Question

A researcher has four different true-breeding strains of pea plants; all of which have white flowers. They know that wild type plants make purple flowers, and that each of their four strains is only mutated in one locus in the pathway that produces the purple pigment. They attempt to determine if these four strains are mutated in the same gene or different genes from each other by preforming complementation tests. To do this, they cross every strain with every other strain, and get the following offspring:

Strain 1 x strain 2 gives purple offspring

Strain 1 x strain 3 gives white offspring

Strain 1 x strain 4 gives purple offspring

Strain 2 x strain 3 gives purple offspring

Strain 2 x strain 4 gives white offspring

Strain 3 x strain 4 gives purple offspring

How many different genes (complementation groups) are mutated in the four strains?

Answer 1

Hi

The question needs you to find out whether the mutations are from same gene, if not, then how many genes are involved? This is called complementation. All alleles of one gene belong to one complement group. The logic to solve such question is, to know that alleles of same gene cannot complement each other to give wild type. Only when two different genes come together they complement each other and give rise to wild type phenotype.
In this question, the wild type phenotype is Purple color. The mutant color is white. So when two mutant crosses yield a purple color, their mutations are present in two different genes, thus different comlementation grous. if cross yields white colour then those strains belong to same group.
complementation group 1: strain 1, strain 3
group 2: strain 2 and 4
so 2 genes or groups.