Before we look at how to construct and use a Punnett Square we need a way to write down the names of the alleles. In particular we need a way to record whether or not a gene is mutated.

The easiest way to record the names of alleles when working with Punnett Squares is to use letters, and typically we use capitals for dominant alleles, and lowercase for recessive.

Therefore, 'A' would denote the dominant form, and 'a' the recessive.

As we are working with diploid organisms (so they carry two copies of the gene (unless it occurs on a sex chromosome)) we would write the genes as 'Aa', 'aa' or 'AA'.

If we were following a characteristic that was conferred by two alleles then we would use a different letter for the second gene (say B), and so we would note different organisms as AaBB, aabb, AAbb etc.

We have now looked at how to write down alleles for use in Punnett Squares. We are now going to look at how to construct a Punnett Square.