Question for any genetics expert

[hmb]

In translocation cases of downs syndrome, does all the chromosome become attached to another, or is part left behind? Does the fragment (if there is one) separate during meiosis? If only a fragment is attached does that affect the symptoms produced?

And do translocation cases happen with Klienfeltners?

(Just teaching this to sixth form and hoping that Encyclopedia Mumsnetica can help me out again!)

Hello! Just a quick response, as I'm just off to take ds swimming, and because I am completely useless at cytogenetics because it involves spatial awareness. I think in most cases the translocation is a Robertsonian, end-to-end fusion, so there's no fragment. The person you want is jimjams...help!! She knows all about Rb translocations. There must be reciprocal ones though, because that's how they defined the DSCR (Down's syndrome critical region) in which the most important genes must lie. I'm not helping much, am I? I'll try again later. Pubmed is down, but the "books" link on there would probably help. See you later!

Ugh Rb fusions yuck yuck yuck! (PhD was on Rb fusions-although in mice who are quite happy to undego fusion). Yes usually the fusion is Rb ie 21 fuses to another uniarmed chromosome, loss can occur but only of small amounts.

The exact breakpoints can vary, and small amounts of chromosome can be lost (usually its centromeric heterochromatin- although in the house mouse Rb fusion usually involves he loss of telomeric sequences and some satellite DNA sequences). Humans have been shown to sometimes have dicentric chromosomes, so in that case the loss if any would be very small indeed.

i guess if too much was lost then the embryo would be non-viable. The main effect of losing some sequences is that it makes fission unlikely (although in this case I'm thinking of balanced arrangements- it wouldn't really be relevant for DS).

Kleinfelter's won't involve Rb fusions as the X chromosome is metacentric so it can't fuse to another chromosome iyswim.

HTH

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I knew that Encyclopedia Mumsnetica would come up with the goods. Thank you ladies! Re the Klienfelter's, yes, and 'Dur! Stupid me!'.

Twiglett