Pseudoalleles

 If we assume that the mutant genes like vestigial and antlered are not allelic but located at different loci instead of locating at the same locus in different chromosomes, so closely linked that there is no crossing over between them, the mutant gene will suppress the expression of the adjacent normal allele to a certain extent. These closely linked genes are called pseudo alleles and this suppression is the result of the position effect. Thus, visible or apparent cases of allelism may be explained on the assumption of close linkage.

Pseudoalleles refer to closely linked and functionally related genes. A cluster of pseudoalleles is known as pseudoallele series or a complex locus or a complex region.

Characteristics of pseudoalleles

1. Pseudoalleles govern different expressions of the same character. In other words, they are functionally related.

2. Pseudoalleles are considered to occupy a complex locus which is divided into sub loci. Thus, they occupy different positions, but on the same complex locus.

3. They exhibit a low frequency of genetic recombination by crossing over. In other words, crossing over occurs between pseudoalleles, but at a very low frequency.

4. They exhibit a cis-trans position effect. In trans heterozygotes, such mutants produce a mutant phenotype, but in cis-heterozygotes, they produce a wild phenotype.

Examples of pseudoalleles

There are several examples of pseudoalleles. The well-known examples are the lozenge gene and star asteroid in Drosophila.

1. Lozenge eye in Drosophila

Green and Green (1949) studied the lozenge locus in Drosophila. The mutant gene produces an eye with a glossy smooth surface. Several alleles of the lozenge gene were identified and all mapped at one locus. All heterozygotes carrying two different mutants were lozenge in phenotype. But the progeny of such heterozygotes produced wild-type recombinants at a frequency much higher than expected spontaneous mutation. This indicated that Iz1 and Iz2 were pseudoalleles.

Pseudoalleles are closely linked genes that have similar phenotypic effects but can still be recombined with each other. The recombination between pseudoalleles is very rare. Such alleles are considered to be occupying a complex locus divided into sub loci between which recombination can occur.

Iz1 +/ Iz2 + Trans-heterozygote (mutant phenotype)

Iz1Iz2 /+ + Cis-heterozygote (wild phenotype)

2. Star asteroid eye in Drosophila

Star (S) is a dominant mutant that produces slightly smaller eyes than wild type in S/+ flies. Asteroid (ast) is a recessive mutant that gives a much smaller eye in homozygotes (ast/ast). The S/ast heterozygotes have still smaller eyes.

Thus, the order of eye size is + / + > S / + >ast / ast / ast> s /ast

The S and ast maps are in the same region. From the cross between star and asteroid (Star x asteroid), 16 wild-type flies were recovered in a population of 57,000. This indicated intragenic recombination between S and ast which could produce wild type. Hence, these S and ast alleles are called pseudoalleles. Thus, studies of pseudoalleles provided strong evidence in favor of intragenic recombination. A pseudoallele complex locus has several units of function, mutation, and recombination. It means that a gene can be divided into subunits.