While there are many loci and alleles involved in the Weimaraner’s coat color, there are only two loci that are important for the Blue versus Gray coat color in the Weimaraner, the Locus B pair and Locus D pair.
The B genes work in pairs, with one of each of these two alleles inherited from each parent. The D genes also work this way. Each of these genes may be dominant or recessive. Dominance is noted with a capital letter and recessives with a lowercase letter for that allele. In other words, B is dominant to b, and D is dominant to d. If present, the dominant gene will determine what you see and will always “override” the paired recessive.
Locus B Pair
The dominant B produces a black coat color whereas the recessive b produces a liver or chocolate coat color. Since these genes come in pairs, a dog could be BB, Bb or bb. BB individuals will be black. bb individuals will be chocolate. Bb individuals will also be black because the dominant B “masks” the b. The b is still there and can and does get passed onto the dog’s offspring, but it is not visible as the dogs phenotype (the physical characteristic that you can see).
All Gray Weimaraners are bb. Blue Weimaraners are BB or Bb.
Locus D Pair
This pair controls dilution. The dominant D causes full pigmentation whereas the recessive d produces a dilute pigment. Because D is dominant, a Dd (or DD) individual will be fully pigmented and there would be no dilution of coat color. It is commonly accepted that all Weimaraners are dd; that is, all Weimaraners, both Blue and Gray, are diluted and never fully pigmented. The dd pair makes the bb chocolate/liver into the light tan that we call Weimaraner Gray, and it makes Bb of BB black into a charcoal colored dog that we call Blue.
The dilution can work in degrees. In other words, it can make some dogs lighter than others. Some Blue Weimaraners can be so light as to appear gray. We must remember that the difference between blue and gray in Weimaraners is tonal, not the degree of dilution. We have Silver-Gray, Gray and Mouse-Gray that vary in how dark they are, but they are all bbdd. Blues too can range from very dilute to a very dark Blue. Thus, very light Blues may look lighter than a Mouse-Gray.
How to Determine the Inheritance of color in a Weimaraner Litter
Two Gray Weimaraners will never produce a Blue Weimaraner. This is because there is no dominant B gene that any puppy can inherit from either parent.
When considering inheritance of coat color if there is a Blue parent, we merely need to look at the locus B pair in Weimaraners since all Weimaraners are dd. A simple way to determine what a mating would produce is to do a Punnett square. With this tool we can predict the statistical outcome of genotype and phenotype. An empty Punnett square looks like this:
Let’s say we are breeding a homozygous (having the same alleles, so BB) Blue Weimaraner to a Gray Weimaraner (bb). We would note the geneotype of one parent at the top of the Punnett square and the other parents on the side. The empty squares represent the puppies. Since we know that each puppy inherits one of the alleles from each parent, we move one allele down into the offspring’s squares.
The completed Punnett square would appear like this:
As we can see from the completed Punnett square, a Gray Weimaraner bred to a homozygous Blue Weimaraner will produce 100% Blue Weimaraners, and further, all of them will be heterozygous Blues. In other words, all of the dogs will be blue and carry the gray recessive.
When we breed a Gray to a Blue who is carrying the recessive b, we can use the Punnett square to determine that we will statistically get 50% Blues and 50% Grays:
Thanks to this simple tool, it is now perfectly understandable how two Blues can produce Gray pups. Statistically we would see 75% Blues and 25% Grays:
Punnett squares only help predict probability, and results are not statistically significant when dealing in small numbers. Therefore it would be theoretically possible for two Blues to produce an all Gray litter. However because Blue is dominant, it is never possible for two Grays to produce a single Blue in a litter.