You can use any letter you like, but select one that has a clearly different lower case, for example: Aa, Bb, Dd. Split the alleles for each parental type and add them into your Punnett square ...

You can use any letter you like but select one that has a clearly different lower case, for example: Aa, Bb, Dd. Split the alleles for each parental type and add them into your Punnett square ...