Punnett squares are based on the principles of Mendelian inheritance, which state that each individual has two copies of each gene, one inherited from each parent. The genotype of an individual is the combination of the two alleles that they have for a particular gene.
To construct a Punnett square, the alleles of each parent are written along the top and side of the square. The possible offspring genotypes are then written in the boxes of the square.
For example, if a heterozygous parent (Aa) is crossed with a homozygous recessive parent (aa), the Punnett square would look like this:
```
| | a | A
|---|---|---|
| a | aa | Aa
| A | Aa | AA
```
The Punnett square shows that there are two possible offspring genotypes: Aa and aa. The probability of each genotype is 50%.
Punnett squares can be used to predict the genotype of offspring for any cross between two parents with known genotypes. They are a valuable tool for geneticists and breeders.