Punnett squares for breeders: predicting trait ratios

A Punnett square is a 2×2 grid that predicts the offspring of a single-gene cross. It's the breeder's back-of-the-napkin math, and it explains why you see what you see in the tent.

The 3:1 ratio

Cross two heterozygotes (Aa × Aa) — say, two plants both carrying one purple allele and one green allele. Their seeds split:

Three out of four offspring show the dominant trait. One in four shows the recessive trait. That's the 3:1 ratio, and it's why you're never quite sure which seedling is going to surprise you.