Codominance occurs when both the alleles of a gene are expressed equally in heterozygous genotype. Here, none of the alleles of a gene masks the expression of the other. It is one of the deviations of Mendel's law of dominance and gives three different phenotypes in the F2 generation of a monohybrid cross.
For example, ABO blood type in humans is regulated by three alleles of a gene. The alleles I^A and I^B are codominant and are equally expressed in the genotype I^AI^B.
A is the answer. You would need soil with different amounts- including no amount- to determine how the fertilizer affects the plants Since you are measuring the height a ruler would be a good idea as well.
