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I am a high school student and I am a little confused in a topic related to dizygotic twins, I get to know that dizygotic twins are genetically dissimilar because they are formed as a result of two different fertilizations but my question is that we know sperms are formed by meiosis in germ cells and each germ cell produces 4 sperms which are genetically different from each other but there is not only a single germ cell ,they are in millions so every time meiosis occurs 4 sperms are produced from each in millions so 1/4th of them should share common genetic material? so dizygotic twins can be genetically similar ,isn't it?

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  • $\begingroup$ When sperm cells are formed via meiosis, all resulting mature sperm cells are unique from each other due to meiotic recombination; their genomes are shuffled in such a way that they will never be identical, even between daughter cells of the same germ cell. The deck of cards is always shuffled uniquely, so even if the same daughter pair of sperm meet with a pair of same-daughter eggs, it will still produce a unique individual (zygote). See here. $\endgroup$
    – S Pr
    Apr 26, 2021 at 14:32

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Dizygotic twins are formed as a result of fertilization of 2 different eggs by 2 different sperms. For the 2 zygotes to be genetically identical it is necessary that both eggs and sperm have exactly same genetic material.

You cannot say that 1/4 th of sperms will share same exact same genetic material..during meiosis there is recombination..now there is no defined way that only a certain gene will recombine with the homologous pair..these recombinations are quite random..so each time a new recombinant is formed. Further variation comes during Anaphase as the sister chromatids seperate hence generating even more variant sperm. Even if 2 in million sperms are same..it is not certain that these 2 will fertilize eggs..it can be any other sperm

Moreover for dizygotic twins to be genetically identical the 2 eggs should also be genetically identical..but again the above reasons show that this is quite difficult..(here the eggs won't be in millions.. instead 2 eggs will be released during ovulation but recombinations happen still)

Now probability of fertilization of these 2 similar eggs by 2 identical sperms out of millions is almost nil

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    $\begingroup$ It would be best to include citations. Also, while you talk about meiotic recombination you completely neglected to mention the more fundamental concept of independent assortment, which in and of itself yields an identicalness probability of roughly 1 out 70 trillion on humans. $\endgroup$
    – rotaredom
    Apr 29, 2021 at 10:38

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