Gregor Mendel came to understand the basics of trait inheritance by crossing a “pure” yellow pea breed with a “pure” green pea breed. The term pure simply means that this breed of peas only produces more peas of the same colour when cross-pollinated with itself. When these pure yellow and green peas where crossed, all of their offspring were yellow. What’s even more interesting, is that when this next generation was interbred, their offspring were 75% yellow and 25% green. So why did the green peas go missing for a whole generation?

To understand what is going on under the hood, let’s draw and fill in the Punnett squares for these 2 generations! As you can see, when crossing the 2 pure breeds, the resulting offspring are all heterozygotes, or carriers of the recessive green allele. However, since all the pea plants from the F1 generation are yellow, the yellow phenotype must be the dominant one.

When crossing the F1 generation with itself, we pair up 2 heterozygotes and the resulting offspring will be 25% homozygous dominant, 50% heterozygous and 25% homozygous recessive. The resulting phenotype will therefore be 75% yellow peas and 25% green peas.