Labrador retrievers come in three solid colors: black, yellow and chocolate. Black and chocolate Labradors are eumelanistic colors, with their genes residing on the Brown locus. Yellow labs, however, are a completely different animal. Their color relies on alleles located on the Extension locus, the sole purpose of which is to dampen the effect of black and chocolate genes. With 81 combinations of these three colors being possible, the puppies can be any of a variety of colors.
Black Labrador Genetics
The black color gene is located at the B locus, the location on the genetic strand for the Brown or eumelanistic series of genes. This color is dominant to chocolate, which resides on the same locus. Because all genes are paired, a dog can appear to be black but still possess a gene for chocolate. A black dog, therefore, can possess two black genes (BB) or one gene for black and one gene for chocolate (Bb). It is the gene at the Extension (E) locus that controls whether a Labrador appears to be black, chocolate or yellow, however. If a Labrador retriever possesses even one dominant of the dominant E allele, then his genes always will produce eumelanin; he will appear to be black or chocolate. A black Labrador can possess either two dominant Extension genes (EE) or one dominant and one recessive gene (Ee).
Yellow Labrador Genetics
Yellow Labrador retrievers always have two recessive genes at the E locus (ee), meaning that their ability to produce eumelanin is disabled. Such dogs produce a pigment called phaeomelanin in their coats, which causes the coat to have a yellow appearance. Yellow dogs, despite having two recessive genes controlling their coat color, can carry genes for either black or chocolate, as the genes on the E locus only block the black or the chocolate from being expressed.
Yellows Carrying Black or Chocolate
As noted above, all yellow Labrador retrievers possess a pair of e alleles on the Extension locus. However, a dog who appears yellow also possesses the color genes on the Brown locus, even if the color cannot be expressed. A yellow dog, therefore, possesses the genetic code for black (eeBB or eeBb) or for chocolate (eebb). A Labrador with the double recessive on the E locus still can produce eumelanin in his nose, flews and eye rims, generally causing them to be black in color. A dog who has brown pigment in those areas can be assumed to carry chocolate.
The color black is always dominant to yellow in Labrador retrievers. A black Labrador who possesses two copies of the dominant allele at the E locus (EEBB) only can produce black or possibly chocolate puppies when bred to an eeBB, eeBb or eebb yellow Labrador. Yellow puppies only become possible if the black Labrador possesses a recessive copy of the E allele (EeBB) or (EeBb). The percentage of black, chocolate or yellow puppies in any given litter is difficult to predict without knowing what genes are carried by the parents.