Mutation Rates

The frequency with which a wild-type allele at a locus changes into a mutant allele is referred to as the mutation rate. It is generally expressed as the number of mutations per biological unit, which may be mutations per cell division, per gamete, or per round of replication. For example, achondroplasia is a type of hereditary dwarfism in humans that results from a dominant mutation. On average, about four achondroplasia mutations arise in every 100,000 gametes, and so the mutation rate is 4/100,000, or 0.00004, mutations per gamete. The mutation rate provides information about how often a mutation arises.

Mutation rates vary among genes and species (Table 13.3), but we can draw several general conclusions about mutation rates. First, spontaneous mutation rates are low for all organisms studied. Typical mutation rates for bacterial genes range from about 1 to 100 mutations per 10 billion cells (from 1 × 10–8 to 1 × 10–10). The mutation rates for most eukaryotic genes are a bit higher, from about 1 to 10 mutations per million gametes (from 1 × 10–5 to 1 × 10–6). These higher values in eukaryotes may be due to the fact that the rates are calculated per gamete, and that several cell divisions are required to produce a gamete, whereas mutation rates in prokaryotic cells are calculated per cell division.

TABLE 13.3 Mutation rates of different genes in different organisms
Organism Mutation Rate Unit
Bacteriophage T2 Lysis inhibition

Host range

 1 × 10–8

3 × 10–9

 Per replication
Escherichia coli Lactose fermentation

Histidine requirement

 2 × 10–7

2 × 10–8

 Per cell division
Neurospora crassa Inositol requirement

Adenine requirement

 8 × 10–8

4 × 10–8

 Per asexual spore
Corn Kernel color 2.2 ×10–6 Per gamete
Drosophila Eye color

Allozymes

 4 × 10–5

5.14 × 10–6

 Per gamete
Mouse Albino coat color

Dilution coat color

 4.5 × 10–5

3 × 10–5

 Per gamete
Human Huntington disease

Achondroplasia

Neurofibromatosis (Michigan)

Hemophilia A (Finland)

Duchenne muscular dystrophy (Wisconsin)

 1 × 10–6

1 × 10–5

1 × 10–4

3.2 ×10–5

9.2 × 10–5

 Per gamete

354

The differences in mutation rates among species may be due to differing abilities to repair mutations, unequal exposures to mutagens, or biological differences in rates of spontaneously arising mutations. Even within a single species, spontaneous rates of mutation vary among genes. The reason for this variation is not entirely understood, but some regions of DNA are known hotspots for mutations.

Recent research suggests that fewer mutations occur in DNA sequences that are associated with nucleosomes (see Chapter 8). Reduced mutation rates may occur in these sequences because DNA associated with nucleosomes is less exposed to mutagens, but they could also be explained by the effect of nucleosomes on DNA repair, recombination, or replication, all of which influence the rate of mutation.

CONCEPTS

Mutation rate is the frequency with which a wild-type allele at a locus changes into a mutant allele. Rates of mutations are generally low and are affected by environmental and genetic factors.