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Down syndrome, or trisomy 21, is a genetic condition that occurs when a person has a part of, or an extra chromosome with the 21st pair of chromosomes. This essentially gives them three chromosomes where only two should be present. In interphase, considered the normal life cycle phase the cell goes through, there would be three chromosomes present. In prophase, all the chromosomes are copies, so there would be six chromatids present. In metaphase, the chromatids align themselves along the "equator" of the cell. In anaphase, the six chromatids migrate along spindle fibers to opposite ends of the cell, giving each of two daughter cells three chromosomes again. In telophase I (mitosis), the nuclear membrane forms around each all the chromosomes, the 21st trisomy trio included. In telophase II (meiosis), the chromosomes have undergone another separation and migration, one germ cell will have a "normal" count of one choromsome from the 21st pair, while the other will have a pair of chromosomes, where there should be only one.
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