What is a mutation?
A mutation is a sudden and permanent change in the genetic material of a cell. If it occurs in a body cell it affects only that individual, but if it occurs in a gamete it can be passed to offspring. Mutations create new alleles and are an important source of variation. They usually occur randomly and at a low natural rate. There are two main kinds: gene mutation and chromosomal mutation.
Gene mutation
A gene mutation is a change in the base sequence of DNA within a single gene. It may involve the substitution, insertion or deletion of one or more bases, which can change the protein made. Sickle cell anaemia is caused by a single base substitution that changes one amino acid in haemoglobin.
Example
In sickle cell anaemia, one base change in the haemoglobin gene replaces one amino acid. The red blood cells become sickle-shaped and carry less oxygen — a whole body effect from a single base change.
Chromosomal mutation
A chromosomal mutation changes the number or structure of chromosomes, often caused by errors during meiosis (non-disjunction). Down syndrome results from an extra copy of chromosome 21 (trisomy 21), giving 47 chromosomes instead of 46. Other examples are Turner syndrome (XO), where a female has only one X chromosome, and Klinefelter syndrome (XXY), where a male has an extra X. Most mutations are harmful or neutral, but a small number are beneficial and, if they occur in gametes, can be passed on and provide the variation on which evolution depends.
Key idea
Gene mutation = change in DNA base sequence (e.g. sickle cell anaemia). Chromosomal mutation = change in chromosome number or structure (e.g. Down syndrome, trisomy 21).
Remember
Mutagens increase the mutation rate. Examples include ionising radiation (X-rays, UV, gamma rays) and chemicals such as tar in cigarette smoke.