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What is Trisomy and explain in detail?
A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy (an abnormal number of chromosomes).

Description and causes
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent.  In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes.  The number of chromosomes is different for different species