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Who can go thru the mechs for Down's as trisomy 21 and via the translocation way? I am very poor in genetics, and don't get the mech of the latter--the former is meiosis 1 where someone (usually mommy) does not disjoin. But I don't get how the translocation works--
as far as I remember the mechanism of translocation involves 2 acrosomic chromosome(like 14 and 21). they loose their p-ends and join. So, a Down child gets 3 doses of chromosome 21 - 1 from 1 parent, another from the second parent, and the third one connected with chromosome 14.
I am not sure though...please correct me if I am wrong.
I like to remember Down's syndrome as 1)blaming it on the mum and 2) blaming it on the dad
Now as you know Down's is trisomy of chromosome 21....meaning 3 copies of this chromosome.
The most likely way of this happening is non-disjunction of Chr.21 in meiosis..........I think you must be with familiar with this happening due to the mum's meiotic non-disjunction.
In the 2nd type(I think its called non-robertsonian or unbalanced translocation)??In this there is crossover of genetic material between chromosome 21 and 14(usually).......now usually crossover occur among homologous chromosomes not non-homologous ones.The abnormal crossover leads to information from one of the chromosome 21 pairs to be located on chr. 14 now.So during meiosis the chromosome 21 tht didn;t participate in the translocation may end up with the chr.14 which has gens of chr.21(during the trnaslocation)....and so the new daughter cell has 2 copies of chr.21 genes during meiosis.During fertilization another chr.21 is introduced from the ovum and so the total is 3 copies....trisomy 21.....and thats why i call this one blame it on the dad!!
Now I'm not sure if its robertsonian(balanced) or non-robertsonian translocation that plays a part....I hope someone can help me out on that one.However to the best of my knowledge its Robertsonian.
I believe its robertsonian as robersonian is always unbalanced, and a reciprocal translocation is balanced, could be wrong tho??
OK I'll take your word for it....so in down't its robertsonian and its unbalanced whereas the balanced one is called reciprocal right??OK thanks.
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