The cross counter

You will write three small functions that count a cross with one gene. The genotypes are written as two letters, such as "Tt". Capital T is dominant.

The idea. Pair every allele of one parent with every allele of the other. Sort each pair so that the capital letter comes first, so "tT" becomes "Tt".

Your tasks.

1. Finish cross. It gives a dictionary: the genotype and the number of the 4 boxes that hold it.
2. Finish dominant_share. It gives the share of the offspring that show the dominant trait, as a fraction.
3. Finish unknown_plant. In a test cross of a tall plant with a dwarf plant, it names the genotype of the tall plant from the numbers of tall and dwarf offspring.