# The incredible unlikelihood of being

*By Shane Fletcher On August 11, 2013 · Leave a Comment*

Anyone that has one or more brothers or sisters, or is a parent of multiple children knows that siblings are all unique individuals. The different genetic material in each egg and sperm that conceives them produces vastly different children in looks and personality. There are no doubts that boys are different to girls, but I have 3 daughters that are all quite different from one another. Why is there such a genetic variety in children with the same mother and father? I’m glad you asked. 🙂

Humans have 46 chromosomes, grouped as 23 pairs, each of which is a long string of DNA, the genetic information that makes us the individuals that we are. These 23 pairs of chromosomes are found in the nucleus of every cell in your body, and they are made up of so much DNA that laid end to end it is about 2 meters in length. 2 meters of DNA in every cell in your body. You have ten trillion cells (10 000 000 000 000) so if all the DNA in your body was laid end to end it would make the 300 million kilometre round trip journey from the earth to the sun and back … 66 times. This is not really relevant to the point I’m going to be making, but it blows my mind and I’m sure yours as well. Back to genetic diversity …

Each sperm and egg has only 23 chromosomes, or 1 of each pair that lives in the body of the man or woman. Chromosome 1 can be either of the pair, chromosome 2 can be either of the pair, chromosome 3 can be either of the pair, etc. The number of different possible combinations is calculated by multiplying the number of choices for each chromosome together, or 2x2x…x2 (23 times) or 2^23. There are **8 388 608** different chromosome combinations for each egg. Also each sperm.

When a sperm fertilises an egg, these two single lengths of 23 chromosomes combine into a pair totalling the 46 chromosomes that humans have. The total number of genetically different children a man and a woman can have is calculated by multiplying 8 388 608 (for the number of different eggs) by 8 388 608 (for the number of different sperm) to give a total of **70 368 744 177 664** different combinations. [This doesn’t take into account that there are over 100 genetic mutations in each generation, slight differences in the DNA of the egg and the sperm that make each combination more than the sum of its two parts. This is how evolution works, and it means you can add a couple of zeroes to that total making the unique combinations over 7 quintillion, but let’s just keep it to the simple math for the moment.]

There are over **70 trillion** different brothers or sisters you could have. Just as importantly there are over 70 trillion different siblings *they* could have. There are 70 trillion other people that could have been born instead of you. To put it another way, and perhaps put an unwanted image in your head, if the time of conception was just a fraction of a second earlier or later a different sperm could have reached the egg, *you* would not have been conceived and instead a brother or sister would have taken your place in the family. The fact you were born means you won the lotto. Actually the odds of winning Saturdays lotto, picking 6 numbers out of 45, is a paltry 1 chance in 8 145 060 so the fact that you were born and are reading this, means you won that lotto *8.5 million times*. The chances of you not being here are so very great. And that is just looking at your conception. Let’s go back a generation.

Each of your parents are also a 1 in 70 trillion miracle, dependent entirely on your grand parents conceiving children at exactly the right instant in time. This means there is less than 1 chance in 348 449 000 000 000 000 000 000 000 000 000 000 000 000 000 that both sets of grand parents would have your parent who then in turn would have you. A very big number that we write notationally as 3.48 x 10^44 (for the number of digits to the right of the 3). That’s 2 generations. Going back 3 generations to include when your grandparents were conceived is 8.5 x 10^96 combinations. To compare, there are only 10^80 atoms in the entire universe and 1 chance in 10^50 is sometimes quoted as being a statistical impossibility. The odds get exponentially more minuscule as you go back each generation, doubling the number of people that must have been conceived at just the right time with the right genetic material. 100 years. 1000 years. 10 000 years. The number of combinations get impossibly large and the odds of success get impossibly small.

I will talk about this more in my next post, but for now I just wanted to leave all the parents out there with the absolute knowledge that each of their children is, as they always suspected, a one of a kind miracle.

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