An individual’s growth and development is affected by the genes they inherit from their parents.
Sometimes chromosomes can change. These changes are called mutations and can occur quite naturally. A mutation can cause changes in an organism’s characteristics and alter its growth and development.
An example of a human chromosome mutation is when a child inherits an extra copy of chromosome 21. Each cell has three copies of chromosome 21 instead of two. This causes a condition called Down's syndrome. People with Down's syndrome have a characteristic appearance and may have reduced mental abilities.
Not all chromosome mutations are harmful. In many crop plants, for example in wheat and barley, the cells have complete extra sets of chromosomes due to a chromosome mutation. This gives them a higher yield, extra disease resistance and hardiness.
The frequency with which mutations happen can increase if the chromosomes are exposed to certain factors, called mutagenic agents. Mutagenic agents include: