Effect of Genotypes on Induction of Mutations(Mutator Gene & Paramutations)
Fig. 21.9. Colourless and spotted maize kernels. Kernel at left is homozygous aa without any Dt locus and that at right is homozygous aa with Dt mutator gene causing recessive allele a to undergo somatic mutation leading to spotting.
In Drosophila, frequency of recessive lethals has been shown to differ in different laboratory stocks, from 0.1% to 1.0%. Sometimes, high mutability of a stock could be found to be due to a recessive mutator gene on chromosome 2, so that when this gene was absent, the frequency of sex linked recessives fell down to 0.07%. Other mutator genes have also been identified both in Drosophila and other organisms like Escherichia coli and are related with DNA polymerase enzyme involved in DNA replication and repair (see Chemistry of the Gene 2. Synthesis, Modification and Repair of DNA).
Fig. 21.9. Colourless and spotted maize kernels. Kernel at left is homozygous aa without any Dt locus and that at right is homozygous aa with Dt mutator gene causing recessive allele a to undergo somatic mutation leading to spotting.
A more important system than the above is Ds-Ac system in corn studied in detail by Barbara McClintock. Ds is a Dissociation gene on chromosome 9 and causes chromosome breakage but is active only in the presence of Ac or Activator gene, which can be located on any other chromosome. These examples of a-Dt and Ds-Ac system have been discussed in Plasmids, IS Elements, Transposons and Retroelements on Plasmids and Transposons, due to their relevance with transposable elements. As earlier mentioned, Barbara McClintock (a lady) was awarded Nobel Prize in 1983 for her work on Ds-AC system. She died in 1992.
It is usually accepted that the two alleles A and a present in a heterozygote (Aa)remain uncontaminated and that their temporary association does not affect each other. This basic assumption was questioned when Brink in 1956 discovered the phenomenon of paramutation in corn. It was shown that at R locus for pigment there are different alleles having different phenotypic effects as follows :
Rr = full colour
Rst = stippled effect
Rmb = marbled effect
Paramutations, though rare, are also found in Oenothera, tomato and ferns. The segregation distorter (SD)gene earlier discussed in Lethality and Interaction of Genes can also be explained on the basis of paramutations.