Do siblings share 50% of their genes? (This is my first question on Maths Stack Exchange, I'm not sure if this is a maths question or a physics question ... or something else.)
I was watching this video about psychology, and it it the presenter asserts (at about 5:45) that:

Identical twins share ... 100% of their genes ... whereas non-identical twins, ... just like any brother and sister, share only 50% of their genes.

Now this doesn't seem quite right to me. I can accept that:


*

*Identical twins share 100% of each other's genes (being split from the same egg).

*Brothers and sisters (twins or not) share 50% of their parents' genes

Am I wrong in thinking that brothers and sisters don't necessarily share 50% of each other's genes?

Assuming the parents share no common alleles , then each twin will inherit $50\%$ of each parent's set.  So for every gene, there is $50\%$ probability of both twins inheriting the same allele from a parent. There for the expectation is that the twins share half of their alleles.
However, since it is likely that the parents actually do share some (indeed, rather many) alleles, the expectation should be somewhat higher than one half.
