Interviewee: Matt Ridley. People with Down Syndrome (DS) have an excess of genetic material in their cells. The vast majority of people with this syndrome have an extra copy of chromosome 21. Down syndrome is one of the most commonly occurring genetic birth defects, and it is the most common genetic basis for mental retardation. (DNAi Location: Genome >Tour > Genome spots > Chromosome 21: Down syndrome > A Chromosome 21)

Chromosome 21 is one of the smallest chromosomes. Some people are born with three copies of chromosome 21 instead of two. You might think this wouldn't matter, but it unbalances the development of the embryo, and the result is Down Syndrome. Down Syndrome is commonest in women bearing children later in life. It's one of the commonest genetic prenatal tests available to women who have a concern over this problem.

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