6 The Punnett square

A Punnett square is a tool for predicting the possible genotypes and phenotypes of the offspring of a particular cross.

How to build one:

1. Work out the gametes each parent can make; the number of boxes depends on the number of gamete types.
2. Draw a grid of rows and columns.
3. Write the gametes of one parent across the top, and the gametes of the other parent down the left side.
4. Fill each box by combining the allele above with the allele at the side.
5. Count genotypes and phenotypes and give the ratio or percentage.

Worked example 1: purple × white (Check your understanding)

Purple (P) is dominant over white (p). Cross pure purple PP × white pp. Gametes: P and p. F₁: all Pp, purple (100%). F₁ × F₁: Pp × Pp

F₂: 1 PP : 2 Pp : 1 pp, i.e. 3 purple : 1 white (75% purple, 25% white).

Worked example 2: a heterozygous axial plant × a terminal plant

A = axial (dominant), a = terminal. Cross Aa × aa. Gametes: A and a (from Aa); a only (from aa).

Offspring: 2 Aa : 2 aa → 50% axial (Aa), 50% terminal (aa); phenotype ratio 1 : 1. Such a cross (with a homozygous recessive) is useful for finding out whether a dominant-looking plant is pure or hybrid.

Worked example 3: unknown purity

A tall pea plant (T = tall, t = short) is crossed with a short plant. Half of the offspring are short. What is the genotype of the tall parent? Short offspring are tt, so each parent gave a t; so the tall parent must be Tt (heterozygous). If the tall parent had been TT, all offspring would have been tall.