If the height of pea plants is controlled by a single gene, where the tall allele, T, is dominant to the short allele, t, how many different possible phenotypes of height would exist in total?

The correct answer is - two

Explanation:

If the height of pea plant is controlled by a single gene, which has one dominant tall allele (T) and one recessive short allele (t) then only two different types of phenotypes of height can exist in total because the height trait is controlled by only two type of allele.

T t

T TT Tt

t Tt tt

The phenotype can be tall or short. The genotype can be of three type TT, Tt, and tt. TT and tt genotype will produce tall offspring and tt genotype will produce short offsprings. So the right answer is two.