Crossing over happens in pachytene, specifically when homologous chromosomes are fully synapsed. I know this chala well because I just finished sketching pachytene chromosomes for my master’s thesis poster, labelling those chiasmata. It’s not zygotene—by then, we’re only just pairing, and my late nights in the lab staring at slides proved you won’t see crossover events until those bivalents are tight.
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