Epistasis
Epistasis is a genetic interaction where one gene (called the epistatic gene)
masks or modifies the effect of another gene (called the hypostatic gene).
Unlike simple dominance, where alleles of the same gene influence a trait,
epistasis occurs between different genes, affecting how certain traits are expressed.
Types of
Epistasis
There are several types of epistasis, each with different interactions between
the genes involved:
- Recessive
Epistasis
In recessive epistasis, two
copies of the recessive allele of one gene can mask the expression of another
gene.
Example: Coat
color in Labrador retrievers. The coat color is influenced by two genes: one
gene (B) determines black or brown color, while another gene (E) controls
pigment deposition. If a dog has two copies of the recessive allele (ee), it
will have a yellow coat, regardless of whether the B gene is for black or brown
coat color.
- Dominant
Epistasis
In dominant epistasis, a single
dominant allele of one gene can mask the effect of another gene.
Example: Color
of squash. The color is determined by two genes, where a dominant allele (W)
suppresses the expression of the other color gene (Y). If the W allele is
present, the squash is white, regardless of the alleles at the Y locus.
- Duplicate
Recessive Epistasis (Complementary Gene Interaction)
In this form, two recessive
alleles at either of two different loci can prevent the expression of a trait.
This interaction often results in a 9:7 phenotypic ratio in the offspring.
Example: Flower
color in sweet peas. Two genes are needed to produce purple flowers, but if
either gene has two recessive alleles (aa or bb), the flowers will be white due
to the lack of pigment production.
- Duplicate
Dominant Epistasis
Here, a dominant allele of either
of two genes is sufficient to produce the same phenotype, resulting in a 15:1
ratio.
Example: Fruit
shape in certain plants, where the presence of a dominant allele at either of
the two loci (A or B) results in a particular shape, even if only one dominant
allele is present at one of the loci.
- Polymeric
Gene Interaction
In this case, two genes interact
additively to enhance the trait. When both genes have dominant alleles, they
produce a cumulative effect, resulting in a distinct phenotype.
Example: Kernel
color in wheat, where the presence of two dominant alleles produces a deeper
color compared to the color produced by each gene individually.
Genetic
Ratios in Epistasis
Epistasis can alter Mendelian phenotypic ratios due to the interaction of
different genes. Typical modified ratios seen in epistatic interactions
include:
9:3:4 for recessive epistasis
12:3:1 for dominant epistasis
9:7 for duplicate recessive
epistasis
15:1 for duplicate dominant epistasis
Significance of Epistasis
- Genetic
Variation: Epistasis contributes to genetic diversity
by modifying phenotypic expressions and creating unique trait
combinations.
- Evolution
and Adaptation: Epistatic interactions play a role in
evolution, as certain gene combinations may be advantageous and thus more
likely to be passed on.
- Complex
Traits: Many complex traits, such as susceptibility
to diseases, are influenced by multiple interacting genes. Understanding
epistasis helps in studying polygenic traits and diseases.
- Agriculture
and Breeding: Knowledge of epistasis is useful in
selective breeding and genetic modification, where specific gene
interactions are utilized to produce desired traits in plants and animals.
