Frequency of Mutations
Frequency of Mutations
Although mutation occurs randomly, different mutation rates prevail at different loci. Some kinds of mutations are more likely to occur than others, and individual genes differ considerably in length. A long gene (more base pairs) is more likely to have a mutation than a short gene. Nevertheless, it is possible to estimate average spontaneous rates for different organisms and traits.
Relatively speaking, genes are extremely stable. In the well-studied fruit fly Drosophila there is approximately one detectable mutation per 10,000 loci (rate of 0.01% per locus per generation). The rate for humans is one per 10,000 to one per 100,000 loci per generation. If we accept the latter, more conservative figure, then a single normal allele is expected to go through 100,000 generations before it is mutated. However, since human chromosomes contain 100,000 loci, every person carries approximately one new mutation. Similarly, each ovum or spermatozoon produced contains, on the average, one mutant allele.
Since most mutations are deleterious, these statistics are anything but cheerful. Fortunately, most mutant genes are recessive and are not expressed in heterozygotes. Only a few will by chance increase enough in frequency for homozygotes to be produced.
Although mutation occurs randomly, different mutation rates prevail at different loci. Some kinds of mutations are more likely to occur than others, and individual genes differ considerably in length. A long gene (more base pairs) is more likely to have a mutation than a short gene. Nevertheless, it is possible to estimate average spontaneous rates for different organisms and traits.
Relatively speaking, genes are extremely stable. In the well-studied fruit fly Drosophila there is approximately one detectable mutation per 10,000 loci (rate of 0.01% per locus per generation). The rate for humans is one per 10,000 to one per 100,000 loci per generation. If we accept the latter, more conservative figure, then a single normal allele is expected to go through 100,000 generations before it is mutated. However, since human chromosomes contain 100,000 loci, every person carries approximately one new mutation. Similarly, each ovum or spermatozoon produced contains, on the average, one mutant allele.
Since most mutations are deleterious, these statistics are anything but cheerful. Fortunately, most mutant genes are recessive and are not expressed in heterozygotes. Only a few will by chance increase enough in frequency for homozygotes to be produced.
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