Epistasis:
Epistasis is a phenomenon in genetics where the expression of one gene masks or modifies the expression of another gene. In other words, the presence of one gene affects the phenotype produced by another gene.

Explanation:
When two nonallelic genes produce a new phenotype only when they are present together, but fail to do so independently, it is called epistasis. Let's understand this in detail:

1. Nonallelic Genes:
- Nonallelic genes refer to genes that are located on different chromosomes and do not have a direct genetic relationship.
- These genes can independently assort during meiosis and segregate into different gametes.

2. New Phenotype:
- The combination of the two nonallelic genes produces a new phenotype that is not observed when either gene is present alone or when they are present independently.
- This new phenotype is a result of the interaction between the two genes.

3. Failure to Produce Phenotype Independently:
- When the two genes are present independently, they do not produce the new phenotype.
- This means that the presence of one gene alone is not sufficient to produce the new phenotype.
- The interaction between the two genes is required for the new phenotype to be expressed.

4. Epistasis:
- Epistasis occurs when the expression of one gene interferes with or modifies the expression of another gene.
- In this case, the presence of one gene masks or suppresses the expression of the other gene, resulting in the failure to produce the new phenotype independently.

Example:
One example of epistasis is the coat color in Labrador Retrievers. The gene responsible for coat color has two alleles: B (black) and b (brown). Another gene, called the E gene, determines whether the pigment produced by the coat color gene is deposited in the hair or not. The E gene has two alleles: E (allows pigment deposition) and e (prevents pigment deposition).

- When both genes are present together (genotype BBEE or BbEE), the dog has a black coat.
- If the E gene is not present (genotype bbEE or bbEe), the dog has a yellow coat, regardless of the coat color gene.
- However, if the coat color gene is not present (genotype BBee or bbee), the dog will have a chocolate coat, regardless of the E gene.

In this example, the presence of one gene (E or B) is necessary for the expression of the other gene (B or E), resulting in the production of a specific coat color phenotype. This is an example of epistasis.

Epistasis is the phenomenon of suppression of phenotypic expression of gene by a non-allelic gene which shows its own effect The gene which masks the effect of another is called epistatic gene while the one which is suppressed is termed hypostatic gene. Epistasis is of three types - dominant, recessive and dominant-recessive.