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What are chromosomal conditions?

When cells divide to make egg and sperm, each new cell usually ends up with 23 chromosomes, one from each pair. Sometimes, however, part or all of a chromosome is missing, rearranged or duplicated. Most fertilised eggs that have more or fewer than 46 chromosomes will not survive. In some cases, however, the embryo will survive, but the chromosomal alterations may affect the way it grows and develops.

Chromosomes from a person with Down syndrome, showing an extra chromosome 21.

What is Down’s syndrome?

Most people with Down’s syndrome have 47 chromosomes, rather than 46. They have an extra copy of chromosome 21, the smallest human chromosome. This is responsible for the features of Down’s syndrome, including delayed development, some learning difficulties and possible heart problems. The chances of a couple having a baby with Down’s syndrome depend on the mother's age. For women under the age of 25, the chances are about one in 1400, rising to one in a 100 by age 40.

This boy has Down's syndrome.

What is Williams syndrome?

Williams syndrome is a rare chromosomal condition, affecting about one in 25,000 people in the UK. People with Williams syndrome are usually missing a small part of chromosome 7, which probably causes the symptoms of this condition. These include some learning difficulties, slight muscle weakness and possible heart problems. Children with Williams syndrome may also be unusually talkative.

Chromosomes from a person with Williams syndrome, showing the gene (pink spots) missing from one of the pair of chromosome 7s.