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To be specific: Assume chromosomes A and B are homologous. They've both replicated into A1, A2 and B1, B2 and have formed a tetrad at the equator (synapsis). Most textbooks show either A1 and B1 OR A2 and B2 exchanging genetic material, but I've never seen an experiment/paper proving that this is actually the case, and now I've come across this figure showing both A1, B1 and A2, B2 crossing over https://web.archive.org/web/20160314020121/http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm see ("crossing over" under meiosis)

enter image description here

So which version is correct, or do we simply not know for sure? Thanks in advance.

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    $\begingroup$ Welcome to Biology.SE. You might want to correct. It is sister chromatids and homologous chromosomes (not homologous chromatids). $\endgroup$ – Remi.b Jan 13 '16 at 15:50
  • $\begingroup$ In the case of meiosis genetic material is not being exchanged between sister chromatids, and to simply say that crossing over happens between homologous chromosomes doesn't really denote which of the two cases listed in the question is happening. I dunno what else to call this :/ $\endgroup$ – E.L. Jan 14 '16 at 10:16
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Yes, it is perfectly possible for both pairs of non-sister chromatids to cross over in a single tetrad. The "standard textbook" depiction is a simplification, true tetrads can sometimes be very complicated and the nested crossing-over would be difficult for entry-level textbooks to cover.

enter image description here

This microscope image clearly shows two chiasmata occurring on two sets of chromatids, with both of the "blue" chromosome's chromatids undergoing crossing over.

In fact, even more complicated chiasmata complexes can form during meiosis. For example, this slide depicts many chiasmata occurring in a single tetrad. More examples can easily be found by searches.

enter image description here

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One or both pairs of homologous chromosomes could exchange genetic material during the process of crossing over. The number of cross-over events depends on the number of chiasmata. There are also ways for meiosis to occur with no exchange of genetic material between homologous chromosomes.

The image you reference at uic.edu is incorrect. The purple arms and red arms would not be "swapped" like that until anaphase of first division. The sister chromatids are bound together prior to anaphase of first division. Many textbook and internet depictions of this are incorrectly drawn.

At diplonema of first division, sister chromatids A and A (and B and B) have axial cores holding them from separating. There are chiasmata holding the bivalent (all four chromatids) together that end up being cross-over sites. For each chiasma (singular) two of four strands are affected. There can be multiple chiasmata per bivalent. Humans average two chiasmata per tetrad (four chromatids). Without at least one chiasma, fertility would suffer.

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    $\begingroup$ Can you provide references for your claims? $\endgroup$ – March Ho May 9 '16 at 14:53

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