I just started reading Blueprint by Robert Plomin. He makes some well-known statements about genetic similarity which we all have read somewhere but I would like to know what the criteria for genetic similarity actually are:
He writes that all humans are the same for 99% of the 3 billions base pairs of our DNA. I take that to mean something like this: If our DNA had only 100 base pairs and we compare two individuals, those two individuals' DNA would differ in only one place out of the 100 base pairs.
But he also writes that siblings are (on average) 50% similar genetically. I know the rudimentary story of two chromosome pairs, one from mother, one from father, etc. My question is simply how this can be spelled out precisely. I mean, if any two humans are 99% similar in terms of their DNA, how can two siblings be only 50% similar genetically?
I have a slight suspicion: 99% similarity between the DNAs of any two human beings leaves out 30 million base pairs where they are different. Two siblings then are, in addition to the 99& similarity in base pairs, also 50% similar regarding the remaining 30 million base pairs. Thus, two siblings differ from each other in only 15 million base pairs. SO the 99% shared DNA is what makes us human, as opposed to apes, and the rest constitutes the arena in which individual differences take place?