Answer: Codominance.
Explanation:
Codominance is a relationship between two parts of a gene. Offspring get one part of a gene, known as an allele from each parent. When the alleles are different, a dominant allele is usually expressed and the effect of the allele is usually covered or masked which is recessive. However, when Codominance occurs, neither of the alleles is recessive and phenotypes of both alleles are expressed.
In the above case, mating the yellow and white corn by the farmer resulted in offsprings that have nearly an even mix of both white and yellow corn. This is known as Codominance because no allele appeared to be recessive. The phenotype of both the alleles for yellow and white corn are expressed in the offsprings.