Fur color in Labrador retrievers is determined by two genes

Fur color in Labrador retrievers is determined by two genes (gene B for coat color and gene E for deposition of pigment). Nose, lip and gum color is determined only by gene B. A black female with black nose and lips (she has a brown mother and a yellow father) is mated with a brown (chocolate) male whose mother was yellow. What is the probability that they will have a puppy that is yellow with a brown nose

Solution

The coat color of Labrador Retrievers (Black, brown (chocolate), or yellow) is controlled by two different genes. The first gene controls the color of the coat, and is either B=Black or b=brown. Each parent provides the offspring with either a \"B\" gene or a \"b\" gene. Black is dominant over brown. This means if either gene from the parent is \"B,\" the offspring\'s genetic code for coat color will be Black. The genetic combinations include:

An offspring with one or two \"B\'s\" will have a Black nose; one with two \"b\'s\" would have a brown nose.

The second gene regulates whether the color pigment will actually be deposited in the hair, and is either \"E\" = permits pigment deposition, or \"e\" = prevents pigment deposition. As above, each parent provides the offspring with either an \"E\" gene or an \"e\" gene. The \"E,\" which permits pigment, is dominant over \"e,\" which does not permit pigment. Thus, if either gene from the parent is \"E,\" the offspring\'s genetic code for coat color pigment (Black or brown) will be deposited. The possible combinations include:

As a result, any Lab that has at least one \"B\" and one \"E\" (BBEE, BBEe, BbEE, or (BbEe) will be Black.

A Lab that has two \"b\'s\" and at least one \"E\" (bbEE or bbEe) will be brown.

A Lab that has two \"e\'s,\" regardless of whether it has \"B\'s\" or \"b\'s\" (BBee, Bbee, or bbee) will be yellow.

You may wonder why some yellow Labs have Black noses and lips. It turns out that the \"e\" gene does not prevent deposition of color in the nose or lips as it does in the coat. Therefore, \"BBee\" and \"Bbee\" yellow Labs have Black noses and lips, while \"bbee\" yellow Labs have brown noses and lips. Note, though, some Labs lose their nose color as they age, so nose color may not be a good indication of the actual genotype of a Lab.