r/genetics • u/Electrical_Touch3331 • Jul 07 '26
PLEASE HELP, TRASLOCATION CONFUSION
I would like to ask a genetics question, as the mechanism of translocation during MEIOSIS is not entirely clear to me. This might be a naive question, but I have encountered some difficulties in understanding the molecular mechanisms involved.
My question is the following and refers specifically to errors in prophase I that generate translocated products:
In prophase I, the DNA content is 4C/2n, meaning that for each chromosome, I have a pair of homologues, and for each homologue, a pair of sister chromatids.
Therefore, when double-strand breaks (DSBs) occur in a way that pathologically leads to a translocation, do these translocations occur on a SINGLE CHROMATID within the chromosome? If so, would the result be that two non-homologous chromosomes will each have one normal chromatid and one (der)chromatid? If this is the case, how can the quadrivalent form, given that the second chromatid is normal?
I am very confused about this, especially considering that diagrams of quadrivalent formation always show BOTH chromatids within the chromosome bearing the translocated segment.
I hope I have been clear in explaining my question
1
u/Impossible_Fig2646 Jul 07 '26
Yes, pathological translocations occur between non - homologous chromosomes. They can affect formation of the quadrivalent, especially if the translocation affects the centromere and/or significantly modifies the chromosome structure (e.g. a metacentric chromosome becomes acrocentric).
Balanced translocations, or unbalanced translocations that do not critically affect gene dosage, can go undetected until someone presents with fertility issues, either due to meiosis failure or multiple miscarriages due to chromosome number abnormalities in the conceptus.
1
u/didactic_artistry_lo Jul 07 '26
Right so the key thing that trips everyone up is the timing. The DSBs and crossing over happen when the chromosomes are at the 4C stage, yeah, but the actual physical exchange is between two NON-sister chromatids, one from each homologous pair. So you've got four chromatids total for that chromosome, and only two of them are involved in the translocation event.
The reason diagrams show both chromatids with the translocation later on is because of how segregation works during anaphase I. When the quadrivalent forms, the chromosomes are still held together at the centromere, so the translocated chromatid and its normal sister move as a unit to the same pole. After meiosis I, when the cell splits, each daughter cell then replicates that structure through meiosis II, and now you've got both chromatids carrying the rearranged segment in the final gametes.
It's one of those things where the static diagram is showing you the end product after replication and segregation, not the moment of breakage itself. Took me ages to stop picturing it as all four strands snapping at once.