Hybrid seeds over the years

A hybrid plant is heterozygous for one gene. If its seed is made by fertilisation and the plants self-pollinate, the share of heterozygous plants halves each year. If its seed is apomictic, the share stays the same. You will write the two models and ask when the share falls below a limit.

The idea. Use exact fractions, so that no rounding hides the answer.

Your tasks.

1. Finish sexual. It gives the share of heterozygous plants in a year, as a fraction.
2. Finish apomictic. It gives the share for apomictic seed.
3. Finish first_year_below. It gives the first year in which the share from fertilised seed is below a limit.