a. Humans have 23 pairs of chromosomes (because each human got one set of 23 chromosomes from mom and one set of 23 chromosomes from dad) that duplicate and then divide twice during meiosis. At the end of meiosis, each cell (whether that be an egg or a sperm) has only one of each of the 23 chromosomes, which is half of what the cell started with. It is random which chromosome of the original...
a. Humans have 23 pairs of chromosomes (because each human got one set of 23 chromosomes from mom and one set of 23 chromosomes from dad) that duplicate and then divide twice during meiosis. At the end of meiosis, each cell (whether that be an egg or a sperm) has only one of each of the 23 chromosomes, which is half of what the cell started with. It is random which chromosome of the original pair ends up in an individual egg or sperm. Because there are two possibilities for each chromosome and 23 different chromosomes, there are
`2^23=8,388,608`
possible combinations of the 23 chromosomes for an egg or sperm.
b. Now, to make an individual one sperm meets up with one egg. An individual has a `1/2^23` chance of getting a certain combination of chromosomes from dad and a `1/2^23` chance of getting a certain combination of chromosomes from mom. Rules of probability tell us to multiply those together when we want both events to occur, so that leaves a `1/2^46 = 1/70368744177664` chance of an individual getting the specific set of chromosomes they have.
In order for this to happen again for an identical sibling, we would then multiply this by itself (using the same rule of probability), and we have a `1/2^92 = 1/4951760157141521099596496896` chance of another individual having the identical combination of chromosomes. And if you take crossing over into account, the chances go lower still. This is why it is essentially impossible to have two identical individuals from the same parents (excluding identical twins of course).
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