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In most cases of Down syndrome, a person has an extra chromosome 21, equating to a total of 47 chromosomes instead of the usual 46. Sometimes, only part of chromosome 21 attaches to another chromosome. This extra genetic material causes the physical features and developmental delays seen in Down syndrome.
Normally, a person has 23 pairs of chromosomes or 46 chromosomes. In down’s syndrome, there is an extra chromosome, Chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy.’ That is why down’s syndrome is also referred to as Trisomy 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for a person with down’s syndrome.
Depending on the type of down’s syndrome, there would be different genetic material. In Trisomy 21, each cell in the body has three copies of chromosome 21 instead of the usual two copies. When a person suffers from Mosaic down’s syndrome, they have an extra chromosome 21 in only some of their body’s cells. In Translocation down’s syndrome, part of chromosome 21 becomes attached (translocated) to another chromosome before or at the time of conception. These individuals have two copies of chromosome 21, and additional material from chromosome 21 that is attached to another chromosome.
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