Predicting Inheritance: Identifying Epistasis
Epistasis
- In some cases, one gene can affect the expression of another gene
- Epistasis is when two genes on different chromosomes affect the same feature
- If epistasis is present it needs to be taken into account when determining the phenotypes of individuals
- The whole combination of alleles from the different genes dictates the phenotype
Worked example
Explaining epistasis
- There is a gene that dictates the feather colour of pigeons
- The gene has two alleles (R or r):
- Allele R codes for a pigment that produces grey feathers
- Allele r doesn’t produce a pigment, resulting in white feathers
- Another gene has also been found to have an effect on feather colour
- This gene has two alleles (F or f):
- Allele F codes for the production of an enzyme that stops grey feathers from being produced even if the allele R is present
- Allele f doesn’t produce an enzyme
- The possible phenotypes are, therefore, as follows:
- RRFF (white feathers)
- RrFF (white feathers)
- rrFF (white feathers)
- RRFf (white feathers)
- RrFf (white feathers)
- rrFf (white feathers)
- rrff (white feathers)
- RRff (grey feathers)
- Rrff (grey feathers)
Identifying epistasis from phenotypic ratios
- In the same way that a deviation from the expected phenotypic ratios suggests that there is linkage (i.e. the genes being inherited are linked), phenotypic ratios can also be used to identify if epistasis may be occurring
- The example that is given above (of feather colour in pigeons) can be used to demonstrate this by comparing the phenotypic ratios of offspring when epistasis is not occurring with the phenotypic ratios of offspring when epistasis is occurring
Without epistasis
- If there is no epistasis, the second gene does not affect the expression of the first gene
- If two heterozygous pigeons (RrFf) are crossed with each other and no epistasis is occurring (i.e. if the enzyme coded for by F does not affect feather colour), we would expect the following:
Parental phenotypes: grey feathers, produces enzyme x grey feathers, produces enzyme
Parental genotypes: RrFf x RrFf
Parental gametes: RF or Rf or rF or rf RF or Rf or rF or rf
- As allele F does not affect feather colour, the genotypes and phenotypes of offspring are as follows:
- RRFF (grey feathers)
- RrFF (grey feathers)
- rrFF (white feathers)
- RRFf (grey feathers)
- RrFf (grey feathers)
- rrFf (white feathers)
- rrff (white feathers)
- RRff (grey feathers)
- Rrff (grey feathers)
- Predicted ratio of phenotypes in offspring = 6 grey feathers : 3 white feathers
With epistasis
- If epistasis is occurring, the second gene does affect the expression of the first gene
- If the same dihybrid cross is carried out (two heterozygous pigeons (RrFf) are crossed with each other) and epistasis is occurring (i.e. if the enzyme coded for by F affects feather colour), we get a different phenotypic ratio
- In this case, we would expect the following genotypes and phenotypes of offspring:
- RRFF (white feathers)
- RrFF (white feathers)
- rrFF (white feathers)
- RRFf (white feathers)
- RrFf (white feathers)
- rrFf (white feathers)
- rrff (white feathers)
- RRff (grey feathers)
- Rrff (grey feathers)
- Predicted ratio of phenotypes in offspring = 2 grey feathers : 7 white feathers
- In this example, the occurrence of epistasis has greatly changed the phenotypic ratios of the offspring
- Unexpected phenotypic ratios can therefore be used to help identify whether or not epistasis may be occurring
