Given: Three positive integers `k`, `m`, and `n`, representing
a population containing `k + m + n` organisms: `k` individuals are
homozygous dominant for a factor, `m` are heterozygous, and `n` are
homozygous recessive.

Return: The probability that two randomly selected mating organisms will produce an individual possessing a dominant allele (and thus displaying the dominant phenotype). Assume that any two organisms can mate.

Note: formula derived on paper