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Coat Genetics...

Article: Colour Genetics
Author: MindiBown Kennels

Probably more than any other area of breeding, the study of colour inheritance is the most fascinating, and probably the most misunderstood. A relevant observation is that it is also harmless. ANY colour is acceptable in our breed. Therefore if we try to figure the genetics of colour and miss, it really doesn't matter. So adopting the principle adage of the medical profession, "First of all, do no harm" this is a good place to experiment. You won't end up with a deformed or crippled pup. I'm certainly not suggesting that anyone should ever put two dogs together purely and simply to chase a certain colour, but trying to predict Colours before the whelping is an interesting diversion.

The genetics of colour is also a logical next step in this series of articles as the predominate nature of these genes are still of the old popular recessive/dominant variety.

In this area however, we learn of two extra components of Genetics.

The first is an extension of the study of coat inheritance factors. Genes in this area still have only two parts to them, one from Mum and one from Dad, but we are now faced with a lot more choices of possibilities that can be passed on.

The second is the discovery that one gene can have an effect on another gene further down the DNA trail. In fact, in colour, several different genes can act upon each other to produce a colour. In some cases, one gene will mask, or counteract the presence of another.

Oh, before we start. From here on I'm going to use the accepted terminology in genetics to signify recessive or dominant. Namely, a capitol for dominant. "B" or lower case for recessive "b".

Let's start with the pattern or presence of white.

There are 4 parts to this allele.

In order of dominance:

S : "Self coloured", No white showing. (Most Dominant)
sⁱ : "Irish Marked". i.e. white feet, tail and collar
s^p: "Parti colour" (two or more solid colors, one of which must be white)
s^w : Almost all white, very little, if any colour.

Now these are all listed in order of dominance. A solid colour dog is probably carrying the "S" allele. Because this is the most dominant. However, the other half of the gene may be any of the others.

On the other hand, if you had two parti colour dogs and bred them, they could only produce more parti's or an almost pure white. If you had your heart set on an Irish marked pup, forget it. It's genetically impossible. Of a more practical nature. If you are offered an Irish marked pup supposedly from two parti colour parents, walk away. That breeders pedigrees aren't worth the paper they're written on.

Next, let's look at the pattern of black.

Again, in order of dominance.

A: Results in pure black. The strange thing is that despite this being the most dominant, an all black Chihuahua is seldom seen.
ay: This is the gene (Allele) that produces the "Sable" effect. Black hairs interspersed through another colour.
a^s: Produces a black blanket, usually on a tan body. Very similar to a GSD.
a^t: The most recessive, although not at all uncommon. This allele produces the common "Black & Tan" markings as seen on a Doberman, or English Toy Terrier.

A Black & Tan dog can ONLY carry a gene made up of a^t/a^t . No other combination is possible. If this statement makes sense to you than you are getting a good grasp of the basic concepts of Recessive/Dominant Genetics.

There are a number of interesting points about this set of alleles. (By the way, a set of alleles such as this is referred to as a "lotus") The first is that despite the A allele being the most dominant, an all black Chihuahua is seldom seen. Why? My personal theory is that it's because black dogs are not popular in the show ring. Their colour tends to disguise their good points and accentuate the bad. For this reason a lot of breeders won't breed an all black dog. The danger here is that it would be easy for the option of a solid black dog to completely and IRREVERSIBLY disappear. The easiest inherited trait to breed out is one caused by a dominant allele. It is my personal belief that this is an example of breeders choosing to breed for success in the show ring, rather than a genuine desire to improve the breed. E.O.S. (End Of Sermon)

There is one more lotus of alleles that affect the black on a Chihuahua.

E^m No matter what other genes are active, this allele will put a black mask on it.
E Next most dominant, this results in no black mask.
e^br Brindle. To those unfamiliar with the term, sort of irregular vertical black stripes. Not particularly rare in Chihuahuas but not overly common either.
e The most recessive of this lotus, this allele MASKS black anywhere on the dog.

Now think about this last allele for just a minute. This is a wonderful example of genetics disguising things, and of the ability of one gene to affect another totally separate one. Let's say you have a dog that is a^t/a^t . Black and Tan. (In accepted genetic terminology we say the the dog is of the a^t/a^t genotype) Now if the e allele is also present you will have an all tan dog. One might say a Black & Tan for people who don't like Black & Tans.

So far we have covered the gene that controls white (S) and black (A) & (E)

Now let's have a look at the genes that control colour and it's shading.

There are basically 3.

The first

B Allows black to be black. (Sort of the opposite of e)
b Makes all black hairs appear brown. (Chocolate)

Let's stop here and have a closer look at this b gene. The Chocolate Gene. It's another example of one gene altering another one someplace else.

In this particular case we can see that two totally separate and distinct genes must be present for the chocolate colour to appear. The first one is any member of the A lotus. There has to be some black present. The second is the b allele, which will turn that black to chocolate.

There's an interesting marker or flag for the b allele. It gives the dog a brown nose and yellow eyes. This then makes the breeding of chocolates rather easier to predict. If you have a cream or tan dog (or any other colour) with yellow eyes and a brown nose, it carries the chocolate gene. All you need now is some black in it's breeding partner and hey presto, a percentage of the litter will be chocolate.

If only all recessive genes carried a flag like the chocolate gene does. Life would be so much easier.

It's also interesting to note that I recently saw an ad on an American Chihuahua breeders Web Site offering a pup for sale as a pet. The reason given was that it had a self coloured or brown nose, and she was trying to breed this feature out of her kennels.

I had to ask myself whether she was saying that chocolate was an unacceptable colour to her, or did she not realise that the brown nose was merely part and parcel of the chocolate gene?

I tend to think that it was the latter. That is rather unfortunate as it means that a quite experienced breeder in fact has no knowledge of even basic genetics in her breed. Sadly, this is the case more often than you may think.

Next we come to:
D Makes all Colours intense and full. e.g. Black is Black
d The opposite. Makes all Colours less intense, or diluted. This gene in an otherwise black dog will wash the black out to a grey colour. Our so-called "Blue"

And lastly:
C Similar in effect as D. Allows Colours to be fully saturated.
cch Decreases the saturation of colour while allowing black to remain black. Most rare in Chihuahuas, this gives us a Chinchilla effect. Hairs that are black at the tips but light at the roots. Most often the root colour is either a blonde or smoky grey or white. VERY pretty, and very rare.

So there you have it. Do you think you now know all about predicting Colours in a Chihuahua? Fantastic! E-mail me will you and explain it to me!

The more I study genetics, the more questions I seem to come up with.

For example. Is there an order of dominance between the lotus'? There must be. Has anyone EVER seen a parti colour with a black mask? That would suggest that if the S^p allele is present, it is dominant over the E^m allele.

Can anyone explain cream? I know from observation that it is very dominant. (There hasn't been a single ChaChi litter that isn't 50% creams) But it's not mentioned anywhere in the text books. Is it in fact Tan diluted by the d allele. If this was so, we'd expect more blues from cream dogs and that doesn't seem to be the case.

I'll close this subject with a final observation. I made an earlier comment on the number of breeders whose knowledge of genetics is sadly lacking. If you are a student of our breed however, do NOT dismiss what an experienced breeder says even if they come out with a statement that proves their technical knowledge is minimal. (I have a long coat girl that ALWAYS throws longs no matter who you put her with) What you'll find is that a lot of breeders have noticed trends over the years that although they may not be able to explain genetically, they are none the less valid.

A good example is blues. We now know that a Blue Chihuahua is a combination of the A, (Black) and the d (dilution) allele. I have not been able to find a single reference in any book, article or journal that would even remotely suggest that the dilution or d allele is in any way connected to the chocolate or b allele. However, breeder after breeder has stated quite categorically that "blues come from chocolates".

What do you make of this contradiction? In my case, I take the view that what these breeders have seen is valid and probably correct. We just can't explain it yet.

Listen to what they have to say and then study some more and see if you can then logically explain what they have seen. Or, perhaps break new ground and prove them correct scientifically.

(C)Copyright 2007, this article is copyright protected
Special thanks to MindiBown Kennels for permission to display this article on RoyaltyChi Chihuahuas website