Epistasis

You may be familiar with Mendel’s laws of inheritance and how they influence an organism’s phenotype. Yet, we see variations in height, color, and other traits in organisms belonging to the same species. Why does this occur? Why are some traits expressed while others aren’t?

Some genetic traits don’t follow Mendel’s laws of inheritance. A trait is sometimes controlled by more than 1 gene. Eye color is an example of this, as it is controlled by over 8 genes. Here, we will discuss epistasis, or how some genes control the expression of other genes.

Epistasis Definition

There are cases when a gene may oppose the expression of another gene, or sometimes a gene may not be expressed without another. This is how the genes in our bodies interact with one another to express a trait. We call this epistasis.

The word “epistasis” has Greek roots that essentially mean “standing upon.” The gene that is masked by the epistatic gene is called hypostatic. It was earlier thought that genes used to work independently, but now, it is well known that almost all genes interact epistatically.

These gene interactions are why there are continuous variations in the characteristics of plants and animals because there are multiple genes involved for each trait expressed.

Epistatic gene

The epistatic gene is the gene that determines whether a particular trait will be expressed. A common way to understand is by taking the example of the albinism gene in humans.

Before discussing how epistasis works and the ways it can be expressed, it is important for you to know what dominant and recessive alleles are.

Dominant alleles get expressed even if there is only one copy of the allele, whereas, recessive alleles need two copies to get expressed. This way, epistasis can either be dominant or recessive.

Is epistasis the same as dominance?

Epistasis is the relationship between alleles of two different genes. This means that the alleles at one locus can either be dominant or even recessive to the alleles at the second locus. Dominance, on the other hand, defines the relationship between two different alleles on the same gene. If one allele on a gene is dominant to the other, the recessive allele will not be expressed unless the gene is homozygous recessive.

Dominant Epistasis

Dominant Epistasis Example

Have you ever eaten a squash? This fruit comes in many colors, white, yellow, and sometimes green. There are a lot of genes that control the color of a squash. This genetic inheritance is called polygenic inheritance.

Squash color is also a heterozygous trait, meaning, it has two different alleles. Let’s consider yellow color (G) to be dominant over green color (g). Another gene at a second locus has a White color gene (W) and is considered dominant over no change (w). The gene at the second locus controls the expression of the gene at the first locus. This is dominant epistasis and the dominant allele (W) at the second locus is epistatic to alleles at the first locus (G/g)

In this case, the dominant allele at the second locus can mask the expression of the gene at the first locus, even if the alleles are dominant. A dihybrid cross between WwGg and WwGg shows the following results (Table 1).

Table 1. Shows the dihybrid cross between two squash plants with genotype WwGg. Y = Yellow, W = White, G = Green. (12/16) squash plants are white, (3/16) are yellow, and (1/16) are green. StudySmarter Original

Dominant epistasis phenotypic ratio

You can deduce from the above table that the phenotypic ratio in the case of dominant epistasis is 12:3:1. This is because when the dominant allele (W) at the second locus is expressed, it inhibits the expression of the alleles (G or g) even when the allele at the first locus is dominant. The only time the gene at the first locus will be expressed is when the genotype at the second locus is homozygous recessive (ww).

Therefore, from the above table, you can see that there would be 9 white, 3 yellow, and one green squash. This is how dominant epistasis works. Now you know why there are so many colors of squash growing in a field! You should note that this type of gene interaction is also observed in cases of the color coat of barley and skin color in mice.

Recessive Epistasis

Recessive epistasis, on the other hand, occurs when a pair of homozygous recessive alleles at one locus mask the expression of alleles at another locus. To understand how this works, let’s take the example of Labradors!

Recessive Epistasis Example

Who doesn’t love Labradors? Labradors are a breed of dogs that have different fur coat colors, black, brown, or sometimes yellow. These coat color variations are a result of recessive epistasis.

Figure 1. Recessive epistasis affects the fur coat of Labradors and as a result, the Labrador may have black, brown, or white fur Credit: Jeremy Seto

Let’s consider Labradors with black fur dominant to Labradors with brown fur. Let’s assign B allele to black fur and b allele to brown fur. A Labrador with black fur has BB/Bb genotype and a Labrador with brown fur has bb genotype.

There is another gene at the second locus which decides whether this pigment will be delivered in the fur. E denotes the dominant allele, and the recessive allele is denoted by e. If the gene at this locus is homozygous recessive (ee), then the pigment will not be delivered and as a result, the fur will be yellow.

This is recessive epistasis, as the homozygous recessive gene at the second locus will mask the expression of the gene at the first locus, regardless if the genotype is dominant (BB/Bb) or recessive (bb). Drawing a dihybrid cross between BbEe and BbEe gives us the following results (Table 2).

Table 2. Shows a dihybrid cross between Labradors of genotype BbEe. Drawing a Punnett square, we get the results: (9/16) will have black fur, (3/16) will have brown fur and (4/16) will have yellow fur. StudySmarter Original

Recessive epistasis phenotypic ratio

The table above clearly shows that the phenotypic ratio for recessive epistasis is 9:3:3:1. The reason is the gene at the first locus will always be expressed as long as the gene at the second locus isn’t homozygous recessive (ee). In the case of a homozygous recessive gene expression, the pigment will not be deposited in the fur and the Labrador will have yellow fur.

Therefore, Table 2 shows that there would be 9 black, 3 brown, and 4 yellow Labradors. Recessive epistasis is why there are so many variations in the fur coat color of Labradors.