Thursday, August 8, 2013

Understanding Mismarks

By: GDB Breeding Manager Jenna Bullis

Recently we posted a litter announcement for the Auberge x Tom litter and shared with you all that “Jicama” was career changed due to a prominent facial mismark.  Since then we have received many questions and comments so we thought we would give you some more “Fun Facts from the Breeding Department”. Get ready for your biology lesson for the day!

In order to understand what caused Jicama’s mismark, we have to cover some basics first…there are spots on the canine genes called “loci” (or “locus” for a single spot) that deal with different coat colors. There are thousands of these loci, and it gets pretty complicated really quickly. Fortunately for us, in Labradors (and Goldens) we really only need to focus on two loci to determine whether a dog will be black, chocolate, or yellow: B and E.

Jicama puppy shown with prominent black facial mismark
B comes in two varieties: black and brown. Black (B) is dominant, brown (b) is recessive, and the color applies not only to the dog’s fur, but to some extent all of the areas of pigment we see: nose, lips, foot pads, and around the eyes. If the dog in question has even one copy of the dominant (B) gene, s/he will have a black coat and black nose, etc. Only if the dog has two copies of the recessive gene (b) will their coat and nose look brown.

So where do yellow Labradors and Golden Retrievers fit in? For them, we need to go to a different locus: E, which works a little differently. In recessive form (e), it suppresses or prevents the coat color of the B locus from expressing itself. In other words, the black or chocolate color won’t show up in the fur if the dog is carrying e/e. Instead, their coats will be yellow. Recessive (e) doesn’t remove the other areas of pigment however – they should have black noses, or at least a black rim around their noses, if they have B/B or B/b on that first locus. If they have b/b on that first locus, then those other areas of pigment will be liver colored. Couple that with e/e for recessive yellow coat color and we see a yellow coated dog with liver b/b pigment.

In a way, the coat colors are like a ladder. The first rung (or loci) tells you if the dog is black or brown, then the 2nd rung takes that black or brown dog and if double recessive, turns its fur yellow.
Golden Retrievers are genetically black (BB ee) but look golden (yellow) to reddish due to their (ee) genes restricting the development of black pigment.

So how does all this relate to Jicama? Sometimes when an embryo is developing one of its skin cells undergoes a mutation. Any cell that is produced by this mutant cell dividing also contains the mutation.  Jicama had mutation in a skin cell in which (ee) became (Ee). This allowed the black pigment to form in cells descended from that one original mutant cell. This phenomenon is well documented in Labrador Retrievers and Golden Retrievers. These dogs are sometimes referred to as Mosaics. This same phenomenon has also been observed in domestic cats and in ranched foxes. It’s not a really a birthmark, it’s just a somatic mutation.

The cells responsible for reproduction originate from a different place than the skin cells therefore are not affected by this mutation and thus a dog which has a somatic mutation (or mismark) will not produce its dual color in offspring. 

These types of color mismarks are not incredible unusual in our colony, but they are not typically as prominent and noticeable as Jicama’s. Phew!  As you can see, genetics is a complicated business!  In the end, as we said in the original post, we felt that Jicama’s mismark was prominent enough to draw significant comment from the public, which could be a distraction at best, and a potential burden for a graduate.  She is enjoying her life as a pet in a loving home. 

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