1. The colour of a tomato can be represented by RR, Rr or rr. Why do we use capital letters for some symbols and lower case letters for others? Why are there two
symbols for one trait (characteristic)?

The capital letter is used to show a dominant trait, while a lowercase is used to represent a recessive trait.

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!

b a mature ovary of a plant.