The rate of spontaneous mutations is known to change under the influence of various external and internal factors. One of the external factors which might bring about spontaneous mutations is a change in the temperature range (normally a higher temperature makes the genes susceptible for change).
Disturbed metabolic rates may also bring about mutational changes. Age and sex may also have a bearing on the mutational rate. Increased age may render the genes prone for mutational change.
In Drosophila, it has been observed that males are more susceptible to mutational alterations than the female.
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One of the important factors bringing about spontaneous mutations may be specific mutation inducing and mutation susceptible genes. As has already been pointed out, these are called mutator and mutable genes.
The mutator genes have been discovered by Me Clintock in maize. She described some genes which had the capacity to influence other genes to mutate. Rhodes has described the role of a mutator gene which influences the functioning of a color gene.
In maize the color is governed by three recessive genes a, c, and r, while the dominant alleles of these govern the color other than green.
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In addition to these there is a mutator gene) (dotted color) which controls the variegated color in the grain kernel.
This gene influences the gene as to mutate so that red patches are produced. Usually the plants with the RR will be green (all the three dominant gene pairs are required produce color other than green), but the presence of makes it variegated.
Mutable genes other than the one mentioned above (a) have been reported in maize by Emerson.
He investigated a variety of maize with white pericarp having several red spots of varying sizes; the homozygous recessive allele reducing the white color.
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According to Emerson, these red patches are produced by the unstable white gene undergoing spontaneous mutation each red spot in the endosperm indicates an area where a mutation has occurred.
Jones further continued the studies of Emerson and found that there are six red spots, each having 10-45 colored cells. These colored cells were exactly identical with those produced by the pure red gene.
Jones was conducting an experiment with a homozygous plant and he believed that the six red spots indicated that the gene for white had mutated six times producing a red cell every time.
These cells further multiplied and produced the red patches. The gene for white endosperm color is thus a typical example of a mutable gene.
Several such mutable genes have been reported in Drosophila, sweet pea, 4 o’ clock plant, snapdragons etc.
The genes for miniature wings, gene for purple eye color in Drosophila are known to be mutable genes. In maize plant itself, Stadler has observed that the gene R for aleuron color mutated some 492 times in about a million gametes that he studied.