes. Coat length allele is C for the smooth coat and is dominate.

For the long coat the allele is c and it is recessive. Smooth coat can have C/C or C/c but a long

coat can only have 2 alleles alike c/c. 2 smooths can produce C/C, C/c, and c/c.

Unlike coat length there are 4 alleles associated with color, however a dog will only have 2 of

the 4.

Although coat color is immaterial to quality, it's human nature to prefer certain colors. The

Chihuahua comes in one of the greatest assortment of colors of any breed. This makes predicting

the coat color a little more complicated but a lot more fun!

The genetics of coat color inheritance have yet to be studied conclusively in Chihuahuas. to

complicate matters, many colors can be difficult to describe accurately, especially in long-coated

Chihuahuas. However, coat color inheritance is similar in most dogs of any breed, and it can be

assumed that Chihuahuas share this hereditary pattern. Coat color is determined by the

interaction of several genes. When a locus has several alternative alleles, a dominance hierarchy

exists. The distribution of white spotting in Chihuahuas provides a good illustration of a

dominance hierarchy involving several different alleles at one locus.

Chihuahuas have four different alleles for different degrees of white spotting. Alleles for more

white are always recessive to those for less white. In decreasing order of dominance, the alleles

at the S locus are:

S Solid. These dogs have no white on them. A dog without any white must have at lease one

copy of the S allele. However, because S could mask the presence of any of the less-dominate

pattern. The feet (and perhaps the legs), tail tip, muzzle, and collar are white. Irish-marked dogs

Because this is the most recessive allele such dogs have 2 copies of it, sw/sw.

The presence of several modifier genes can affect exactly how much color exists within ech

genotype. In general, though, if you want a solid-colored puppy, at least one parent must be solid

colored. You cannot get an S allele from spotted dogs, and you cannot get a solid dog without an

S allele.

Now that the white part of your Chihuahua is described, what about the parts with color? Think

of a spotted dog as being splashed with white paint to varying degrees. The underlying color and

pattern is genetically still there. For example, a tri-colored dog (black, tan, and white) is simply a

black-and-tan dog partially covered with a white muzzle and legs. The distribution of dark hairs

is controlled by the interaction of genes at several different loci. Therefore, inheritance can, at

times, seem very complicated. One major locus is the A locus, which determines the distribution

pattern of dark hairs. The Chihuahua has the following alleles at that locus:

A: Dominate black. A is a solid, dark colored coat. A solid black, brown, or gray dog (or a blackand-

white, brown-and-white, or gray-and-white dog) without tan points would have at least one

A allele. These dogs can have the genotype A/A, A/ay, A/at, or A/ad.

ay: Sable. The ay allele produces the tan or red coloration most often seen in the breed. These

dogs are often born with a darker overlay that fades to a greater or lesser extent with maturity.

The hairs may be mixed with darker hairs, somewhat like the color seen in lassie-colored collies.

A Chihuahua with Lassie's sable-and-white coloring would have alleles producing both the sable

color at the A locus and the Irish-marked white pattern at the S locus. Sables can have the

genotype as/as, as/at, or as/ad.

at: Tan point. The at allele produces dogs with a dark body coat and tan points above each eye,

on the feet and under the tail base, similar to the familiar pattern seen in Doberman Pinschers.

Depending on genes at other locations, these dogs can be black and tan, chocolate and tan, or

blue and tan. They can also have various amounts of white on them depending upon interactions

These dogs have a pattern in which dark hairs are tipped with black or brown and are lighter near

the skin. They also have lighter legs, underside, and face, usually with a widow's peak or mask

such as that seen in Siberian huskies. they come in a variety of shades and colors. They may be

combined with varying degrees of white spotting depending on interactions with the S locus. A

domino Chihuahua can only have the genotype ad/ad at this locus.

The distribution of colored hairs determined by the A locus is complicated by interaction with

alleles at the E locus.

Its alleles have the following effects:

Em: Mask. This allele adds a dark mask. Black- or chocolate-masked dogs can have the

genotypes Em/Em, EmE, Em/ebr, or Em/e.

E: Extension. this allows the formation of whatever pattern is determined at the A locus. These

dogs can have the genotypes E/E, E/ebr, or E/e.

ebr: Brindle. The ebr allele produces dogs with irregular, vertical dark stripes running down the

sides of the body over a lighter background. These dogs are either ebr/ebr or ebr/e.

e: Restriction. Dogs with two e alleles have no black hairs on their body, no matter what the

alleles at the A locus dictate. Examples are cream Chihuahuas. These dogs must be e/e.

Besides the distribution of light and dark hairs determined by the alleles at the S, A, and E loci,

the actual hue of color of the darker hairs is influenced by alleles at other loci.

The B locus determines whether dark hairs will be black or chocolate.

B: Black. The B allele allows black pigment to be black. Dogs with black pigment may be B/B

or B/b.

b: Chocolate. The b allele makes all black pigment appear brown. Dogs with b/b also have liver colored

noses and light-colored eyes.

The D locus acts in a similar manner as the B locus but determines if dark pigment will be black

or gray.

D: Dark. The D allele allows black pigment to be black. Dogs with black pigment may be D/D or

D/d.

d: Dilute. The d allele makes all black pigment appear gray. Dogs with d/d also have slightly

lighter-colored noses and eyes.

Note, dogs with both b/b and d/d have a diluted tan color. This is appropriately termed lilac.

The C locus acts on red pigment.

C: Full Color. The C allele allows for fully saturated reds. These dogs may be either C/C or C/c.

c: Chinchilla. The c allele decreases the intensity of the red or tan pigmentation. Cream

Chihuahuas could thus be c/c.

To see how these alleles interact, consider these examples:

The at/at would mean that the dog has the black-and-tan pattern. The E would allow black

pigment to be expressed. The D and the C would mean the black parts would be black rather

than gray. The b/b would mean that the normally black parts would be brown, however (as in red

Dobermans). The si/sp would determine that a white collar and extremities would overlay the

brown and tan colors. This dog would be chocolate and tan, overlaid with a white collar, legs,

muzzle, and tail tip.

The A would mean the dog has a solid, dark coat color. The Em would dictate that it also has a

black mask. Although, since the muzzle is already dark, the mask would not be noticeable. The B

would determine that the dark hairs were black rather than brown, but the d/d would turn them

all gray (or blue). The S would determine that no white was on the dog. This dog would be solid

gray.