Answer:
The loss or addition of one or more chromosomes to the basic chromosome complement is called aneuploidy. Failure of segregation ofchromatids during cell division give rise to aneuploidy. In humans there are 46 chromosomes or 23 pairs. Out of these, there are 22 pairs ofautosomes and one pair of sex chromosome. Sometimes an additional set of chromosomes are present or a set of chromosomes is lacking which leads to an abnormal condition in organisms like Down's syndrome, Turner's syndrome, Klinefelter 's syndrome etc. All these are chromosomal disorders.
Down's syndrome (also called mongolism) is due to presence of an additional copy of chromosome 21 which is also called trisomy of 21 i.e., (2n + 1). It leads to an abnormal condition in organisms called mongolism or mongolian idiocy. This disorder was first discovered by Langdon Down (1886). The affected individual is short statured with small round head, furrowed tongue and partially open mouth. The individual is physically and mentally retarded. Heart and other organs may be defective. Gonads and genitalia are underdeveloped.
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