Two strains of true-breeding maize that both produce ears of corn with white kernels are crossed and found to produce F1 plants that all make ears of corn with red kernels. If these F1 plants are backcrossed to one of the parents, what proportion of the offspring should have white kernels

50% or 1/2

Explanation:

Since the two parental strains are true-breeding, that is, homozygous; it means that the red kerneled ears of corn offspring produced are heterozygous and one of the parental strains must have been homozygous recessive and the other homozygous dominant.

Assuming the ears of corn's colour is coded for by the allele A, it means that one of the parents has the genotype AA while the other has aa with the offspring having genotype Aa.

AA and aa genotypes produce white ears of corn while Aa genotype produce red ears of corn.

Thus, if Aa is backrossed to either of AA and aa:

Aa  x  AA = AA (white), AA (white), Aa (red) and Aa (red)

Aa x aa = Aa (red), Aa (red), aa (white) and aa (white)

Hence, the proportion of the offspring with white kernels will be 50% in each case.