Punnett squares

A grid used to determine genotype frequencies.

Example - plant height

  • T = tall allele
  • t = short allele

Purebred tall plants can be obtained by only allowing tall plants to breed with other tall plants for a period of time.

The same can be done with short plants to provide purebred short plants.

Crossing purebred tall (homozygous dominant – TT) and short parent plants (homozygous recessive – tt) results in the first generation (F1) offspring all being tall (heterozygous – Tt).

Genetics back cross 1

Example - eye colour

1:0 ratio

In these offspring the dominant allele (B) overrides the recessive allele (b), resulting in the offspring phenotype having brown eyes – a 1:0 ratio.

Punnett squares 1

1:1 ratio

Punnett squares 2

3:1 ratio

When both parents are heterozygous (Bb in this example) the offspring have a 75% chance of having brown eyes and a 25% chance of having blue eyes – a 3:1 ratio.

Punnett squares 3

To note:

  • Large numbers of offspring need to be produced to provide an accurate ratio. This is because fertilisation is random.