Question

2. In complementation tests, a) mutations that fail to complement are considered to be in the same gene and b) mutations that complement eachother are considered to be in different genes.

i. Explain (concisely) what the terms ‘fail to complement’ and ‘complement’ mean. An operation definition please, i.e. what data corresponds to each of these terms.

ii. Provide a biochemical explanation for the complementation test. i.e. why to mutations in different gene complement and why do mutations in the same gene fail to complement?

Answer 1

2) i) Complement means when 2 mutants are crossed the progeny has normal wildtype phenotype and fail to complement means the progeny of 2 mutant cannot produce the normal wildtype phenotype.

ii) if the 2 mutations were on the same gene, mutant 1 is homozygous for mutation 1 that is aaBB and mutant 2 is homozygous mutant for the other gene AAbb, so when we cross this the progeny is AaBb, hybrid having functional alleles of both genes,

so if the pathway is a white pigment is converted to another white intermediate by gene A and the intermediate is converted to purple pigment, which is required for purple coloured flowers by another gene B then aaBB is a white mutant plant and AAbb is another white mutant then if these plants are crossed, progeny is AaBb, hybrid having functional alleles of both genes, so have purple flowers.

If a and a` are different mutant alleles of gene A then a`a`BB and aaBB are both white plants so their progeny is a`aBB, this lacks functional A gene so purple pigment cannot be produced so mutations of the same gene fail to complement.