Siblings

[MCB]

Okay, so i know that full siblings share on average 50% of their DNA. What i can't seem to find an answer to is why is this only on average? Can the DNA shared differ to amounts greater than or less than 50% in full siblings (i.e. a brother and a sister actually share only 42% of genes)? How much can full siblings dna differ from 50% of genes shared, and what is the reason that siblings may share more or less than the average 50%? I am asking about regular siblings and not identical twins.

[-Demosthenes-]

They can theoretically share no genes, if I'm not mistaken. Half of my genes are from my mother and half from my father, and half of my brother's genes from from his mother and half from his father, but not the same halves. Every gamete (sperm and egg) contain half the genome of the parent, but a random half. So I can be genetically closer to my sister than my brother -- if she happened to get a more similar gamete (similar to the gamete I got or came from whatever) from our mother or father or both. And the opposite can be true as well.

[Mr Skeptic]

Or if you want an experiment to do to, flip a coin 10 times, writing down a row of H or T for heads or tails. Then do it again, writing the second row under the first. On average, half the flips will match, but sometimes they will match more and sometimes less. This is analogous to the genetics as described by Demosthenes.