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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.

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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

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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

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