Punnett Square Method

The steps

1. Define the gene symbol: choose a capital letter for the dominant allele and the same letter in lower case for the recessive allele.
2. Write the genotype of each parent (e.g. Tt x Tt).
3. Work out the gametes each parent can produce by splitting its genotype into single alleles, and write each gamete inside a circle.
4. Draw a grid: write one parent's gametes across the top and the other parent's gametes down the left side.
5. Fill in each inner cell by combining the gamete of its row with the gamete of its column, this gives the offspring genotype for that cell.
6. Count the genotypes and phenotypes across all the cells and express the result as a ratio.

Worked example: Tt x Tt

Reading the grid

The four inner cells give the genotype ratio 1 TT : 2 Tt : 1 tt. Since TT and Tt are both tall (T is dominant) and tt is short, the phenotype ratio is 3 tall : 1 short.

Always read the ratio from the completed grid, not from memory, the ratio changes if the parents' genotypes are different, for example a test cross of Tt x tt gives a 1:1 ratio, not 3:1.

Setting up gametes correctly

The number of gamete types a parent produces equals 2 raised to the power of the number of heterozygous gene pairs. A monohybrid heterozygote (Tt) produces 2 gamete types; a dihybrid heterozygote (TtYy) produces 4 gamete types, so a dihybrid Punnett square needs a 4 x 4 grid instead of 2 x 2.

Worked problem 1, easy: RR x rr

Question. In pea plants, round seed (R) is dominant to wrinkled (r). A pure-breeding round-seeded plant is crossed with a wrinkled-seeded plant.

Give the parent genotypes, complete the Punnett square, and state the offspring phenotype.

Model answer. The parents are RR and rr. The round parent forms only R gametes and the wrinkled parent only r gametes, so every box is Rr.

The offspring are all heterozygous round: the genotype is all Rr and the phenotype is all round.

Worked problem 2, medium: Rr x rr

Question. One of the round Rr offspring above is crossed with a wrinkled (rr) plant. Predict the genotype and phenotype ratios of this second cross.

Model answer. The Rr parent forms two gamete types, R and r; the rr parent forms only r. Combining them gives a genotype ratio of 1 Rr : 1 rr and a phenotype ratio of 1 round : 1 wrinkled.

The reappearance of wrinkled seeds shows the round parent carried a hidden recessive allele.

Worked problem 3, SPM-level: incomplete dominance

Question. In a hypothetical flowering plant, flower colour shows incomplete dominance: genotype C^R C^R is red, C^W C^W is white, and the heterozygote C^R C^W is pink. Two pink plants are crossed.

Draw the Punnett square and give the genotype and phenotype ratios of the offspring.

Model answer. Each pink parent forms C^R and C^W gametes. The grid gives a genotype ratio of 1 C^R C^R : 2 C^R C^W : 1 C^W C^W.

Because neither allele is fully dominant, the phenotype ratio is the same as the genotype ratio: 1 red : 2 pink : 1 white. This is the point to remember, under incomplete dominance the heterozygote has its own appearance, so the 3 : 1 pattern does not apply.

Practice problems

• 1. In pea plants, purple flower (P) is dominant to white (p). A heterozygous purple plant is crossed with a white plant. Draw the Punnett square and state the phenotype ratio of the offspring.
• 2. A pure-breeding tall pea plant (T dominant) is crossed with a heterozygous tall plant. State the genotype ratio and the phenotype of the offspring.
• 3. A plant has the genotype TtRr. List all the gamete types it can form, and state how many boxes a Punnett square would need to cross it with another TtRr plant.

Practice answers

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions