🎓 Prepared by students from Boğaziçi University

What is a Punnett Square and How Do You Use It?

A Punnett square is a grid that predicts offspring genotypes from parent alleles. It shows all possible combinations of gametes, helping you calculate inheritance ratios and understand genetic variation. Essential for predicting trait inheritance.

Short answer

A Punnett square uses parent alleles to show all possible offspring genotypes. For a monohybrid cross (one trait), list each parent's alleles on the grid axes, multiply rows and columns, and read the genotype ratios (e.g., 1:2:1 or 3:1 phenotype ratio).

How to Use a Punnett Square

1
Step 1: Write parent alleles
Identify each parent's genotype (e.g., Aa and Aa)
↓
2
Step 2: Set up the grid
Draw a 2×2 (monohybrid) or 4×4 (dihybrid) grid. Label rows and columns with parent alleles
↓
3
Step 3: Fill each box
Combine row and column alleles. E.g., A (row) + a (column) = Aa
↓
4
Step 4: Count genotypes
Tally: AA, Aa, aa. Count identical genotypes to find ratios
↓
5
Step 5: Determine phenotypes
Dominant allele = dominant phenotype. Recessive (aa) = recessive phenotype

Try it: interactive calculator

Dominant phenotype %

75%

= ((4 + 8) / 16) * 100

Dominant homozygous (AA) count4 boxesHeterozygous (Aa) count8 boxesTotal boxes in square16 boxes

Step-by-step worked examples

Complete a Punnett square for Aa × Aa. What are the genotype and phenotype ratios?

Grid (2×2):
       A    a
  A  | AA | Aa |
  a  | Aa | aa |

Genotypes: 1 AA : 2 Aa : 1 aa
Phenotypes: 3 dominant (AA, Aa) : 1 recessive (aa)
Ratio: 3:1

Cross AA (tall plant) with aa (short plant). What are all offspring?

Grid:
       A    A
  a  | Aa | Aa |
  a  | Aa | Aa |

All boxes: Aa (100% heterozygous)
All offspring are tall (dominant phenotype)
Genotype ratio: 0:4:0 (all Aa)

If both parents are Aa, what percentage of offspring will be homozygous?

Aa × Aa Punnett square:
1 AA + 1 aa = 2 homozygous out of 4
2/4 = 0.5 = 50%
Heterozygous (Aa) = 2/4 = 50%

Flashcards

Quick quiz

Q1.In a Punnett square, what do rows and columns represent?

Correct answer: B. Rows and columns show alleles each parent can pass. Their intersection shows offspring genotype.

Q2.If a Punnett square shows 16 boxes (4×4), how many traits are being analyzed?

Correct answer: B. 4×4 is a dihybrid cross (two traits). Monohybrid (one trait) is 2×2.

Q3.Tt × Tt Punnett square. What percentage are homozygous?

Correct answer: B. 1 TT + 1 tt (homozygous) = 2 out of 4. 2/4 = 50%.

Q4.AA × aa. What is the genotype of all offspring?

Correct answer: B. A from first parent + a from second parent = 100% Aa offspring (all heterozygous).

📄Download this topic as a printable worksheet (PDF)Summary + 10 questions + answer key — print it, share it in class.↓

Study better with Bounlu apps

Notek

The full card deck, worked steps and AI-tutor support for “What is a Punnett Square and How Do You Use It?” are in Notek — study by hand before your exam.

Get it free

PromyNever miss an exam or homework deadline with Promy: AI turns the dates in your inbox into reminders automatically.

Common mistakes

✗

Assuming a 2×2 Punnett square shows two traits. — Correct: A 2×2 square is monohybrid (one trait). Each parent has two alleles for that ONE trait.

✗

Adding up all genotypes instead of counting them. — Correct: Count identical genotypes in the grid, then write the ratio (e.g., 1 AA : 2 Aa : 1 aa).

✗

Confusing the grid labels. Parents' alleles are rows/columns, offspring are in the boxes. — Correct: Label rows and columns with parent alleles. Fill boxes with offspring genotypes by combining.

✗

The 3:1 ratio applies to all crosses. — Correct: 3:1 phenotype ratio appears in Aa × Aa (monohybrid). Different crosses give different ratios (1:1 for Aa × aa, 9:3:3:1 for dihybrid).

FAQ

What is a monohybrid vs. dihybrid cross?

Monohybrid tracks ONE trait (2×2 Punnett square). Dihybrid tracks TWO traits (4×4 grid). Dihybrid is more complex but follows the same principle.

Can I predict the exact offspring phenotype?

No, the Punnett square shows PROBABILITIES. A cross with 3:1 ratio means roughly 75% will be dominant phenotype, but any individual is unpredictable.

How do test crosses work?

A test cross is Aa × aa (or AA × aa). It pairs a dominant individual with a recessive to reveal hidden recessive alleles in the dominant parent.

What if I don't know a parent's full genotype?

Use available information: if they show a recessive phenotype, they are homozygous recessive (aa). If dominant phenotype, they could be AA or Aa—you may need more crosses to determine.