Epistasis- An Overview

Epistasis is a dihybrid cross with a difference. Instead of each of the genes at different loci affecting separate characteristics, the action of one gene will inhibit or affect the expression of the other gene. The gene that has the effect on the other is called the epistatic gene, and the one that is being acted upon is called the hypostatic gene. Epistasis changes the typical phenotypic ratio observed, and also reduces phenotypic variation.

Dominant Epistasis

In dominant epistasis, the dominant allele at the epistasic locus affects the expression of the gene at the hypostatic locus. A recessive allele at the epistatic locus will have no effect. The example used to illustrate this is feather colour in chickens. The colour of a chickens feathers is determined by the interaction of two gene loci I/i and C/C.

• I/i is the epistatic gene. Any chicken with a dominant I in their genome will have white feathers. Being homozygous recessive ii at this locus enables the expression of genes coded for at the hypostatic locus.
• At the hypostatic locus C/c the dominant allele C codes for coloured feathers while the recessive c codes for no colour. Hence, a chicken that is homozygous recessive cc will also be white.

Crossing a pure-breeding White Leghorn chicken (homozygous dominant at both loci IICC) with a pure-breeding white Wyandotte chicken (homzygous recessive at both loci iicc) produces an F1 generation that all have the genotype IiCc. These are all white as they possess the dominant I allele. If you are not sure how this works think back to normal monohybrid and dihybrid crosses - each of these parents can only produce one sort of gamete as they are homozygous: (IC) and (ic) respectively.

Now, crossing two F1 chickens which are heterozygous at both loci will produce this result:

	(IC)	(Ic)	(iC)	(ic)
(IC)	IICC white	IICc white	IiCC white	IiCc white
(Ic)	IICc white	IIcc white	IiCc white	Iicc white
(iC)	IiCC white	IiCc white	iiCC coloured	iiCc coloured
(ic)	IiCc white	Iicc white	iiCc coloured	iicc white

You can see that our usual Mendelian 9:3:3:1 dihybrid cross ratio has been change by epistasis to 13:3 (white:coloured). In practice, this may sometimes be 12:3:1 if the double recessive genotype (iicc or equivalent) produces a phenotype different to either of the other ones. It just so happens that in this example iicc chickens are also white. 12:3:1 or 13:3 are typical phenotypic ratios of dominant epistasis.

Recessive Epistasis

In recessive epistasis, the epistatic gene will only affect the hypostatic gene if the organism is homozygous recessive at the epistatic locus. An example of this is the control of flower colour in Salvia. Once again, the colour is controlled by two genes A/a and B/b.

• A/a is the epistatic gene. Organisms homozygous recessive aa at this locus will be white. The presence of the dominant allele A enables the expression of the colour coded for at the hypostatic locus.
• B/b is the hypostatic gene. The dominant allele B codes for purple flowers, while the recessive allele b codes for pink flowers. Organisms will only have pink flowers if homozygous recessive bb at this locus, and if the epistatic locus enables the expression of this gene (i.e. a dominant A must be present).

Crossing two flowers heterozygous at both loci AaBb (which have the phenotype purple) produces the following results:

	(AB)	(Ab)	(aB)	(ab)
(AB)	AABB purple	AABb purple	AaBB purple	AaBb purple
(Ab)	AABb purple	AAbb pink	AaBb purple	Aabb pink
(aB)	AaBB purple	AaBb purple	aaBB white	aaBb white
(ab)	AaBb purple	Aabb pink	aaBb white	aabb white

Our usual Mendelian 9:3:3:1 dihybrid cross ratio has once again been modified by epistasis. The typical phenotypic ratio for recessive epistasis is 9:3:4 as showed by this diagram.

Modified dihybrid Mendelian ratios

Dominant ´ Recessive 9 :6: 1

Single Dominant 12 :3: 1

Duplicate Dominant 15: 1

Single Recessive 9: 3 :4

Duplicate Recessive 9: 7

No Epistasis 9: 3: 3: 1

A-B- A-bb aaB- aabb

Type of gene interaction

1. Duplicate Recessive (9:7)

 Recessive at one either locus masks the expression of the dominant phenotype at the other locus.

2. Single Recessive (9:3:4)

 Recessive Trait at one locus masks the effect of the second locus.

3. Duplicate Dominant (15:1)

The dominant trait at either locus will express one phenotype, the other phenotype is homozygous at both loci.

4. Single Dominant (12:3:1)

Dominant trait at one locus masks the expression of the second locus.

5. Dominant ´ Recessive (9:6:1)

w Duplicate effects from the two loci. The phenotypes are: Two dominant, One dominant, and None dominant.

Multiple Alleles: The ABO Blood Group

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Among the variety of more complex situations, one of the most common is multiple alleles. Typically, we teach with genes for which only two alleles are known, but many genes have more than two different alleles--thus, "multiple" alleles.

One such gene which is of great interest to humans is the ABO blood group gene. This particular gene has three alleles, rather than two. Of course, each of us has only two sets of chromosomes, so any one individual has only two of these alleles at once. But the presence of three different alleles means that there are six possible genotypes, rather than the three possible for the more familiar two-allele situation.

For the ABO gene, the three alleles are the I^A, I^B and i alleles. We typically call these alleles "A," "B," and "O," but of course our rules for assigning symbols to alleles demand that all three be represented by some version of the same symbol. In this case, that common symbol is the letter "I," which stands for "immunoglobin."

By now, the concept of dominance should be familiar to you. Of course, things get a bit more complicated when there are three alleles instead of just two. As the symbols above should suggest, the i allele (the "O" allele) is recessive to both the I^A and I^B alleles (the "A" and "B" alleles). The I^A and I^B show co-dominance. This means that in an individual who is heterozygous for these two alleles, the phenotypes of both alleles are completely expressed, thus producing blood type AB.

Thus we have the following:

Genotype	Phenotype
ii	Type O
IAIA or IAi	Type A
IBIB or IBi	Type B
IAIB	Type AB

Clearly, the additional genotypes produced by having three alleles rather than just two make for much more fun when it comes to figuring out genetic interactions between two people :^)

Other Blood Groups:

The ABO blood group isn't the only aspect of our blood type chemistry. There are about a dozen known genes which affect "blood type" activity. Because the ABO blood type has been known the longest, and has such a powerful effect, it is the one we focus on. The genes for the others work in similar fashions, though certainly not all have three alleles.

The other aspect of blood type which is of most interest to us is the Rh factor. Genetically, this is much simpler than the ABo system. It has only two alleles, one dominant (Rh-positive) and one recessive (Rh-negative). However, this facet of our blood type has some serious implications when we reproduce, and so has come under pretty heavy scrutiny. You can explore more about this part of your blood type by reading the Rh Disease essay.

Just what is blood type, anyway?

Our blood type is part of the marvelous protective machinery in our bodies called the Immune System.

Our immune system's task is to identify and destroy biological materials which are foreign to our own bodies. In order to perform this task, of course, your immune system needs to know how to distinguish between what belongs to you and what is foreign. This isn't the easiest of tasks, as you are made out of precisely the same materials that would compose any living invader. So a vital part of your immune system is the tagging of your own cells and tissues, so your antibodies won't destroy your own cells.

Blood type is part of this much larger self-tagging system aspect of your immune system. "Blood Type A" actually means "blood cells tagged with antigen A." Antigen A is a specific protein marker found on the surfaces of all Type A blood cells. And the task of the I^A allele is to cause the creation and attachment of this specific kind of antigen. The I^B allele causes the creation and attachment of a different protein marker, the B antigen.

Knowing this about blood type can also explain to you why the I^A and ^B alleles are co-dominant, and why the i allele is recessive. The I^AI^B genotype results in both A and B antigens on the cell surfaces. The i allele causes no antigen to be produced, and is thus a "silent" allele.