Gene vs allele : A new concept of allelomorphism

Classical concept
In classical genetics, a distinction was made between gene and allele on the basis of following two criteria.

Recombination test.
Recombination was believed to take place between two genes but not between two alleles. In other words, intragenic interallelic recombination was not conceived. For instance; a hybrid aB/Ab between mutants aa (aaBB)and bb (AAbb)for two linked genes A and B could give rise to wild type progeny on a test cross. Since A and B are linked, this would be possible only due to recombination (Fig. 14.1). On the other hand if two mutants a₁a₁ and a₂a₂ belonging to same gene A were crossed and F₁ (a₁/a₂)is test crossed (a₁a₂ x aa)no wild type progeny would be expected (Fig. 14.2). It was thus, earlier believed that different genes or loci could recombine with each other by crossing over but different alleles of a gene could not. This test of allelism is illustrated in Figure 14.3.

Complementation test.
It was also shown that mutant alleles of two different genes coming from two parents, thus being in repulsion phase (also known as trans configuration), will complement giving rise to wild type in F₁ generation. But the mutant forms allelic to each other, will never complement (Fig. 14.4).

Thus, in its classical concept, the gene recombined as a unit, functioned as a unit and also changed (mutated) as a unit. This concept was consistent with the view of Morgan and his group that genes were like beads on a string, where one bead could change independently and recombine with its neighbouring beads. It will be seen in this section that such a view where genes corresponded to beads on a string was an oversimplification. The first exception to this came, when it was shown that Bar locus in Drosophila controlling size (number of facets) of eye, contained more than one units of function and could undergo intralocus recombination. As will be seen in Organization of Genetic Material 1.  Packaging of DNA as Nucleosomes in Eukaryotes, a concept, of beads on a string or better called as string on beads was revived in connection with chromatin structure and the nucleosome. In this case beads correspond to nucleosomes and not to genes.