In peas, the round allele is dominant over the wrinkled allele. If a plant with round peas is crossed to a plant with wrinkled peas, all of the resulting plants have round peas. What is the genotype of the parents in this cross?

A) RR × rr

B) RR × Rr

C) Rr × rr

D) Rr × Rr

E) rr × rr

a) RR x rr

Explanation:

First, we need to glean as much information as we can from the initial statement. If one of the plants is wrinkled, and ‘wrinkled’ is a recessive trait, then we know that that particular plant must have and homozygous recessive (rr) genotype. As for the other plant: Since it has a round phenotype, it may have either an ‘Rr’ (option C) or ’RR’ (option A) genotype. We may then build the corresponding Punnett charts:

Option C:

       |      r      |       r

R     |     Rr     |     Rr

r      |      rr     |      rr

Option A:

       |      r      |       r

R     |     Rr     |     Rr

R     |     Rr     |     Rr

As we can see, only option (A) will lead to the entirety of the descendants having a round phenotype.