Fig. 1.

Changes in the frequency of the extra (duplicate) allele (A²). According to the MDN model, the gene (A) duplicates at t = 0, and one copy (A¹) is maintained by selection for its original function. The extra allele (A²) is subject to loss from the population by drift and inactivation by common mutations. Frequent loss of A² will continue until a rare mutation provides a new selectable function.