The achievement by British scientists provides the best experimental tool yet available for studying Down’s syndrome, which affects about one in 750 babies, and promises to transform understanding of the chromosomal disorder.

Healthy people have 23 pairs of chromosomes — the packets in which genes are carried — and Down’s syndrome occurs when an extra copy of chromosome 21 is inherited by mistake.

The surplus genes seem to interact with other parts of the genome to trigger health problems, such as learning difficulties, heart defects and a greater susceptibility to leukaemias and Alzheimer’s disease.

Research has been hampered by a lack of adequate animal models. Mouse models are available, but they do not mimic Down’s well, as the genes found in the human chromosome 21 are scattered across several chromosomes in mice.

The new study, led by Victor Tybulewicz, of the National Institute for Medical Research, and Elizabeth Fisher, of University College London, has produced the first mouse model that comes close to replicating the condition as it appears in human beings.

Professor Fisher said: “People with Down’s syndrome have particular susceptibilities for some diseases like leukaemias and auto-immune disorders. We believe this new technology will help us work out why this is, and what to do about it . . . It is not going to lead to a cure, but we hope it will help us to improve therapies for the health issues that result.” The work, she said, would also have implications for wider medical research.

She said: “People with Down’s syndrome suffer from the same diseases as the rest of the population, but they are particularly susceptible . . . Research that helps people with Down’s syndrome will also help the rest of us.”

The team, whose results are published today in the journal Science, added a complete human chromosome 21 to mouse embryonic stem cells, which were then merged with mouse embryos. Some of these then gave rise to mice that carried 92 per cent of the added human chromosome.

This strain of mice, known as Tc1, has memory and learning deficits similar to those seen in people with Down’s as well as heart abnormalities that are typical of the human disorder.

Stylianos Antonarakis, of the University of Geneva, said: “That’s a first. No other mouse so far has the heart defect.”

Julie Korenberg, of the University of California, Los Angeles, said: “This is the first mouse I would consider a superb model.” Richard Reeves, of Johns Hopkins University in Baltimore, Maryland, said: “This will have a huge impact on Down’s syndrome research.”

The work should also improve understanding of other human chromosomal disorders, which are collectively known as aneuploidies.

Edwards syndrome, which is caused by three copies of chromosome 18, and Patau syndrome, caused by a triple chromosome 13, typically kill people before the age of five. Other aneuploidies are thought to cause miscarriage in at least 5 per cent of pregnancies.

Dr Tybulewicz said: “Aneuploidies are seen in at least 5 per cent of all pregnancies and are therefore a big cause of human illness, death and miscarriage. This technology will provide a crucial genetic tool in understanding this complex human syndrome.”

Carol Boys, chief executive of the Down’s Syndrome Association, said: “We welcome any research that may have a positive impact on the lives of people with Down’s syndrome, and this would appear to be a significant breakthrough.”

CHARTING CHROMOSOME 21

• The syndrome takes its name from John Langdon Down, an English doctor who described the condition in 1866
  
  
• It affects up to one in 750 births worldwide. There are about 60,000 people in Britain with Down’s
  
  
• Life expectancy in Britain is typically about 50. Most people with the condition live rich and fulfilled lives
  
  
• Older mothers have a higher chance of conceiving a child with Down’s. Four out of five children with the condition, however, are still born to mothers under 35 because these women have many more babies than older groups
  
  
• Older mothers are usually offered pre-natal screening to determine whether a foetus has Down’s, and many opt for an abortion if the condition is diagnosed
  
  
• There is little evidence that the incidence of Down’s is changing. The effect of pre-natal screening to prevent Down’s births is largely balanced by the trend for women to have children later in life
  
  
• Women who have had a Down’s syndrome pregnancy have a higher chance of having another one

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