How to Do the Punnett Square for a Dihybrid Cross

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    • 1). Assign a letter to both genes to be studied. In Mendel's classic experiment, R represented the dominant allele for round peas, while r represented the recessive allele for wrinkled peas. Similarly, Y was used to represent the dominant allele for yellow peas, while y represented the recessive allele, green peas. Write out the genotype of both parent organisms. In Mendel's example, a parent heterozygous in both genes would have the genotype RrYy.

    • 2). List off all possible gametes resulting from the parent organisms. In the above example, the heterozygous parent's possible gametes would be RY, Ry, rY and ry.

    • 3). Construct a table of 16 squares (four rows, four columns) with one parent's gametes on the x-axis and the other's on the y-axis.

    • 4). Fill in the table's blank squares with the combinations of alleles representing the offspring. For example, the combination of Ry and ry gametes would result in a Rryy offspring.

    • 5). Count the frequency of each phenotype to obtain a phenotypic ratio. Remember, a heterozygote will express the dominant allele. The offsprings' phenotypic ratio should be 9:3:3:1 (in the above example, 9 round, yellow peas, 3 round, green peas, 3 wrinkled, yellow peas and 1 wrinkled, green pea).

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