Each box of a Punnett square is one equally likely outcome. That turns genetics into the theoretical probability you know from math class.
Key idea
Chance of a genotype or phenotype = (number of boxes with it) ÷ 4. One box out of four is 1/4, or 25%; three boxes is 3/4, or 75%. The chance applies to each offspring separately.
Why it works
Each offspring is a new, independent draw, like a new roll of a die. A 1/4 chance of a white-flowered plant does not mean one plant in every four will be white. In a small family or litter the count can easily differ from the ratio. With hundreds of offspring, the share gets close to 25%. Mendel counted thousands of pea plants for this reason.
Worked example: Read the chance
Two Pp plants are crossed. What is the chance that one offspring has white flowers (pp)?
- The square has PP, Pp, Pp, pp.
- Boxes with pp: 1 of 4.
Answer: 1/4, or 25%.
Worked example: Expected count
From the same cross, a gardener grows 200 seeds. About how many plants will have purple flowers?
- Purple boxes: PP, Pp, Pp = 3 of 4.
- 3/4 × 200 = 150.
Answer: About 150 (the real count will be near 150, not exactly).
Worked example: Each one is new
Two Bb guinea pigs have had three black pups. What is the chance the fourth is white?
- Each pup is a new draw from the same square.
- White (bb) is 1 box of 4. Past pups do not change it.
Answer: 1/4, or 25%.
Common mistake
Students think a 3:1 ratio means exactly 3 of every 4 offspring will show the trait, or that after three black pups a white one is "due." Each offspring is a new chance. Ratios describe what to expect on average over many offspring.