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First cousins share 1/8 of their DNA, which is sort of meaningful. Second cousins share 1/32 of their DNA or about 3%, which is really marginal in terms of thei

First cousins share 1/8 of their DNA, which is sort of meaningful. Second cousins share 1/32 of their DNA or about 3%, which is really marginal in terms of their being any resemblance or affinity due to DNA. By the time you get to third cousins the shared DNA is down to 1/128 or less than 1%. So I've lost interest in tracking down any relatives beyond second cousins.

scandox 5y ago
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?
- zzzzzzzza 5y ago
  he's probably talking about snps, single nucleotide polymorphisms, which are pretty much the places where human dna is different from one another.
- Merrill 5y ago
  A child inherits half of the mother's DNA and half of the father's DNA. This is because in somatic cells chromosomes come in pairs, but when sperm and eggs are produced, the pairs divide and each sperm or egg contains only one chromosome from each of the 23 pairs. 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.
- haihaibye 5y ago
  When talking about relatives the fraction refers to the amount of variation in the population. 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

scandox 5y ago

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?

zzzzzzzza 5y ago
he's probably talking about snps, single nucleotide polymorphisms, which are pretty much the places where human dna is different from one another.
Merrill 5y ago
A child inherits half of the mother's DNA and half of the father's DNA. This is because in somatic cells chromosomes come in pairs, but when sperm and eggs are produced, the pairs divide and each sperm or egg contains only one chromosome from each of the 23 pairs. 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.
haihaibye 5y ago
When talking about relatives the fraction refers to the amount of variation in the population. 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