Next in the genotype we have "B". "B", or 'brown', is another altering factor of dogs expressing black. This 'browning' factor is the part that is affected when you see a chocolate Labrador. This alteration I will explain further in detail. Here I will give a brief explanation. As in the example of the punnet square Labradors, we began with a genetically homozygous "BB" dog (that is, solid black), which was then bred to another homozygous dog. This dog, however, was not dominant black, but rather, recessively so. This dog's genotype was "bb". This "bb" is the execption to dominant blacks on this influence. Any "B" ("BB" or "Bb") results in a dominantly black dog. When no "B" is present, the colour is actually expressed as 'brown', 'red', or 'liver'. All are shades of red and range greatly.
But back to our original phenotype. Our dog, case #1 Black Labrador, has been established to have "KK". He is also homogynous for "BB". Because of this, our dog is not chocolate, nor will he ever have chocolate offspring. Any resulting offspring will all be Black. Our dog's phenotype so far is: "KK" "BB".
Here is our 'chocolate' Labrador. This dog expresses the result of the alleles "bb". Distincly different to our dominant black Labrador, this dog is still similar in shade, despite being brown. There are some distinct differences though, which will be further discussed.
We can establish that this dog's phenotype is "KK" "bb" - "KK" because no agouti patterns are present (agouti patterning plays on the 'k' influence) and "bb" because there is no dominant, black producing "K".
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