"9-3-3-1" Ratio

Please note that bearing only four children when crossing two traits is technically incorrect. When you cross one trait, you'll have 2²=4 distinct results. When you cross two traits, you'll have 2⁴=16 distinct results:

The 9:3:3:1 ratio resulting when crossing two traits.

However, for simplicity and ease of use, only four children are shown when crossing peas in the Pea Experiment.