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26 year old woman has produced two children with Down syndrome,and she has also two miscarriages.Which of the following would be the best explanation?

A.Woman > 35 years of age

B.Robertsonian Translocation between 14 and 21

C.Meiotic Non-dysjunction

D.Mitotic non-Dysjunction




I too went for C,But the answer given was B-Robertsonian transolcation...

Any idea??The explanation goes like this :

Mother transmits the translocated chromosome to each of her offspring.If she also transmits her normal copy of chromosome 21,then she will effectively transmit two copies.By fertilisation it turns into a trisomy.The miscarriages may represent fetuses that inherit 3 copies of the long arm and were spontaneoulsy aborted.


Lol , you have to miss some of them :-(damn thats like 2 in a row wrong )....Oh well I knew that robersonian translocation caused downs but I would think that sometimes meiotic disjunction could cause miscarriages too. The reason being since monosomy in autosomes are not compatible with life, in meiotic nondisjunction 1 gamet would be created with one extra, and other gamete one less. Was it a usmle world question ?


No,Not in USMLE World.I am little confused with the explanation given for the miscarriage.In robertsonian translocation,Miscarriage is mostly due to monosomy and not due to trisomy I suppose..and also,If miscarriages inherited 3 copies ,how the two Down's children ,with the same 3 copies,Survived??


3 copies of 14 or mono 21, or mono 14 - not compartible with life
and when she has three 21s - Down
if you have Kaplan you can find explanations in Cytogenetics...
honestly, my first thought was B, but when i started to read notes, it could be C too, because in non-disjunction there are potential trisomies and monosomies and last are not compatible with life. given that she has either Down or miscarriage - it most likely C
with RT there is a window for normal child, in nondisjunction on Ch21 - only 2 choices
but i found this question in Bio notes, and choices there are completely different, theonly one is RT, there is no choice of ND. go figure


besides, 90-95% Downs are ND and only 5% - RT



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