Down Syndrome

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• Chromosomes fail to separate properly, causing a nondisjunction. This results in cells having an extra copy of a chromosome, specifically Chromosome 21. It is the MOST COMMON cause for Down Syndrome.

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• A whole Chromosome 21 is attached to another chromosome (i.e. an extra copy is attached elsewhere), which results in a structural abnormality causing Down Syndrome. It is much rarer than Trisomy 21.

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• After the fertilization of egg and sperm cells, one line of cells replicate properly. However, the other line of cells doesn't, and they have an extra copy of Chromosome 21. This results in Down Syndrome, but the effects are less severe than the other causes as the DNA is incorrect in only some cells. This is the RAREST method of inheriting Down Syndrome.