For every trait, there are two types of characters - dominant and recessive which are determined by the combination of the alleles for a given gene.
Each gene has 2 types of alleles- one is the dominant allele which always expresses itself that is in both homozygous and heterozygous conditions whereas the other one is the recessive allele which can only express itself in the homozygous conditions.
In Mendel's experiments when he crossed plants with different traits he observed that in the F1 generation there was always a single trait that was getting expressed and it was the trait that was being coded by the dominant allele.
You would need two plants at the least, one that you kept out side of sunlight and one in sunlight. I don't know how you would figure out how much Carbon Dioxide is used but the plant in the sunlight i believe would be the control. <span />