From The School of Biomedical Sciences Wiki
The concept of wildtype can refer to a gene, phenotype or genotype, and is the most typical form of a species in nature/ a natural breeding population- i.e it is unmodified and so not a mutant.
An allele that is deemed wildtype is the one which shows the highest gene frequency and its characteristics are assumed to be non-harmful to the organism.
The wildtype allele is indicated by a superscript +. Wildtypes are very useful in research, as specific mutations can be introduced and their effects can be compared to naturally occurring genotypes and phenotypes.