A Punnett square function

You will write the three pieces of a one-gene cross for flower colour in peas: purple (P) is dominant over white (p).

The idea. A child takes one letter from each parent, sorted so the capital comes first. One P makes a purple flower. Counter counts the results.

Your tasks.

1. Write cross(mum, dad): a list of the four children, one for each pair of letters.
2. Write colour(g): "purple" if the genotype holds a P, else "white".
3. Set counts to the Counter of colours for Pp with Pp.