euploidy - recognised as a conception • Down syndrome...

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euploidy Euploidy is the category of chromosome changes which involve the addition or loss of complete sets of chromosomes. triploidy The possession of one complete extra set of chromosomes is usually caused by polyspermy , the fertilisation of an egg by more than one sperm. Such embryos will usually spontaneously abort. tetraploidy This is usually the result of a failure of the first zygotic division. It is also lethal to the embryo. Any other cell division may also fail to complete properly and in consequence a very small proportion of tetraploid cells can sometimes be found in normal individuals. aneuploidy Aneuploidy is the category of chromosome changes which do not involve whole sets. It is usually the consequence of a failure of a single chromosome (or bivalent) to complete division. monosomies All autosomal monosomies are lethal in very early embryogenesis. They do not even feature in the table above because they abort too early even to be
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Unformatted text preview: recognised as a conception. • Down syndrome, trisomy 21 The incidence of trisomy 21 rises sharply with increasing maternal age. Incidence of Down's syndrome with increasing maternal age Incidence Maternal Age At chorionic villus sampling At Birth 20 ~1 in 750 1 in 1500 30 ~1 in 450 1 in 900 35 1 in 240 1 in 400 40 1 in 110 1 in 100 45 1 in 13 1 in 30 Most cases arise from non disjunction in the first meiotic division, the father contributing the extra chromosome in 15% of cases. A small proportion of cases are mosaic and these probably arise from a non disjunction event in an early zygotic division. About 4% of cases arise by inheritance of a translocation chromosome from a parent who is a balanced carrier. The symptoms include characteristic facial dysmorphologies, and an IQ of less than 50. Down syndrome is responsible for about 1/3 of all cases of moderate to severe mental handicap....
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This note was uploaded on 11/22/2011 for the course CHEMISTRY CHM1025 taught by Professor Laurachoudry during the Fall '10 term at Broward College.

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euploidy - recognised as a conception • Down syndrome...

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