We had earlier shown that the mutation frequencies, both spontaneous and induced, differ from locus to locus, as observed by L.J. Stadler in maize. Differences in frequencies of mutation at a locus are observed even between sexes and different culture conditions. In maize mutator gene and paramutation are two such popular examples where genetic differences influence induced mutations.

Mutator gene

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). The most popular example of mutator. gene known in corn relates to a gene a which is located on chromosome 3 and causes absence of anthocyanin leading to loss of purple colour in corn kernel endosperm and other parts of the plant. However, it was noticed that if chromosome 9 has a dominant gene Dt, then a may mutate to one of the other A alleles, permitting anthocyanin production in heterozygote (Ad). This mutation can occur even in somatic tissues, leading to spots in kernel aleurone layer (Fig. 21.9) or stripes in stalk and leave. Since kernel aleurone is triploid, the effect of as many as three doses of Dt can be observed on mutability of a as shown in Table 21.8.

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.

If the dose of Dt is kept constant and dose of a varies, the mutation rate increases proportionately for each a allele added.

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.

Paramutations
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 :

R^r = full colour
R^st = stippled effect
R^mb = marbled effect

It was also shown that one dose of R^r produces dark mottling and two to three doses produce full colour. It was found that when R^r allele from one locality was associated with R^st or R^mb from a different locality (outside Andes mountains) in heterozygous condition like R^r R^st or R^r R^mb, then R^r gets altered, so that after extraction from heterozygous condition, it now produces much lighter colour than before. This effect was transmitted from generation to generation proving that R^r has definitely undergone some change. This change was called paramutation and R^sl and R^mb alleles causing this change were called paramutagenic.

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.