Punnett squares
A grid used to determine genotype frequencies.
Example
Gene:
- Plant height
Alleles:
- Tall = T
- Small = t
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 鈥 TT) and short parent plants (homozygous recessive 鈥 tt) results in the first generation (F1) offspring all being tall (heterozygous 鈥 Tt).
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 鈥 a 1:0 ratio.
1:1 ratio
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 鈥 a 3:1 ratio.
To note:
- Large numbers of offspring need to be produced to provide an accurate ratio. This is because fertilisation is random.