Mendelian Genetics

Epistatic Gene Interactions

Chickens can have various comb types on their heads.  One of the earliest experiments was to sort out the genetic interactions of the genes controlling this trait.

Chicken Varieties	Phenotype
Wyandotte	Rose Comb
Brahmas	Pea Comb
Leghorns	Single Comb

A cross was among two of the chicken varieties and the following results were obtained.

Parental cross	Rose comb (Wyandotte) x Pea Comb (Brahmas) (both parents were pure lines)
F1 result	All Walnut Comb (This is a new phenotype not previously seen.)
	(intermate among sibs)
F2 result	9 Walnut:3 Rose:3 Pea:1 Single

Unusual Results:

1. The F₁ differed from both parents.
2. Two new phenotypes not seen in the parents appeared in the F₂

Further analysis revealed the following phenotypic and genotypic frequencies.

Phenotypes	Genotypes	Frequency
Walnut	R_P_	9/16
Rose	R_pp	3/16
Pea	rrP_	3/16
Single	rrpp	1/16

These analyses also determined that the genotypes of the original pure line parents were:

Rose	RRpp
Pea	rrPP

Therefore, genotypically the cross was:

Parental cross	RRpp x rrPP
F1 genotype	RrPp
F2 genotypes	See above table

The 9:3:3:1 ratio has been seen before.  This is the ratio that is obtained when a dihybrid is selfed.  This further suggests that two genes are controlling the comb phenotype.  And indeed this is the case.  As we will see, more than one gene can be involved in the expression of a single phenotype.  The term for this interaction is epistatis, and depending upon the interaction of the genes different ratios will be obtained.  

Epistasis - the interaction between two or more genes to control a single phenotype