Punnett squares are probability tables

A Punnett square does not predict the exact order of children. It describes probabilities for each independent offspring.

Use this post as a small study note for making better decisions while solving, not as a quick definition dump. Read the idea, pause at the example and try to move one line forward yourself, then use the checklist on a real problem.

The main idea

Put one parent's gametes across the top and the other's down the side, then fill combinations carefully.

The important part is not memorizing the method; it is noticing what calls the method into action. Which expression, diagram, unit, variable, or code behavior made this approach the right one? If you can name that trigger, a similar problem becomes much less stressful.

A short example

For $Aa\times Aa$, the genotype ratio is $1AA:2Aa:1aa$ and the dominant phenotype chance is $\frac{3}{4}$.

How to approach it step by step

1. Write the givens cleanly and fix units, symbols, or variable names before calculating.
2. Identify the intermediate fact you need before trying to jump directly to the answer.
3. After each line, run a small check: do the units match, is the sign correct, is indentation or scope correct in code?
4. When you get an answer, return to the original question and check that it answers exactly what was asked.

Check while you solve

• Separate genotype from phenotype.
• Treat each child as a new probability event.
• Use ratios only after all boxes are filled.

The common mistake

A 25% chance does not mean exactly one out of every four children in a real family.

The practical way to catch this mistake is to check the decision points, not only the final answer. Ask questions like: Why this formula? Why this component? Why this loop condition? That makes the answer stronger in content, not just in arithmetic.