Epistasis
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of key inheritance terms. You can test your knowledge on these below.
What is a genotype?
The genetic makeup of an organism - the alleles it possesses.
What is meant by a dihybrid cross?
A genetic cross involving two different genes, each with two alleles.
What is codominance?
Both alleles are expressed in the phenotype of a heterozygote.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains epistasis or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is Epistasis?
Epistasis occurs when the expression of one gene is affected by another gene at a different locus. One gene can mask or modify the expression of another, altering the expected phenotypic ratios in genetic crosses.
Epistasis can be:
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Recessive: Two copies of the epistatic allele are needed to mask the other gene.
- Dominant: Only one copy of the epistatic allele is needed to mask the other gene.
Recessive Epistasis
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The epistatic gene masks the effect of another gene when homozygous recessive (e.g. aa).
Example: Coat colour in mice
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Gene A: Determines pigment colour (A = agouti brown, aa = black)
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Gene B: Controls pigment production (B = pigment, bb = no pigment)
So, if bb is present, the mouse is albino, regardless of the A/a alleles.
Genotype → Phenotype:
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AaBb / AABB → agouti brown
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aaBb / aaBB → black
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aabb / AAbb / Aabb → albino (as they are homozygous recessive for Gene B, no pigment is produced)
Dominant Epistasis
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The epistatic gene masks the effect of another gene when at least one dominant allele is present.
Example: Fruit colour in squash
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Gene W: Controls pigment production (W = no pigment, ww = pigment)
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Gene Y: Determines pigment colour (Y = yellow, yy = green)
If W is present, the squash is white, regardless of Y/y alleles.
Genotype → Phenotype:
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wwYY / wwYy- → yellow
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wwyy → green
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WWYY / WWYy / WWyy/ WwYY / WwYy / Wwyy → white (The dominant W allele means no pigment is produced)
Using the flow diagram above, you can see that the presence of the W allele means the plant cannot produce Enzyme 1, this means they cannot produce compound B that makes the plant green.
If they cannot produce the green compound (B) it does not matter what alleles they have for the Y gene, as there will be no green compound present to convert to the yellow compound (C).
Key Terms
- Epistasis: Interaction between genes where one gene influences the expression of another gene.
- Recessive Epistasis: Epistasis occurs when two recessive alleles are needed to mask another gene.
- Dominant Epistasis: Epistasis occurs when one dominant allele masks another gene.
- Phenotypic Ratio: The ratio of different observable traits in offspring.
- Dihybrid Cross: A genetic cross examining the inheritance of two genes.
No answer provided.
Exam Tip
Be strategic with Punnett squares:
- Always define your alleles clearly.
- Use capital and lowercase letters consistently (e.g., A/a and B/b).
- When you spot non-Mendelian ratios (e.g., 9:3:4 or 12:3:1) that you did not expect, it may be that epistasis is occurring!
No answer provided.
Coat colour in mice is controlled by two genes, each with two alleles. The genes are on different chromosomes.
One gene controls the pigment colour. The presence of allele A results in a yellow and black banding pattern on individual hairs, producing an overall brown appearance called agouti. Mice with the genotype aa do not make the yellow pigment and are, therefore, black.
The other gene determines whether any pigment is produced. The allele D is required for development of coat colour. Mice with the genotype dd produce no pigment and are called albino.
a) What type of gene interaction is occurring between the two genes? Explain your answer. (2 marks)
b) Give all the possible genotypes for a black mouse. (1 mark)
a) epistasis
one gene influences the expression of another
b) aaDD or aaDd
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!