Why is it important that Mendel began with purebred plant?

Answer 1

The correct term is "true-breeding". What that means is that if he takes his two pea plants with white flowers and breeds them together, he will always get a pea plant with white flowers. Something that is true-breeding for a particular trait is homozygous, i.e. if the allele for red flowers is R and the allele for white flowers is w, then a true-breeding white flowering plant is ww, and true-breeding red flowering plant is RR.

If you cross-breed a true-breeding red flowering with a true-breeding white flowering plant, you would get 1/4 of the offspring as true-breeding red flowers, 1/4 of the off-spring as true-breeding white flowers, and 1/2 the offspring as heterozygous (not true-breeding) red flowers - Rw. If you don't start with true-breeding plants - say you start with Rw and ww (a red and a white plant) you get 1/2 the offspring heterozygous red, and 1/2 true-breeding white. Thus if you didn't know anymore, you would assume that half the time when you breed a red and a white plant, you would get a red plant, and half the time a white, which is incorrect. Furthermore, if you conducted the experiment again, say with RR and Rw, you would get a different result (in this case, all red). By starting with plants that are true-breeding, you ensure that you get the same results that properly show how the traits are passed on.