Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

My professor was talking about double mutant analysis with null mutants, and how double mutant analysis won't work with hypomorphs. I really don't understand the concept of double mutant analysis.

He was talking about pathways - if A and B are in the same pathway, then you would mutate A and he said something about the phenotype being identical. If A and B are in different pathways, you could mutate both of them and there would be a "stronger" phenotype.

He also said it would only work with null mutants, not with hypomorphs, because with hypomorphs some of the pathway would still be active, not completely shut off.

I really don't understand any of this - what a double mutant is, the whole pathway thing, why the phenotype is "identical" or "stronger", or why you use hypomorphs. I would really appreciate a rundown on the idea of double mutant analysis because my professor's explanation was so unclear. Thanks in advance.

share|improve this question
Do you mean why you DO NOT use hymomorphs ? –  biogirl Dec 9 '13 at 11:45
you may also want to see this –  biogirl Dec 9 '13 at 12:12

1 Answer 1

  • What is a double mutant ?

See this image.

In this mutant 1 has one mutation, mutant 2 has another mutation while the double mutant has "double" or 2 mutations.

Key : A,B means double mutation , A means mutation in A and B means mutation in B.

  • Types of double mutations

a)Additive phenotype: A, B = A + B Implies that A and B function independently and on different processes.
Eg. A = short roots, B = no root hairs A, B = short roots with no root hairs. Conclusion: A and B function independently on different aspects of root development (A is required for normal root growth and B is required for root hair development.

b) Epistatic phenotype: A, B = either A or B

i) If A = B = A, B then epistasis implies that both genes function in the same pathway and that each gene is essential to the process. Eg. A = short roots, B = short roots A, B = short roots. Both A and B are required for normal growth of roots.

ii) If A is opposite in phenotype to B, and A, B = A, then epistasis implies that A and B are in the same pathway and B is a negative regulator of A. Eg. A = long roots, B = short roots A, B = long roots. A is required to suppress root growth and B is a negative regulator of A.

c) Synergistic phenotype: A, B >>> A or B.

Implies that A and B contribute independently to the same process. Eg. A = long roots (wt = 2 cm, A = 4 cm) B = long roots (B = 4 cm) A, B = very long roots (20 cm). A and B negatively control root growth independently of one another and are performing partially redundant functions.

When your professor was saying that if A and B are in same pathway and you mutate A and phenotype comes as if both A and B have been mutated, then he was probably referring to the epistatic phenotype.

When he was talking about having a stronger effect by mutating them both, he was talking about Synergistic phenotype.

Source : Double mutant analysis

share|improve this answer
I do not know about the hypomorph thing. Maybe someone else will help you ! –  biogirl Dec 9 '13 at 12:06
Feel free to ask anything u do not understand. These things are tricky ! –  biogirl Dec 9 '13 at 12:07

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.