Scientists have been able to introduce most of a human chromosome into mice, producing the most successful recreation of Down's syndrome so far.
Mice have been created which show signs of Down's syndrome
The Medical Research Council hopes the step will help research into Down's and other chromosomal conditions.
They say it is a significant technical development, as it had previously been possible to place only fragments of chromosomes into mouse cells.
The UK research is published in the magazine Science.
A person with Down's has three copies of chromosome 21, instead of the normal two.
It belongs to a class of disorders known as aneuploidies in which individuals have the wrong number of chromosomes.
Aneuploidies occur in at least 5% of all pregnancies and are a significant cause of illness, death and miscarriage.
Less well known aneuploidies are Edward's syndrome, where there are three copies of chromosome 18 and Patau's syndrome, where an extra copy of chromosome 13 is present.
Both prove fatal in early childhood.
'A technical step forward'
The Down's research was led by Victor Tybulewicz at the National Institute for Medical Research and Professor Elizabeth Fisher from the Institute of Neurology at University College London.
They were able to add about 90% of the 250 genes on human chromosome 21 into the embryonic stem cells of mice.
They then used these cells to generate a strain of mice that carried the extra human chromosome.
These mice have been shown to have problems with memory, in brain function and in the formation of the heart, similar to those that can occur in people with Down's syndrome.
Professor Fisher said: "This is a technical step forward in stem cell technology. It opens up a lot of possibilities."
She added: "We have created a rather good model for Down's.
"People with Down's syndrome have particular susceptibilities for some diseases like leukaemias and autoimmune disorders, which affect the rest of us as well.
"We believe this new technology will help us work out why this is, and what to do about it."
She added: "We also hope it will help unravel the picture of individual genes responsible for complex conditions, such as diabetes, and to create artificial chromosomes for gene therapy."
'Not a cure'
Professor Karen Steel, of the Wellcome Trust Sanger Institute, said: "In previous mouse models of Down's syndrome, in which extra copies of single genes or relatively small regions of human chromosome 21 have been added, the mice show only part of the spectrum of effects seen in the human.
"The new mouse model carries almost the complete human chromosome 21 and shows a wide range of features, suggesting that the chromosome as a whole may contribute to the syndrome rather than just a handful of the genes it carries.
"The authors rightly caution that we are also introducing human genes and human proteins into the mouse, and so might see additional effects, but this study shows a promising method to help in work to understand and, ultimately, improve our treatment of such complex diseases."
Gerry Coghlan, a consultant cardiologist, and chair of Uxbridge Downs Society (his 10-year-old son Eion has Down's), said: "If you've got a mouse model of Down's, you can take individual genes out and see what happens over a period of time.
"That way, we can find out which genes are responsible for the heart defects, the brain defects, the problems with the thyroid and the tendency toward leukaemia.
"We can then begin to understand what the underlying problems in the cells that lead to these conditions are, and whether or not we can treat them."
The Down's Syndrome Association said the research appeared to be a significant breakthrough.
But a spokesman cautioned: "It must be recognised that this research does not herald a 'cure' or a 'treatment' for Down's syndrome."