Robertsonian Translocation

[Ronin786]

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So I'm having a bit of trouble understanding Robertsonian Translocation, namely this image:

http://www.ncbi.nlm.nih.gov/books/NBK21947/figure/A2884/
I have a few questions about it:
1) I read that theres a 1:3 chance that the child born has the Down syndrome phenotype. Is this because that one dead child doesn't count? So if the question was, what are the chances that a Robertsonian carrier would have a Down child would it still be 1:3, or 1:4? Shouldn't it be 1:4 because that one fatal is still considered an "attempt"?

2) This might not exactly be Robertsonian, but can someone explain the meiotic pairing part? Why do those chromosomes split the way they do into the 4 gametes? Its probably a stupid cell bio concept i'm zoning out on, but a simple explanation would be appreciated.

Thanks!

 

[newmanstep123]

My opinion:
1) it emphasize baby born, 1/3 will be Down's; as for the lethal one, remember that Turner is the only full monosomy that is seen in humans.
2) as for the the meiotic pairing part, the figure is some kind of misleading. Indeed, should be illustrated as following:

Pairing at the first meiotic division can occur three ways, with equal chances for all. When it occurs as shown in A, one secondary oocyte or spermatocyte will get the 14q21q translocation chromosome and the other secondary oocyte or spermatocyte will get a normal 14 and a normal 21. When fertilization occurs, with a normal 14 and a normal 21 from the other parent, the zygotes formed from the gametes with pairing as in A will be either 45, XX or XY, -14, -21, +t(14q21q) a phenotypically normal balanced Robertsonian translocation carrier, or be a 46, XX or XY normal individual. When pairing at first meiotic division is as shown in B, one secondary oocyte or spermatocyte will get chromosome 14 only (no chromosome 21) and the other secondary spermatocyte or oocyte will get chromosome 14q21q and chromosome 21. At fertilization, again the zygote will get a normal 21 and a normal 14 from the other parent. When the gamete from B that got only chromosome 14 unites with a normal gamete, there is monosomy for chromosome 21. That results in very early embryonic death and spontaneous abortion. When the gamete from B that got the 14q21q chromosome plus chromosome 21 unites with a normal gamete, one gets the prober dose of necessary genetic information for chromosome 14 (14 from the normal gamete and 14q of the translocation chromosome) but one has three copies of the genetic information of chromosome 21(21 from the normal gamete, and a 21 and a 21q from the translocation). This results in Down syndrome. When, by chance the chromosomes line up as at C, both gametic products are lethal early in development. One will get only chromosome 21 and will be lacking a chromosome 14. Monosomy for chromosome 14 is lethal. One will get a 14 and a 14q21q, resulting in a zygote that will be trisomic for chromosome 14. This trisomy is also lethal.

 

[Phloston]

All you need to know for the Step1 about Robertsonian's:

1) Involves two acrocentric chromosomes, where the p-arms are lost.

2) Accounts for ~4% of Down syndrome cases.

3) Suspect an RT if a woman has repeated first-trimester miscarriages (don't be fooled; I've seen a vignette where they mentioned this AND also mentioned that the husband was SIXTY-EIGHT years-old, and "husband's age" and "RT" were both answer choices, and the answer was RT.

4) Carriers of RT have forty-FIVE chromosomes, not 46. Makes sense, but when you're rushing through an exam, don't slip on that.

 

[Ronin786]

Thanks for the replies, although I'm going to have to go through Newman's when I'm a bit more clear-headed.

And its 45 chromosomes but 46 chromosomal material, right? Which is why they have no phenotype. And in that case, why isn't it considered a monosomy?

 

[Phloston]

There are 45 in RT carriers and 46 in those with Down's syndrome.

Wikipedia lists [45,XX,der(14;21)(q10;q10)] as the karyotype in Down's, but as far as I'm aware, that's the carrier karyotype, not the diseased one. If you change it to 46, then it's Down's.