When DNA is duplicated, errors may occur that result in a change in a gene. These changes are called mutations and they may have a dramatic effect on cell function. Mutations may occur in egg or sperm cells or in body cells but can only be passed on to a child when they are in eggs or sperm. Mutations are important for three reasons: they form the basis of evolution by providing a species with a better chance of survival if the mutation is beneficial; they account for a fraction of certain inherited disorders, such as haemophilia; and some mutations in body cells cause them to become cancerous.
Most mutations involve a change in just one base (unit of DNA). Mutations may occur spontaneously as random errors in copying or may be caused by exposure to UV light, such as that present in sunlight, certain chemicals (mutagens), and radiation.
A mutation in an egg or sperm cell is transmitted to a child at fertilization, and it then exists in every cell in the child’s body. Some of these inherited mutations are harmless, but some may cause severe abnormalities in the child.
A sperm that contains a gene carrying a genetic mutation fertilizes a normal egg cell.
The mutated gene from the sperm is copied into every new cell as the fertilized egg begins to divide.
The child has the mutated gene in every cell in his or her body. The presence of this gene may result in a genetic disorder.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.