The frequency of a certain gene in a parasite population undergoes sinusoidal cycles as a result of coevolution with its host. It reaches a maximum frequency of 70% and a minimum of 30%, with a complete cycle taking three years. Find a function that describes the gene frequency dynamics over time (measured in years) assuming that it starts at a frequency of 50% at time t = 0.

 (Let f(t) be the percent gene frequency.)