Gene mutations
Every allele in this chapter had to come from somewhere. Alleles are not created at fertilisation — fertilisation only shuffles the ones that already exist. New ones come from mutation.
Gene mutation
A change in the base sequence of the DNA of a gene. Because the base sequence is the information, changing it changes what the gene says, and the changed allele is copied into every cell descended from that one.
What causes them
Some mutations happen simply because copying DNA is not perfect — they occur at a low rate on their own. Others are caused by agents that damage DNA:
Ionising radiation
X-rays, gamma rays and the radiation from radioactive materials.
Ultraviolet light
The short-wavelength part of sunlight, which is why prolonged exposure damages skin cells.
Certain chemicals
Including several found in tobacco smoke and in some industrial processes.
Copying errors
A mistake made when the cell replicates its DNA before dividing, missed by the cell's own correction machinery.
Which cell it happens in matters enormously
A mutation in an ordinary body cell affects only that cell and its descendants — it is not passed to any child. A mutation in a cell that goes on to form a gamete can be inherited, and from then on it is an allele like any other, obeying the same rules of dominance and segregation as the alleles Mendel counted.
Most mutations are not useful
A random change to a working instruction is far more likely to spoil it than improve it, and several of the conditions in this skill began as mutations. But a mutation is not aimed at harm; it is a change, and occasionally a change turns out to be an advantage.
Mutation is the only source of new variation
Meiosis and fertilisation deal out the existing alleles in new combinations, but they cannot invent one. Every allele in every species — including every one of the seven pea characters Mendel studied — began as a mutation in a single individual.