In pea plants, the allele for round peas (R) is dominant to the allele for wrinkled peas (r). A gardener has a plant he got from his neighbor that produces round peas, but he does not know its genotype. What simple procedure could he do to determine the genotype of his new plant

The simplest procedure is to perform a cross test with wrinkled pea plant. If winked pea trait appears in the offspring then it shows that the new plant's genotype is Rr.

a simple test crossing with a wrinkled pea

Explanation:

To detect the underlying genotype of an organism with a dominant phenotype specifically, TEST CROSSING a type of breeding analysis is done. The test cross is one of fundamental tool used and devised by Gregor Mendel.

your answer is "the 1950's"