CELL DIVISION
MEIOSIS
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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21 pairs of chromosomes
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Trisomy 21
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Non-disjunction of sex chromosomes
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Fertilization of the egg by two sperm
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Detailed explanation-1: -About 95 percent of the time, Down syndrome is caused by trisomy 21-the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell. Mosaic Down syndrome.
Detailed explanation-2: -The addition of an extra chromosome usually occurs spontaneously during conception. The cause of this is unknown and prevention is not possible. The most important risk factor for trisomy conditions is maternal age. Women in their late 30s and 40s have a higher chance of trisomy conditions occurring.
Detailed explanation-3: -Abstract. Trisomy 21 (Down syndrome) is the most common autosomal trisomy in newborns, and is strongly associated with increasing maternal age. Trisomy 21 results most commonly from maternal meiotic nondisjunction. Unbalanced translocation accounts for up to 4% of cases.