The reverse, of course, may cause an allele to survive, even though it produces a serious genetic burden on the population. This burden might cause a population to develop a rapid reproductive maturation, but shorten the longevity in order to avoid the mal-adaptive trait from appearing. Phenotypic expressions depend upon conditions of development. It should live longer, for example. Restrictive conditions kill the individuals and permissive conditions allow it to live.
Cancer is an example of a conditional mutant condition that depends upon multiple loci.
Also, cancer may be produced both by multiple mutation, or by somatic crossing over. Mutations occur at rates characteristic of loci , and the type of mutational change . Where detailed experiments have been possible, the mutation rate varies greatly within a locus. Some loci are rarely seen in a mutant state. Others have many mutant alleles present.