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An extra chromosome, making three of a form, known as trisomy (see additionally Overview of Chromosome and Gene Problems ). The most common trisomy in a new child is trisomy 21 (three copies of chromosome 21, which is the smallest human chromosome). Chromosomes are structures within cells that contain DNA and lots of genes. Apollo 11 50th anniversary neil armstrong michael collins buzz aldrin signatures shirt Youngsters with Down syndrome have delayed bodily and psychological development, specific head and facial features, and are sometimes quick.