In genetics, the specific types of genes (alleles) carried by individuals in any population comprise that individual's genotype. The actual expression of those genes produces a set of observable characteristics (phenotype). In any population of organisms, the wild type (also often printed in a hyphenated form as "wild-type") represents the most common genotype. With many organisms, alleles that are not a part of that genotype are often considered mutant alleles. The designation of wild type is based upon a quantitative (numerical) representation or estimation of the norm (normal) or standard in a population.
For example, one of the first descriptions of a wild-type gene was made with reference to the Drosophila fruit fly. In early studies of genetic traits of Drosophila, the American geneticist Thomas Hunt Morgan (1866-1945) noted a white-eyed fly in an isolated breeding population of red-eyed Drosophila flies (the flies were isolated in a bottle). Because the vast majority of Drosophila have red eyes, Morgan considered the white-eyed fly a mutant and termed the gene for red eyes in Drosophila the wild-type gene.
Outside of strict reference to genotype or phenotype, the term wild type is also used to denote the natural state of an organism, or the natural life cycle of an organism. When wild type is used to describe an entire organism, the sub-population of most prevalent phenotypes with the population is often referred to as the wild-type strain.
A genetic complementation test is used to determine the location and nature of mutations. Essentially, a complementation test looks for restoration of the wild-type phenotype in a mating between organisms with mutant genes. Complementation testing also determines the capability of mutants to act independently to supply the genetic information needed to result in the expression of a wild-type phenotype. For example, when two mutations affect the same gene, and neither mutation is capable of generating a wild-type phenotype, if these mutations are combined in the same cell the resulting strain must have a mutant phenotype. On the other hand, if the mutations affect different genes, so that each is able to generate some of the gene products required to produce a wild-type phenotype, then between the two genes the sum of the two gene products might still be able to generate a wild-type phenotype.
Geneticists use a variety of symbols and type scripts (capitals, italics, etc.) to denote wild-type alleles of a gene. One method commonly used indicates a wild type gene by the presence of a plus sign (+). Most often, this symbol is used as a superscript next to the notation for the allele. For example, the notation Pax1+ denotes the wild type allele of a Pax1 gene in mice that is the most prevalent allele for the gene. In contrast, when an organism undergoes a mutation that reverts the gene back to the wild type the plus sign is associated with a superscripted allele symbol. Addition reversions are usually identified by numbers preceding the allele in question. Geneticists also often use the letter "w" to denote the wild type gene. In the case of Drosophila the allele for red eyes is often designated by the letter "w" or the plus sign
A revertant is a mutation that restores the phenotype to the wild type (most prevalent form). In a true revertant, the original mutation itself is mutated back to the original wild type. With pseudo-revertants, or with pseudo reversions, the original mutation remains while another mutation that takes place within the same gene restores the wild-type phenotype. In the case of Drosophilae, a revertant would restore red eyes to the fly regardless whether it was a true revertant or a pseudo-revertant.
Initial forms of gene therapy were essentially gene replacement therapies that sought to introduce complete copies of the relevant wild-type gene into the organism having a genetic disease. The theory was that a wild-type gene, introduced via an appropriate agent (vector), might allow for wild-type (normal) gene expression. In the case of an enzyme deficiency, for example, such an introduction of the wild-type gene for the enzyme would allow the cell to produce the otherwise deficient enzyme.
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