Punnett Squares Predict Ratios, Not Individuals

Lab Brief #2

🎯 Your mission

Master the concept. Use it tomorrow.

⚡ The twist

Nature loves to surprise the careful watcher.

What you'll learn

Crossing two heterozygotes, Bb × Bb, predicts a 3:1 ratio of dominant to recessive phenotype — but that is a probability applying to each offspring independently, not a quota the family must fill. All four children of such a cross can show the recessive trait, because each birth is a fresh 1-in-4. Ratios only emerge reliably over large numbers, which is precisely why Mendel counted thousands of pea plants rather than a handful.

Key words

heterozygous
Carrying two different alleles of a gene, such as Bb.
probability
The chance of an outcome for each independent event, which only produces the predicted ratio over large numbers.

Make a hypothesis first

Before you start: write down what you think will happen, and why.

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "heterozygous" and "probability" mean in your own words before you start.

Step 2 of 4 · Do

Complete a Bb × Bb Punnett square, then test it by tossing two coins 20 times, treating heads as B and tails as b. Pool the class results and compare the small-sample ratio with the pooled ratio against the predicted 3:1.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A Bb × Bb cross predicts a phenotype ratio of:

How sure are you?😐 maybe

Question 2 of 3

Can all four children of a Bb × Bb cross show the recessive trait?

How sure are you?😐 maybe

Question 3 of 3

Mendel counted thousands of plants because predicted ratios only appear reliably in:

How sure are you?😐 maybe

Mind = blown

🔬 The more you look, the more nature shows you.

Nice work!

You got 0 of 3 right.

Next: Genotype, Phenotype and the Environment All lessons

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Punnett Squares Predict Ratios, Not Individuals

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