1. Epistasis Sometimes a phenotype is caused by the interaction of several genes. These genes are separate alleles and segregate independently. In the case of the Labrador Retriever one allele (E/e) actually controls the expression of the other allele (B/b) Visit this URL. Its sponsor will send a kit so you can actually determine the genotype of your Lab. http://www.vetgen.com/color.html http://www.vetgen.com/color.html

2. Labrador Retrievers In Labs, the black coat color (B) is dominant to the chocolate (b). Crosses would be expected to follow a simple monohybrid cross... Black –BB –Bb Brown –bb

3. The Yellow Lab There’s actually a third coat color, yellow. Could this be incomplete or co- dominance? There’s another set of alleles that work with the B/b allele pair. The E/e allele actually controls the expression of the B/b allele.. Yellow –??

4. Epistasis in Labs The E/e allele –The dominant allele (E) allows the B/b allele to function normally. –The recessive allele (e) does not allow pigment to form in the hair. When a lab has the ee genotype: –Coat color is yellow –Regardless of the B/b genotype! When dealing with coat color in labs, first find out the E/e genotype...

5. Dihybrid Cross Here’s a dihybrid cross between two black labs: BbEe x BbEe Remember, any dog with an ee genotype will have the yellow phenotype… What are the phenotypes of the dogs in the Punnett Square?

6. Dyhybrid Ratios Black dogs: BB or Bb and at least one E Chocolate dogs: bb and at least one E Yellow labs will have genotypes with ee Note the 9:3:3:1 ratio, typical of the dihybrid cross... (almost)

7. Labrador Retrievers Got it? So there are three coat colors… Not really. There’s actually another form of the yellow. Note the nose color…. Remember sometimes a phenotype is caused by the interaction of several genes. Graphics downloaded 2/20/02 from http://www.vetgen.com/color.html