The familiar 3:1 phenotype ratio hides a 1:2:1 genotype ratio underneath, which is exactly why a test cross exists: it is the only way to tell whether that dominant-looking plant is AA or Aa. The more offspring are sown, the closer the counted ratios creep to their theoretical values, because Mendel’s laws are laws of probability rather than an exact split in every batch. A Punnett square lays out every gamete combination of a cross, and under incomplete dominance 3:1 becomes 1:2:1 as an intermediate class appears in the middle.
The familiar 3:1 phenotype ratio hides a 1:2:1 genotype ratio underneath, which is exactly why a test cross exists: it is the only way to tell whether that dominant-looking plant is AA or Aa. The more offspring are sown, the closer the counted ratios creep to their theoretical values, because Mendel’s laws are laws of probability rather than an exact split in every batch. A Punnett square lays out every gamete combination of a cross, and under incomplete dominance 3:1 becomes 1:2:1 as an intermediate class appears in the middle.
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