forked-line.html 03_10-forked-line.jpg

To generate phenotypic ratios in the F₂, consider the A/a, B/b, and C/c gene pairs separately. For example, for the A/a pair, the F₁ cross is Aa x Aa.

In the F₂, a phenotypic ratio of 3/4 A: 1/4 a, and a genotypic ratio (not shown here) of 1/4 AA:1/2 Aa:1/4 aa is produced, assuming independent assortment.

The same ratios apply to the BB x bb and CC x cc crosses.

The proportions of organisms expressing each phenotypic combination can be predicted by multiplying the individual probabilities, using the product law.