# ErnstMuller comments on Why is my sister related to me only 50%? - Less Wrong Discussion

4 06 May 2011 06:17PM

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Comment author: 08 May 2011 11:33:18PM *  -1 points [-]

Because it works something like this, in a grossly oversimplified explanation:

Human genes are organized on 23 chromosomes, 2 complete sets of them, so 46 in all, with 50% being redundant. Your DNA is fragmented in 46 stripes of neatly ordered DNA.

Information on these two sets of chromosomes aren't identical, just redundant: On one set could for example be the information to make fair hair, on the other the information for brown hair, with the carrier of those genes getting brownish hair.

If a human produces gametes (cells used for reproduction, sperm and eggs), each gamete will get half of the set of chromosomes.

So, to oversimplify even more, your daddy will put one of its two sets of chromosomes into his sperm, yielding sperms with each half of his whole set of chromosomes. For the sake of clarity, lets call the sets of sperms he can produce D1 and D2. Mommy will do the same, just getting M1 and M2.

If they procreate, the embryo will get chromosomes from 2 randomly chosen gametes, which can be M1 or M2, and D1 and D2.

That leads to following possible genotype-mixtures: M1D1 M2D1 M1D2 M2D1

The outcome is completely random, so you would have a chance to get a sibling with identical genes 25%, with half of your genes 50% and with none of your genes also 25%. Which gets to an average of 50% of identical genes per sibling.

Evolution works with huge numbers and enormous timescales, so it is save to assume that this statistic works out true and is safe enough to bet on.

Less simplified, each the chromosomes which is put into a gamete is randomly chosen. So, each gamete gets 23 completely randomly chosen chromosomes from the whole set of 46 chromosomes. (Keyword here is meiosis, biologogenes for "making gametes"). And, to complicate matters further, in one step of the meiosis the DNA-strands which make up the chromosomes are broken randomly (but at the correct places) and joined. (This process is called crossover.)

So, the body does it's best to make the genetic makeup of any gamete to a very randomly chosen set of all genes in the body. (That's because the body does not know and isn't able to find out which of his genes make him fit in an evolutionary sense. So, randomness is the best chance. Again, that's very simplified.)

That boils down to the fact, that siblings share approximately 50% of their genes, give or take a few.

If you want to know that a bit better, read Dawkins "The selfish gene" (no atheism in there, just good old biology). Really good book, which clarifies many things regarding evolution and genetics with just traces of chemistry.

Or, if you want to know it really good, read a book about genetics, I would recommend the tome "Genes IX" by Benjamin Lewin or similar.) (Serious biologenes and chemistry there, though.)

Hope that helped.