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?
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