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

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

B

B

b b

Bb Bb

Bb Bb

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

–??

–??

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...

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…

BE

Be

BE Be

BBEE

bE be

bE

be

BBEe

BBEe BBee

BbEE BbEe

BbEe Bbee

BbEE BbEe

BbEe Bbee

bbEE bbEe

bbEe bbee

• What are the phenotypes of the dogs in the Punnett Square?

BE

Be

BE Be

BBEE

bE be

bE

be

BBEe

BBEe BBee

BbEE BbEe

BbEe Bbee

BbEE BbEe

BbEe Bbee

bbEE bbEe

bbEe bbee

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)

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