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Concept of Allelomorphism

Concept of Allelomorphism

Concept of Allelomorphism

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

  1. Recombination test-

    Recombination was believed to take place between two genes but not between two alleles. In other worlds, intragenic interallelic recombination was not conceived. For instance, a hybrid aB/Ab between mutants as (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. 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. 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.

  2. 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 F, generation. But the mutant forms allelic to each other, will never complement.

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 chapter 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. A concept, of beads on a string or 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.

Position effect

It was shown early in the present century that due to shift of loci from one position to another, or due to shift of another segment in vicinity of a locus, expression changes. This will be illustrated by following two examples.

  1. Bar eye in Drosophila-

    In 1925, Sturtevant published results of his study on analysis of Bar eye mutant in Drosophila. It was shown that it was a semi-dominant mutation leading to reduction in number of facets from 779 in wild type (B+/B+) to 358 in heterozygote (B+/B) and to 68 in homozygote dominant (B/B). In flies, homozygous for Bar eye (B/B), mutations to wild type occurred at a frequency of one in 1600, and mutation to an extreme type known as ultrabar (with only 24 facets) also occurred, although with a lower frequency. Pure ultrabar cultures (Bu/Bu) could also be established. Ultrabar also gave rise to wild type eye with the same rate with which ultrabar was obtained from them. It was observed by Sturtevant that frequency of these events was much higher than what was expected due to spontaneous mutations. Therefore, Sturtevant had to seek and alternative explanation. He formulated a hypothesis and confirmed that new types arose due to unequal crossing over in the region of Bar locus. The technique used for confirming unequal crossing over, involved use of marker genes on either side of Bar locus, so that if crossing over occurred, then these marker genes on either side of Bar locus would also recombine, and will prove the occurrence of recombination. Asymmetric pairing of homologous chromosomes may lead to unequal crossing over, which is evident from recombined marker genes (forked bristles = f,fused veins=fu). For instance, fB+/+Bfu, when testcrossed with fBfB, gave following four types-(i) ultrabar eye, forked bristles and fused veins (fBufu/fBfu); (ii) wild type eye, wild type bristles and wild type veins (+ Bu +/fBfu); (iii) wild type eye, forked bristles and fused veins (fBufu/fBfu); (iv) ultrabar eye, wild type bristles and wild type veins (+Bu+ /fBfu) {Bu = BB; Bu = absence of B}.

Sturtevant also demonstrated that phenotype of flies with two bar genes on one chromosome and none on the other (double bar heterozygous), was different from those with one bar gene on each of the two chromosomes. This indicates that position of gene with respect to adjacent regions also influences its expression. This was called position effect. Later C.B. Bridges and his group, through a study of salivary gland chromosomes demonstrated cytologically that Bar character was associated with a repeat of 16A region in X-chromosome and that ultrabar (double bar) had two such repeats in the same region.

The following conclusions can be made from this work: first that gene is not a point, but has its dimensions and that its alleles may differ in size; second, that alleles of a gene or locus may recombine with each other, producing new combinations and third, that phenotypes of a heterozygote in cis-configuration (for instance ++/aja,) and that of a heterozygote in trans configuration (a1 +/+ a2) may differ due to position effect.

  1. Variegated position effect-

    Another kind of position effect can be illustrated with the help of an example of eye colour in Drosophila. White eye locus is present on X-chromosome near tip of the end away from centromere. Further, the centromere is flanked on either side with heterochromatin (inert and more condensed chromatin material relative to euchromatin, which is active and less condensed), the remaining region being largely euchromatic, so that white eye locus lies in euchromatin and quite away from heterochromatin. It was shown that when wild type allele of white eye locus i.e. W+, responsible for red eye colour, is transposed to a region near heterochromatin, a mottling of eye is observed. Mottling means that some facets are wild type and some are white giving a variegated appearance. Variegation effects can also b observed, if a gene originally located near heterochromatin is transferred to a position away from it. Such changes in position of genes can result due to structural changes like inversions or translocations in chromosomes.

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