A pea plant that has round seeds has the genotype Rr. It crossed with a pea plant that has wrinkled seeds and the genotype rr. What is the probability that the offspring will have wrinkled seeds

1/2.

the gene Rr separate to R and r.

the gene rr separate to r and r.

then they mix, turn to

Rr, Rr, rr, rr.

because wrinkled gene r is recessive, so only rr would be wrinkled. the probability would be 2/4=1/2.