How to Use a Punnett Square to Predict Offspring Genotypes

If you read this lesson, you'll be able to set up and solve basic Punnett square problems to predict the possible genotypes and phenotypes of offspring from two parents. This is a key skill for genetics questions in school biology.

What Is a Punnett Square?

A Punnett square is a simple grid that helps you work out all the possible combinations of genes (alleles) that offspring can inherit from two parents. It's most often used for traits controlled by a single gene with two alleles (like brown vs. blue eyes).

Step-by-Step Example 1: Monohybrid Cross

Question: Two pea plants are crossed. Both are heterozygous for flower color (Pp), where P = purple (dominant), p = white (recessive). What are the possible genotypes and phenotypes of their offspring?

Step 1: Write Parent Genotypes

Both parents: Pp

Step 2: List Possible Gametes

Each parent can pass on either P or p.

Step 3: Draw the Punnett Square

Step 4: Fill in the Squares

• Top row: PP, Pp
• Bottom row: Pp, pp

Step 5: Analyze Results

• Genotypes: 1 PP, 2 Pp, 1 pp (ratio 1:2:1)
• Phenotypes: PP and Pp are purple (dominant), pp is white
• Phenotypic ratio: 3 purple : 1 white

Step-by-Step Example 2: Homozygous and Heterozygous Cross

Question: Cross a homozygous dominant (BB) plant with a homozygous recessive (bb) plant for seed shape (B = round, b = wrinkled). What are the offspring genotypes and phenotypes?

Step 1: Write Parent Genotypes

• Parent 1: BB
• Parent 2: bb

Step 2: List Possible Gametes

• Parent 1 can only give B
• Parent 2 can only give b

Step 3: Draw the Punnett Square

Step 4: Fill in the Squares

All squares: Bb

Step 5: Analyze Results

• All offspring genotype: Bb (heterozygous)
• All phenotype: round (since B is dominant)

Typical Mistakes and How to Avoid Them

• Forgetting to list all possible gametes: Always check both parents can give each allele.
• Mixing up dominant and recessive: Remember, dominant alleles show if present.
• Miscounting ratios: Double-check your square—there should be four boxes for a monohybrid cross.

Try It Yourself

Cross a heterozygous (Tt) tall plant with a homozygous recessive (tt) short plant. What are the possible genotypes and phenotypes of the offspring?

Real-Life Application

Punnett squares help predict the likelihood of inheriting genetic diseases or traits, such as cystic fibrosis or blood type.

Takeaway

Practice drawing Punnett squares with different parent combinations to master genetic prediction. For more step-by-step biology help, you can try the free TutorAI Solve tool after signing up.