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The cause of Down syndrome is an extra chromosome included with the 21st pair of chromosomes. For this reason, Down syndrome is often called trisomy 21, meaning there are three chromosomes where there should only be two. In meiosis, the process that produces sex cells, disjunction occurs during the end of metaphase and the start of anaphase. During metaphase, the copied chromatids align themselves at the "equator" of the cell. At the conclusion of metaphase, the centriole that binds the chromatids together is dissolved, or broken. Disjunction occurs, and the chromatids migrate toward their respective poles at opposite ends of the cell. So it is at the end of metaphase the non-disjuction occurs for chromosome pair 21, and two chromatids inhabit the space that was to hold only one. When this egg is fertilized by a sperm with a haploid (half) chromosome count, there will be the condition known as trisomy 21 that will present itself, because there is a presence of an extra chromosome on the 21st pair of chromosomes.
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