This is a simple simulation of the spread of lactose persistence alelle. Assume that lactose persistence alelle is a dominant gene, and it increases the survival rate before reproduction. Bad_mortality is the chance a baby with lactose persistence alelle has to die. Good_mortality is the chance a baby without lactose persistence alelle has to die.
Other assumption: Monogamy; Each couple has the same N offsprings at birth; Lactose persistence only affects the survival of pre-puberties.