complementation test

Question

What is a complementation test? When two mutants complement
each other, that is, when you recover the wild type, does this mean
that they are on the same locus, or in different loci?

Answer 1

Hi, Ashley! "Complementation" is the genetic form of the hokey romantic phrase "you complete me". Two strains are said to complement one another if, in combination, they produce a whole/complete/wildtype phenotype or pathway. If you think about it, the only way they can 'cover' each others' defects is if each brings what the other is missing. Consider two possibilities, each discussing two genes, A and B.

CASE I: If organism #1 is defective for gene A (but OK for gene B) and organism #2 is also defective for gene A (but OK for gene B), then combining their resources (such as occurs when making a diploid from two haploids) does them no good--the resulting individual has two bad copies of A and no good ones. These are said "not to complement", and it's generally an indication that they had the SAME defect, or defects in the SAME gene.

 

Case II: If organism #1 is defective for gene A (but OK for gene B) whereas organism #2 is defective for gene B (but OK for gene A) then the combination of them has one good copy of gene A and one good copy of gene B, and everything is fixed!

 

If you think about it, failure to complement is generally more informative than complementation, because it means you have definite information: the two strains in question have the SAME gene defective. If two strains fail to complement, you know that have different defects... but no idea which ones those are.

 

In teaching, I always compare this to the nursery rhyme about Jack Sprat and his wife--"...but between the two of them they licked the platter clean"--Jack and Wife 'complemented' each other to complete the dinner!