Double mutant analysis with null mutants
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