1. Theory of Point Mutation:
According to this theory, multiple alleles arise as a result of mutations of a gene at a given locus producing different effects.
2. Theory of Pseudoallelism:
According to this theory, multiple alleles may occupy separate gen loci which lie in close association in a chromosome and are inherited together because of close linkage (Lewis – 1951). By Lewis and Green, pseudo alleles were regarded as distinct genes with their specific functions which are carried out in a sequence at the same intracellular location influencing the same character. It means that pseudo alleles are functionally allelic but structurally nonallelic. Therefore, functionally related closely linked genes are called pseudo alleles, which because of their close linkage most often are inherited together. These closely linked genes have been called pseudo alleles. These affect the expression of normal genes showing the position effect.
3. Heterochromatin Theory of Allelism:
Chromosomal breakage and rearrangement sometimes bring the heterochromatin next to genes, and suppress their expression. In maize, there is evidence that position effects are sometimes due to the transposition of very minute fragments of heterochromatin. The fragments are too small to be visible under the microscope. It is often difficult to tell which of these three theories explains a particular case of allelism and, in fact, it is possible that all three apply in different cases.