> People share exactly 50% of their genes with their parents and children, but only probably 50% of genes with siblings -- it could be 51%, or could be 48%.
I don't see why the percent shared could not be higher than 50% with your parents or children. You can share a gene with someone that you did not get it from or give it to. For example, suppose some particular gene has three variants, V1, V2, and V3. If your mother has V1/V2, and your father has V2/V3, the possible cases for you are V1/V2, V1/V3, V2/V2, and V2/V3.
In two of those cases you share 50% of that gene with each parent. In the other two cases you share 50% with one parent and 100% with the other parent.
What I wrote is what I got from reading the book a long time ago. Of course biology has advanced since the 1970's -- and of course my memory or understanding of it could be faulty because I read it as a teenager.
What you wrote might explain why parents have "favorite" children they're more likely to give their life for.
I don't see why the percent shared could not be higher than 50% with your parents or children. You can share a gene with someone that you did not get it from or give it to. For example, suppose some particular gene has three variants, V1, V2, and V3. If your mother has V1/V2, and your father has V2/V3, the possible cases for you are V1/V2, V1/V3, V2/V2, and V2/V3.
In two of those cases you share 50% of that gene with each parent. In the other two cases you share 50% with one parent and 100% with the other parent.