Counting a dihybrid cross

Two RrYy pea plants are crossed. You will make the gametes of a parent, count the four kinds of seed in all $16$ boxes, and find the share of seeds with a new combination of traits.

The idea. A gamete takes one copy for shape (R or r) and one for colour (Y or y). The function looks already tells you what a seed looks like from its two gametes.

Your tasks.

1. Make gametes_of return the four gametes of a parent such as "RrYy".
2. Set counts to a dictionary of how many boxes give each kind of seed.
3. Set share_new to the fraction of seeds that are round green or wrinkled yellow.