Maternal gamete and 8 coming from non disjunction in

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maternal gamete and 8% coming from non-disjunction in the paternal gamete. Most non-discjuntion events occur in Meiosis I When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Robertsonian translocations cause 2-3% of Down syndrome cases.
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[email protected] (chromosome not separated) occurs in meiosis I -­૒ homologous chromosome pairs go to the same daughter cell. Result: Two of the gametes have an extra chromosome, and the other two are missing a chromosome. Nondisjunction in meiosis I
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[email protected] occurs in meiosis II -­૒ sister [email protected] segregate to the same daughter cell. Result: Two of the gametes have normal numbers of chromosome, one has an extra chromosome, and the other one is missing a chromosome. Nondisjunction in meiosis II
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Clicker question A non-disjunction of Chromosome 21 occurs in meiosis I of an oocyte (egg cell). Assuming that all 4 gametes were fertilized by normal sperm cells, what would the karyotypes of the resulting zygotes (early embryos)? A. 2 zygotes with normal karyotype, 1 monosomic for Chr. 21, one trisomic for Chr. 21 B. 3 zygotes with normal karyotype, one trisomic for Chr. 21 C. 2 zygotes trisomic for Chr. 21, two monosomic for Chr. 21 D. All 4 normal, because only non-disjunction in meiosis II can result in trisomy
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The risk of trisomy 21 increases with the age of the mother Risk of a 48 year old woman to have a child with trisomy 21 is 1/14, and any chromosomal aberration is 1/10!
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