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Punnett Square - Part 2

Click here for Part 1

Now let's extend our analysis to predict the results of a cross between a dihybrid pea with green, smooth seeds (GgNn) and a homozygous recessive pea with yellow, wrinkled seeds (ggnn). Again, we place all the possible gametes from one parent along the top of our square, and all the possible gametes from the other parent along its left edge. Remember from Mendel's Law of Independent Assortment that each allele from one gene is equally likely to be found in a gamete with each allele from the other gene. Thus, each parent can make four possible gametes, and our Punnett Square will have 16 boxes.

Filling in the boxes as we did in the single gene cross results in the prediction of a 1/4 GgNn (green, smooth): 1/4 Ggnn (green, wrinkled): 1/4 ggNn (yellow, smooth): 1/4 ggnn (yellow, wrinkled) ratio in the progeny.