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

 

 

Gene Interaction

Often times genes display independent assortment, but express phenotypes independently. When genes at multiple loci determine a single phenotype, gene interaction is taking place. The effects of genes at one locus depends upon the presence of genes at different loci. This is termed gene interaction because the effect takes place between genes at different loci, determining the outcome of a phenotypic expression. The interaction between these genes at different loci results in new phenotypes that cannot be predicted based only upon allelic interactions at a single loci.

The excerpt below from Genetics by, Benjamin A. Pierce shows Mendel's cross of the pea  plant where independence is shown in assortment during meiosis and in phenotypic expression.

The R/r alleles influence only the shape of the seed, but does not influence seed color.


Interactions among genes at more than two loci are more common, but the text and most professors will provide examples with genes interacting between two loci.  The following example from the text involves two loci that determine one phenotype (fruit color), but usually, gene interaction takes place between genes at more than two loci.

 

 

Read the following important message regarding working problems with gene interaction:

 

 

 

 

 

 

Determining the number of possible Genotypes at a locus given the number of Alleles present

Genetics page 106- Chapter 5

More than two alleles (multiple alleles) may be present in a group of individual organisms, although each individual diploid organism still has only two alleles at that locus.

Concept Check 5- Genetics -college textbook for Biology 305

How many genotypes are present at a locus with five alleles?

solution:

use the equation on page 104 :   [n(n+1)]/2 

plug in the values:  5(5+1)/2   =  5(6)/2 = 30/2 = 15 (choice C)

The answer is provided at the end of the chapter on page 126, but they don't explain how they got the answer.

Complete and Incomplete Dominance

In the example where the red flower could have the genotype AA or Aa, the Aa genotype shows that the A allele exhibits complete dominance over the white allele because only the allele for red flowers (A) is expressed in the phenotype.

However, Mendel also observed what is called incomplete dominance.  Incomplete dominance is when the progeny exhibits an intermediate phenotype (one that is an intermediate between the two parents.

For example, a cross resulting in intermediate phenotypes between to homozygous true breeding parents for two different flower colors: AA (red)  x   aa  (white)

The resulting Progeny (if incomplete dominance):  The phenotype would be a pink flower (dark or light pink, but not white or red).



Below, I am utilizing the Genetics textbook by Pierce to support the information that I'm providing you.

 

 

 

Here is the answer from Chegg's Cramster on section 5.1 comprhension questions:

1. What is the difference between complete dominance and incomplete dominance?
2. What is incomplete penetrance and what causes it?

However, they don't really answer #2. So, here's my answer:

Penetrance is defined as the percentage of individuals with a specific genotype that express the phenotype expected from having that genotype. Incomplete Penetrance is the term geneticists use to describe the circumstance where the genotype does not produce the expected phenotype in all individuals of a population. Incomplete penetrance and variable expressivity are as a result of the effects of the environment and other genes that may alter or mask the expression of a certain gene.

 

Understanding Dominance in Genetics

Dominance in genetics may be understood by comparing heterozygotes to homozygotes. In a heterozygous individual, only one trait is expressed phenotypically (physical expression of the trait).

For example, say we have two alleles for flower color at a locus- Red flowers are dominant over white flowers:           A- produces red flowers                 a- produces white flowers.

Individuals with the genotypes:

A_  - red flowers

aa - white flowers 

So, individuals with the genotype AA at that locus (position of alleles on the homologous chromosomes-homozygous dominant) are true breeding plants for red flowers.

Individuals with the genotype aa at that locus are homozygous recessive for white flowers, and are therefore true breeding for white flowers.

The plants with the genotype Aa (heterozygous). However, dominance is determined by this genotype because although this plant possesses both alleles for white and red flowers, it only phenotypically expresses the allele for red flowers. Therefore, the allele, A, for red flowers is considered to be dominant.

If we cross two heterozygous plants:   Aa x Aa

Resulting Progeny in F1 generation:  1/4 AA   1/2 Aa 1/4 aa  - Genotypic ratio

                                                     3/4 A_ (red)  1/4 aa (white) - Phenotypic ratio

Since most of the resulting progeny express red flower petals, the red allele is the dominant allele because it will be expressed even when the white allele is present.