A young girl with Down's syndrome (image cropped)

Single-gene and chromosome disorders

Some conditions are caused by genetic ‘abnormalities’

Some conditions are so tightly tied to DNA mutations that they can be called genetic diseases. In a few cases, inheriting just one copy of an altered allele (i.e. a normal allele from one parent and an altered allele from the other) can lead to disease. These are known as dominant conditions, and they include Huntington’s disease, an incurable condition that causes gradual deterioration of the brain (for more on this, see our Real Voices interview with Matt Ellison).

Recessive conditions, such as cystic fibrosis, are a larger group of genetic disorders that only occur when someone inherits two copies of an altered allele (i.e. one from each parent).

There are also conditions caused by larger changes in the chromosomes. Down’s syndrome, for instance, is caused by an extra copy of chromosome 21. In the UK, all pregnant women are offered screening for Down’s syndrome.

Some women may choose to terminate their pregnancies when they learn that their babies are likely to be born with the syndrome. These decisions are difficult and controversial because many people with Down’s syndrome lead happy and fulfilling lives. Richard Dawkins, the scientist and author of ‘The Selfish Gene’, was recently attacked by the media for advising a woman to have an abortion if she found her child had Down’s syndrome.

Genetic diseases can be more complicated than they might seem at first glance. For example, the protein affected in cystic fibrosis can be altered by more than a thousand different mutations. Some have almost no effect, whereas others cause severe lung and stomach problems. The extent of symptoms can also be affected by other gene, which modify the effect of the cystic fibrosis gene. People with Down’s syndrome differ on a range of characteristics, which depend on several factors including how much of chromosome 21 is present in three copies.

Lead image:

Cropped from Andreas Photography/Flickr CC BY NC ND


About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Genetics and genomics, Health, infection and disease, Medicine
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development