Answer:
The capital letter is used to show a dominant trait, while a lowercase is used to represent a recessive trait.
Step-by-step explanation:
If you are to take a Mother and a Father as example, and they have 3 children, the father's brown eyes as example we can show the children with capital and lower case depending on their looks. If the Father had brown eyes and 2 of his children out of 3 have brown eyes, we can represent that as a dominant trait, so it would be R. Then, if we take the father's hair color, brown, and the mothers hair color, blonde, we can say 2 of 3 children have brown hair, so 1 of 3 children have brown hair. So we now know that brown hair is a recessive trait. 2 of 3 children have blonde hair so the child with brown eyes and blonde hair could be represented like "Rr". Hope that helps, this is just my little explanation of this!