Predicting Inheritance: Identifying Epistasis (OCR A Level Biology): Revision Note

Predicting inheritance: identifying epistasis

Epistasis

• Epistasis occurs when two genes on different chromosomes affect the same feature

  • One gene affects the expression of another gene

• In the same way that a deviation from expected phenotypic ratios suggests that genes may be linked, phenotypic ratios can also be used to identify epistasis

• In a standard dihybrid cross between two double heterozygous parents we would expect to see a ratio of 9:3:3:1 in the offspring, but when epistasis is taking place the offspring ratios will differ from this; we might expect ratios such as:

  • 9:3:4

  • 12:3:1

  • 9:7

  • 13:3

Example: epistasis in pigeons

• There are two genes involved in determining feather colour in pigeons; gene R/r and gene F/f

  • For gene R/r:

    • allele R codes for a pigment that produces grey feathers

    • allele r doesn’t produce a pigment, resulting in white feathers

  • For gene F/f:

    • allele F codes for the production of an enzyme that stops production of grey pigment even if the allele R is present

    • allele f doesn’t produce an enzyme

• This means that, for example:

  • Genotypes RRff and Rrff = grey feathers

  • Genotype rrff = white feathers

  • Any genotype that contains the allele F will result in white feathers, regardless of R/r, e.g.

    • RRFF

    • rrFF

    • RrFf