ABSTRACT
Mutations were recognizable only when they were expressed in the phenotype of an individual. With the methods of DNA sequencing and sequence comparisons, the occurrence of mutations can be documented whether or not they are expressed in the phenotype. The modern methods of molecular genetics enable the detection of all types of mutations when two or more sequences of DNA are aligned and compared. If the mutation recurs, it will not be lost and its frequency may theoretically increase, although the process may be very slow and require thousands of generations, depending on the mutation rate. It is accepted that most of the mutations in natural populations are slightly deleterious or neutral. Most of the mutations that people were aware of before the 1960s, in particular in humans and Drosophila had strong phenotypic effects and many were congenital defects.
