Does crossing over always occur in meiosis?

Is it inevitable that crossing over will occur no matter what in prophase 1 of meiosis? Can I assume it will definitely happen? Also my teacher said that crossing over can occur twice on the same pair of homologous chromosomes, and that this can happen at the same spot where it happened the first time, meaning that... show more Is it inevitable that crossing over will occur no matter what in prophase 1 of meiosis? Can I assume it will definitely happen? Also my teacher said that crossing over can occur twice on the same pair of homologous chromosomes, and that this can happen at the same spot where it happened the first time, meaning that the final result is no net crossing over. Is this true? Also why is the highest possible recombination frequency 50%?
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