Since they are from the same parents can siblings or especially have the same DNA. its the same set of 50% genes right? What makes them different if there are not the same since the prerequisites are identical.
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Let’s set aside identical twins for a second, because they are not created by different egg and sperm cells, but in fact split early in the development process after fertilization.
They could, theoretically (we’ll ignoring mutation). But the odds of that happening are so incredibly low that in practice it’s never going to happen.
A person’s genes contain 46 genes in 23 pairs. When the egg and sperm is created (through meiosis) those pairs are split so one of the pair goes in one cell the other pair goes in another. So possible number of different egg/sperm cells you can make is 2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2=8,388,608.
But that is only from one parent. The egg and sperm are separate so for each possible gene combination in the egg you have all those possible combinations for the sperm as well.So the total number of chromosome combinations for a child is 8,388,608*8,388,608=70,000,000,000,000. That is 70,000 BILLION posibilities, all of which are equally likely.
Now for identical twins, they will still have the same chromosome combinations, but there will likely be a small number of generic differences due to random mutations that happen when each is only a small number of cells.
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