Epistasis- A form of Gene interaction

Epistasis is a term used by biologists to describe the occasion where a gene at one locus masks the expression of a gene at a different locus.

I know that the masking of genes sounds familiar to you...dominance! Yes, they are familiar, but there is a difference. Remember that dominance involves a heterozygote where both alleles for the same gene are present, but only one is expressed phenotypically (the dominant one). The difference between epistasis and dominance is that dominance involves masking of allelic genes or genes located at the same locus. The gene causing the suppression of the other gene's expression is called the epistatic gene. The gene whose expression is being masked is called the hypostatic gene.



Recessive Epistasis




Remember that epistatic gene interaction may be recessive or dominant. The Labrador Retrievers are great
examples of recessive epistasis. The allele, e, masks the black and brown alleles when homozygous (for e that is).


A Detailed Explanation of Recessive Epistasis in Labradors:

In Labradors, one locus determines the pigment produced by skin cells and the allele symbols are B (for black) and b (for brown).


The other locus determines whether the black pigment is deposited or not.

E- black is deposited   
 e- black is not deposited, causing yellow hair

If the second locus is homozygous recessive- ee...

the alleles B and b are masked at the first locus masking the presence of Black and Brown phenotypes! I think this is such a cool discovery.


Now, Let's do some Genetics!

Cross 1:   BBEE x bbee  (P1- Parents)






Results of Cross 1:  BbEe (100% heterozygous progeny)

Intercross progeny:  BbEe x BbEe

9/16 B_E_  Black

3/16 bbE_ Brown/Chocolate

3/16 B_ee Yellow

1/16 bbee Yellow


The fact that yellow dogs have either the brown or black allele, but neither of them express it in coat color reveals how the recessive allele, e, is capable of masking the presence of B or b.

Which allele is the epistatic gene?

Which alleles are the hypostatic genes?

Remember that the epistatic gene does the masking and the hypostatic genes are the genes being masked.

Therefore, the epistatic genotype is ee. So, the allele e is epistatic to alleles B and b. Alleles B and b are hypostatic to allele e.

This is an example of recessive epistasis; two copies of the e allele have to exist in order for the masking of brown and black alleles to take place.

The Book Suggests That we try # 29 at the end of the book:

A dog breeder liked yellow and brown Labradors. In an attempt to produce yellow and brown puppies, he bought a yellow Lab and
that was male and a brown female Lab and mated them. Unfortunately, the resulting progeny were 100% Black!

Explain what happened. How can he produce brown and yellow Labradors.

Below, I've posted the Cramster solution http://www.chegg.com/homework-help/genetics-a-conceptual-approach-4th-edition-chapter-5-problem-29aqp-solution-9781429232500, you can visit their website and get a free week trial before you decide to go with the program. There are various packages.

My idea is that the yellow puppy had a B_ee because the brown puppy definitely has bbE_.
I also want to say that the brown puppy is bbEE because all of the puppies are black, and that means
we can't have any ee alleles. The yellow puppy also must have been BB because we can't get any
bb alleles in the F1 progeny.

To get brown and yellow puppies, he can cross the heterozygous black puppies BbEe (all the progeny
are heterozygous black), and he can get a 9:3:3:1 ratio of Black to Brown to Yellow to Yellow