Punnett Square Practice Problems with Answer Key

What Are Punnett Squares and Why You Need to Master Them

Punnett squares are the most straightforward tool in genetics for predicting offspring genotypes from parental crosses. If you're taking biology, this is a foundational skill you cannot skip. Period.

Most students either overthink them or rush through without understanding the logic. This guide fixes that. You'll get real practice problems with clear explanations and a complete answer key at the end.

Punnett Square Basics: A Quick Refresher

Before jumping into problems, make sure you understand these core concepts:

• Genotype — the genetic makeup (letters like AA, Aa, or aa)
• Phenotype — the physical trait you can see (like tall or short)
• Homozygous — both alleles are the same (AA or aa)
• Heterozygous — alleles are different (Aa)
• Dominant allele — masks the recessive allele (capital letter)
• Recessive allele — only shows when paired with another recessive (lowercase)

How to Set Up a Punnett Square

It's simple. Four steps:

1. Write one parent's alleles across the top (one per column)
2. Write the other parent's alleles down the side (one per row)
3. Fill each box by combining the column and row alleles
4. Read the results — count ratios if asked

Practice Problems: Easy Level

Start here if you're warming up or need a confidence boost.

Problem 1: Simple Dominant-Recessive Cross

In pea plants, tall height (T) is dominant over short height (t). Cross a heterozygous tall plant (Tt) with a homozygous tall plant (TT).

Draw the Punnett square and answer:

• What are the possible genotypes of the offspring?
• What are the possible phenotypes?
• What is the genotypic ratio?

Problem 2: Heterozygous x Heterozygous

In humans, free earlobes (F) are dominant over attached earlobes (f). Cross two heterozygous individuals.

Find the:

• Possible genotypes of offspring
• Phenotypic ratio
• Probability of a child with attached earlobes

Problem 3: Homozygous Recessive Cross

In snapdragons, red flowers (R) show incomplete dominance with white flowers (r). Heterozygous plants (Rr) have pink flowers. Cross two pink-flowered plants.

What phenotypes appear in the offspring and in what ratio?

Practice Problems: Intermediate Level

These require more careful analysis and understanding of dominance patterns.

Problem 4: Codominance Cross

In cattle, red coat color (R) and white coat color (r) are codominant. Heterozygous cattle are roan (Rr). Cross a roan bull with a roan cow.

Give the genotypic and phenotypic ratios of the calves.

Problem 5: Dihybrid Cross Setup

In garden peas, round seeds (R) are dominant over wrinkled (r), and yellow seeds (Y) are dominant over green (y). Cross RrYy with RrYy.

This one is tricky. You'll need a 16-box Punnett square. Try it, then check your work against the answer key below.

Problem 6: Sex-Linked Trait

Red-green color blindness is caused by a recessive allele (b) on the X chromosome. A carrier female (X^BX^b) marries a normal male (X^BY).

What percentage of their sons will be color blind? What percentage of daughters?

Practice Problems: Advanced Level

If you can handle these, you've got the concept locked down.

Problem 7: Multiple Alleles (Blood Types)

Human blood types involve three alleles: I^A, I^B, and i. I^A and I^B are codominant; both are dominant over i.

A father has type AB blood. A mother has type O blood. What blood types can their children have?

Problem 8: Dihybrid with Linked Genes (Conceptual)

Two genes are linked on the same chromosome: flower color (P = purple, p = white) and seed shape (R = round, r = wrinkled). Parent genotype: PpRr x pprr.

If the parental (non-recombinant) gametes are most common, what phenotypes appear in offspring and why is the ratio not the standard 9:3:3:1?

Answer Key

Problem 1 Answers

Parent 1 (across): T, t
Parent 2 (down): T, T

Possible genotypes: TT, Tt
Phenotypes: All tall
Genotypic ratio: 1 TT : 3 Tt

Problem 2 Answers

Genotypic ratio: 1 FF : 2 Ff : 1 ff
Phenotypic ratio: 3 free earlobes : 1 attached earlobes
Probability of attached earlobes: 25%

Problem 3 Answers

This is incomplete dominance, so neither allele dominates completely.

Phenotypic ratio: 1 red : 2 pink : 1 white

Problem 4 Answers

Genotypic ratio: 1 RR : 2 Rr : 1 rr
Phenotypic ratio: 1 red : 2 roan : 1 white

Problem 5 Answers

For the dihybrid cross RrYy x RrYy, the standard 9:3:3:1 ratio applies:

Problem 6 Answers

X^BX^b (carrier female) x X^BY (normal male)

50% of sons will be color blind. 0% of daughters will be color blind (though 50% will be carriers).

Problem 7 Answers

Father: I^AI^B (type AB)
Mother: ii (type O)

Children can only be type A or type B. Neither can be AB or O.

Problem 8 Answers

Linked genes don't assort independently. Instead of the 9:3:3:1 ratio, you get mostly parental types and fewer recombinants.

With parental types being PR and pr, and recombinants being Pr and pR, offspring would show:

• ~50% purple, round (parental)
• ~50% white, wrinkled (parental)
• Very few purple, wrinkled or white, round (recombinants)

The actual ratio depends on the recombination frequency between the genes.

Dominance Patterns: Quick Comparison

Know which pattern you're dealing with before you start. This matters more than most students realize.

How to Solve Any Punnett Square Problem

Follow this checklist every single time. No exceptions.

1. Identify the trait — What characteristic are you tracking?
2. Assign letters — Dominant gets capital, recessive gets lowercase
3. Determine parental genotypes — Are they homozygous or heterozygous?
4. Identify the dominance pattern — Complete, incomplete, codominant?
5. Set up the square correctly — One parent across, one down
6. Fill every box — No skipping
7. Count and calculate ratios — Genotypic and phenotypic
8. Answer the specific question — Don't give extra information

Common Mistakes That Will Cost You Points

• Mixing up genotype and phenotype — Genotype is the letters, phenotype is what you see
• Forgetting incomplete or codominance — If heterozygous shows something different, note that
• Setting up the square wrong — Alleles go on the outside, combinations go inside
• Not simplifying ratios — 2:2 becomes 1:1
• Ignoring sex-linked notation — X and Y chromosomes need superscripts

These errors are 100% preventable. Read the problem twice before you start. Draw your square neatly. Double-check your work.

When to Use a 4-Box vs. 16-Box Square

Most crosses use a 4-box Punnett square. You only need a 16-box square for dihybrid crosses — when tracking two traits simultaneously.

For monohybrid crosses (one trait), always stick with 4 boxes. For dihybrid crosses, you need all 16. Don't waste time building a 16-box square if the question only asks about one trait.