Puppies of all colours can potentially occur in the same litter. Colour is determined primarily by three genes. The first gene (the B locus) determines the density of the coat's eumelanin pigment granules, if that pigment is allowed: dense granules result in a black coat, sparse ones give a chocolate coat. The second (E) locus determines whether the eumelanin is produced at all. A dog with the recessive e allele will produce only phaeomelanin pigment and will be yellow regardless of its genotype at the B locus. The genes known about previously[36] have had their number increased by the introduction of the K locus, where the dominant "black" allele KB is now known to reside.[37] Black or chocolate Labradors therefore must have the KB allele. Yellow Labradors are determined at the E locus, so the K locus is irrelevant in determining their colour. Variations in numerous other genes control the subtler details of the coat's colouration, which in yellow Labradors varies from white to light gold to a fox red. Chocolate and black Labradors' noses will match the coat colour.
It has been shown that out of all dog breeds, it is the Labrador Retriever that is most likely to be obese.[73] In a 2016 published study it was shown that out of 310 Labradors, most were missing all or parts of the POMC gene. This gene plays a part in appetite regulation as well as indication of the amount of one's stored fat. The study concluded that the absence of that gene had a significant impact on Labrador weight and appetite.[70][73] The POMC gene mutation is present in only one other breed – the Flat-Coated Retriever.[70]
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