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What happens when two sperm fertilize an egg? This can be either naturally or by experiment.

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up vote 2 down vote accepted

This phenomenon is called polyspermy and results in an embryo which is not viable. This is because it has more than the usual two sets of chromosomes (one from each parent), but one from the mother and multiple (depending on how much spermcells enter the egg) from the father.

To avoid this, most species (including mammals) have protective mechanisms against it. The so called cortical reaction which starts upon fertilization (the first sperm penetrating the zona pellicula) and releases cortical granules to prevent further sperm cells from entering.

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In humans, polyspermy results in molar pregnancy. – Raoul Sep 5 '14 at 0:02
It can also result in semi-identical twins when the egg goes through trisomic rescue to form 2 diploid cells with different paternal DNA but identical maternal DNA. However this is rare – Caters May 12 '15 at 20:24

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