Exercise 3 solution

def step(p, w_AA, w_Aa, w_aa):
    q = 1 - p
    mean_w = p * p * w_AA + 2 * p * q * w_Aa + q * q * w_aa
    return (p * p * w_AA + p * q * w_Aa) / mean_w

for s in (0.2, 0.05):
    q = 0.5
    gens = 0
    while q >= 0.1:
        p = step(1 - q, 1.0, 1.0, 1 - s)
        q = 1 - p
        gens += 1
    print(s, gens, round(q, 4))
q = 0.5
for _ in range(100):
    q = 1 - step(1 - q, 1.0, 1.0, 0.8)
print(round(q, 4))

Output:

0.2 50 0.0983
0.05 203 0.0996
0.051

Reading it: with a 20% disadvantage it takes 50 generations for q to fall below 0.1; with a 5% disadvantage it takes 203, roughly four times longer for a four times weaker disadvantage. After 100 generations with s = 0.2 the allele is still at about 5%, which shows how slowly selection removes a recessive allele once it is rare.