Alleles are alternative states of a gene (e.g., a1 and a2). Hybrids of a1/a2 are commonly of mutant phenotype, although they may show incomplete (allelic) complementation. Two alleles are identical if their base sequences are identical, even though one or both of these may be different from the sequence of the wild type. Non-identical alleles are still in the same gene (and are non-complementary) yet their expression may be distinguishable. Homoalleles are effected in the same codon but a different nucleotide occurs at the same site in each, and therefore the alleles cannot be separated by recombination in a heterozygote for the locus. The differences between heteroalleles are located at non-identical sites within the codon, or in another codon altogether, and the alleles can therefore be separated by recombination. Isoalleles convey wild phenotype, yet under special circumstances can be recognized by appearance. Multiple alleles are more than two alternatives alleles of the same locus. Super alleles are additional mutations in cis to an allele within a gene that reinforces their expression. Codominant alleles both are expressed in the heterozygotes. In some organisms alleles of the a1 locus are symbolized as a1-1, a1-2, etc. Molecular geneticists involved in physical mapping of the DNA use this term for any DNA difference (e.g., restriction fragment) that displays Mendelian inheritance and occupies the same chromosomal site (see Fig. A51). Allelic variation is widespread within a species and knowledge of the nature of these alleles is not just important in biology, but may also be relevant in applied fields such as disease susceptibility, forensic identifications, etc. In yeast, by hybridizing two different strains and through conduction microarray analysis, the frequency of such variation (single-feature polymorphism) can be ∼1% (Winzeler EA et al 1998 Science 281:1194; Gresham D et al 2006 Science 311:1932). gene symbols, RFLP, RAPD, Mendelian segregation, coalescent, mutation age of, SNIPS; Slatkin M, Rannala B 2000 Annu Rev Genomics Hum Genet 1:225.
Mutation in a gene results in an allelic change from A (adenine) to G (guanine). Nucleotide substitution is identified by sequencing of the PCR amplified segment of the DNA. In the heterozygote for the site reduced amounts of both A and G is shown. (Courtesy of Amersham Biosciences, GE Healthcare Biodirectory 2005, p. 310.)
