I'm very ignorant about these matters but I thought we all shared the vast majority of our DNA? I mean as humans are we not all 99.9% the same ... DNA wise?
What does the 1/8 mean in this context?
What does the 1/8 mean in this context?
Therefore, given that you have received one of each pair, the probability is 1/2 that your sibling received the same one of the pair. Thus, siblings share 1/2 of their DNA on average.
When you get to first cousins, each line of descent from the common grandparents is diluted by 1/2 from the grandparents that they do not have in common.
Thus Nth cousins share on average 1 / 2 ^(2N+1) of their DNA in the sense that the shared DNA is inherited from their common ancestor. This is the only part of their genome that would cause their phenotypes to be similar. Any other similarities between cousins is by chance or because their relatives may have married into similar ethnic groups with shared deeper ancestry.
To simplify, say every 100 bases are random and all humans share the 99% that are the same.
A brother and sister have 50% in common out of the 1% that varies, so they are 99.5% identical relative to whole genome or 50% relative to the variation.
It depends on what population you use as a reference too. Relative to white people, two average white people are unrelated. Relative to the 'average human' they are as related as uncle/nephew. Relative to Bushmen/Pygmy/Andaman islanders, closer still. Relative to a banana, much closer than siblings (in the usual measure) though not quite identical twins