Crossing over — how important is it really?
Can someone explain how important crossing over actually is? I know it happens in prophase I but is it just a small detail or something super critical? Does it happen every time? And how much variety does it actually create compared to independent assortment? Thanks for any clear answer!
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Crossing over is extremely important — without it genetic diversity would be much lower. During prophase I of meiosis homologous chromosomes come very close together (synapsis) and form structures called chiasmata. At these points small pieces of DNA are physically exchanged between maternal and paternal chromosomes. One chromosome can end up having some genes from mom and some from dad on the same strand.
This creates brand new allele combinations on single chromosomes that didn’t exist before in parents. Independent assortment already gives a lot of variety (for humans 2²³ ≈ 8 million possibilities), but crossing over multiplies that number enormously because each crossover point creates new haplotypes.
On average 1–3 crossovers happen per chromosome pair in humans, but it can be more. Without crossing over siblings would inherit much larger blocks of DNA unchanged from grandparents.
For very clear pictures and step-by-step description of when and how it happens check this: phases of meiosis