A recessive condition affects 1 baby in 1600 in a large town of 40000 people…

Count gene copies: each individual has two. The frequencies of two alleles add up to 1. At equilibrium the genotypes are $p^2 + 2pq + q^2 = 1$. Start from the recessive trait, which shows $q^2$. The five factors that disturb the equilibrium are gene flow, drift, mutation, recombination and selection.