A grid used to determine genotype frequencies.
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).
In these offspring the dominant allele (B) overrides the recessive allele (b), resulting in the offspring phenotype having brown eyes – a 1:0 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.