Principles of Genetics and Breeding

2.1.11 Allele

• A gene can occur in one or more forms. Alternate forms of a gene are called “alleles”.
• In a population, a gene can have one to perhaps a dozen alleles. If there is only one allele in the population, the gene is said to be “monomorphic”. If there are two or more alleles at a locus (locus=gene; plural is loci), the gene is said to be “polymorphic”.
• Monomorphic genes are not very interesting, because there is no genetic or phenotypic variance associated with that locus. On the other hand, polymorphic genes are of great interest to a geneticist.
• When more than one allele exists at a locus, there is genetic variance, which produces phenotypic variance; and this can be exploited by breeding programmes.