There are monster people that live on an abandon island
¾ of the monster people population are bald and ¼ of the people have hair. This is a mixed gene pool.
After 5 years the monster people on the island reproduce and pass on their DNA for hair to their offspring. The allele frequency for hair starts increasing.Now ⅚ people have the allele that gives them this trait and ⅙ are bald.
And then another 3 years pass and they start losing hair again. When they reproduce, the bald gene starts to get passed along again.
So after another year half, some of the monster people are bald and half have hair
And after 2 more years the ratio is ¾ are bald and ¼ has hair so the cycle will keep reapeating.