These are localized changes, i.e. change from an A to G in a DNA sequence, deletion of one base, addition of three bases, etc.
Mutations occur randomly and without purpose.
Some mutations are recessive and others dominant in diploid organisms, depends on the expression of the mutant allele in the presence of another allele. In some cases the phenotypic effect of a mutation is reversed through the action of a suppressor mutation that compensates for the initial mutation. Evolutionary mutation generates variability among organisms, and this variation is operated upon by natural selection. It is rare, but it happens, that a mutation is beneficial and is a target of positive natural selection. Experimental allows geneticists to study a genetic pathway by altering the genes that are involved.