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First, you need to know the genotypes of the parents involved in the cross. For arguement's sake, we'll say they're both heterozygous for both traits.
Next you want to assign letters to stand for the different alleles. The convention is to use the first letter of the dominant trait, and uppercase for the dominant, and lower case for the recessive. So at your first locus, the dominant trait is blue, so the first allele would be B if for the dominant phenotype, and b for the recessive. For the second locus, yellow is dominant, so you'd have Y (yellow) and y (blue) as your alleles.
So assuming both parents are heterozygous, your cross would be BbYy x BbYy.
Next, you want to determine the possible combinations of alleles which could be found in th gametes produced. Since you have two copies of each allele, your plant would be diploid, and you're looking at the results of two independent genes. To figure out how many types of gametes (allele combinations) are possible, raise the number 2 (for a diploid organism) to the power of the number of traits. In your poblem, there are two traits, so the number of possible alleles is 2 ^2, or 4.
What are the possible combinations is the next step. This is where you use FOIL (First, Outside, Inside, and Last pairs of alleles). For a parent with BbYy, the first set of each trait would be "BY". The outside would give you "By". The inside would be "bY", and the last would be "by". Remember each gamete must have a copy of an allele for each trait, you couldn't have two "B"s or two "Y"s, because then your plant would be missing information about how to reproduce that trait.
So now, you set up a Punnett square, but since you have four possible gametes, your square will be four blocks wide and four high. Because both parents are assumed here to be heterozygous, both parents will have the same possible gametes listed above:
........|..BY..|..By..|..bY..|..by..|
..BY..|
..By..|
..bY..|
..by..|
You can fill in the blanks your self. To check your work, your ratio should be a phenotypic ratio of 10 green:3 blue:3 yellow. The typical ratio of a heterozygous cross is 9:3:3:1, but if both blue and yellow dominant produce green, I'm assuming when both traits are recessive (the "1"), this one will be green as well. See if you can get this answer yourself.
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