Using Punnett squares
Cross each parent's alleles to find the chance of each offspring genotype.
The common mistake
Confusing genotype (Bb) with phenotype (brown eyes), or giving ratios that don't add to 4.
Instead: Put one parent's alleles on top and the other's on the side, fill all four boxes, then count genotypes and the traits they show.
Worked example with Guide me
Cross Bb × Bb (B = brown, dominant). What fraction will have blue eyes?
- Sugar · Fill the four boxes.Student · BB, Bb, Bb, bb.
- Sugar · Which box gives blue?Student · Only bb.
Answer: 1 in 4 (25%)
Try 3 practice questions
1. Bb × bb — chance of bb?
Show answer
50%
2. BB × bb — offspring genotypes?
Show answer
All Bb
3. Bb × Bb — ratio of dominant to recessive phenotype?
Show answer
3 : 1
Quick answers
- What is the most common mistake with using punnett squares?
- Confusing genotype (Bb) with phenotype (brown eyes), or giving ratios that don't add to 4.
- How do I get using punnett squares right?
- Put one parent's alleles on top and the other's on the side, fill all four boxes, then count genotypes and the traits they show.
- Can you show a worked example?
- Cross Bb × Bb (B = brown, dominant). What fraction will have blue eyes? 1 in 4 (25%)
Related concepts
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