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 Down's genetic mechs  

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Hi guys,

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--
Thank you!


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) 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 in down't its robertsonian and its unbalanced whereas the balanced one is called reciprocal right??OK thanks.

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