When mutations are rare in a population, how might a mutation spread through the population and become common? As the population grows larger, the chance that the same allele will mutate in the sex cells of multiple individuals increases. m Because mutations produce dominant alleles, they spread rapidly through the population by masking recessive alleles. If the mutation confers a selective advantage, it will spread through the population because it increases individual fitness. Mutations increase the frequency of crossing over during meiosis, producing more chromosomes that are carrying the mutation.