The answer is 1/2.
If:
R - the dominant allele for red kernels
r - the recessive allele for yellow kernels
Then:
RR - dominant homozygote
Rr - heterozygote
rr - recessive homozygote
The cross Rr x rr will result in 1/2 of offspring with red kernels (Rr) and 1/2 of offspring with yellow kernels (rr):
Parental generation: Rr x rr
Offspring generation: Rr Rr rr rr
So, there are 2 out of 4 Rr offspring (2/4 = 1/2) and 2 out of 4 rr offspring (2/4 = 1/2).