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Obviously I am more related to my sister than to a chimpanzee, so what do these different percentages actually refer to?

Here is my preliminary research:

https://genetics.thetech.org/ask-a-geneticist/siblings-are-around-fifty-percent-related/

https://genetics.thetech.org/ask/ask395/

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    $\begingroup$ Related : biology.stackexchange.com/questions/61206/… $\endgroup$ – user237650 Sep 9 '18 at 15:11
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    $\begingroup$ Possible duplicate of How can someone share 50% of their DNA with their parents yet all humans share 99.9%? $\endgroup$ – Bryan Krause Sep 10 '18 at 16:14
  • $\begingroup$ The obvious answer is that the two sources you mention are using "% related" in different ways, and ones which are probably not clearly defined, althoug I cannot tell without a full quote (no, it's not up to us to read the references). On the one hand this site is concerned with questions to specific biological problems rather than correcting popular science on the internet, but, as two users have pointed out by their comments, this sort of confusing statement is widespread. A wider question with a more comprehensive answer would probably be useful, but it may take a little time. $\endgroup$ – David Sep 10 '18 at 20:37
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50% related to sibling means that, out of the portion of the genome that vary in humans, you share half of them with your sibling. (if you compared total genetic match not just ones that actually vary in humans you are looking at something closer to ~99.95% match) Now because because variations are not evenly distributed and your parents likely came from the same source population you usually share more with a sibling, but recombinations has a random aspect, and sex chromosomes have a large effect as well. So 50% would be the average using the most common methods of comparison, and even then it is only shorthand. It is a calculation more than a conclusion, if you are 50% related to your parents and so is your sibling then the probability calculation is [0.5 X 0.5 = 0.25] Note this is in humans, other animals use different chromosome distributions which can result in different numbers, bees for instance have haploid males making females siblings far more closely related.

95% identical to chimpanzee means out of all the genetic elements only 5% of human ones are unique to humans (or chimps) and not shared by chimps and humans while 95% is shared by both. 95% is a lower estimate because they are using the strictest comparison.

One important thing to consider is there are many ways to compare genetic variance, consider a book as an example, if I switch 2 chapters is that one change, two changes, or hundreds of changes (by the number of letters or words in the chapters). because criteria are different you get slightly different estimates of difference. In the case of the 95% estimate that would be using the "hundreds of differences" method of comparing. Deleting the sentence "Mary had a little lamb" would be 21 changes using this method. Other methods would consider it only one difference since it is the product of a single mutation.

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  • $\begingroup$ What do you mean by "out of the genes that vary in humans"? As far as I am aware all genes vary (i.e. exhibit polymorphism) in humans. So it is the polymorphisms that you share, rather than the presence or absence of a gene. On which point, could you quote a reference to support your assertion that 5% of human genes are not present in humans — the article the poster cites refers to differences in DNA, not genes, and recent questions related to this suggest the percentage of unique genes is much lower. $\endgroup$ – David Sep 9 '18 at 19:00
  • $\begingroup$ No the vast number of genes in humans have no variation, at least ont ones that survive. The human species actually has remarkably little genetic variation. ashg.org/education/pdf/geneticvariation.pdf As for the 5% thats what the 95% similarirty means, it means 95% is shared and 5% is unique. $\endgroup$ – John Sep 10 '18 at 4:34
  • $\begingroup$ The article is not clear about what criteria they are using for comparison so I used the vaguest term available, genes which has both a laymen and more than one technical meaning. I will edit for clarity. $\endgroup$ – John Sep 10 '18 at 4:52
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    $\begingroup$ there are 3 billion nucleotides in the human genome, 10 million/3 billion = 0.003 that's less than 1%. $\endgroup$ – John Sep 10 '18 at 12:48
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    $\begingroup$ Blue eyes, rhesus A-ve, organ donor compatibility. 1 SNP every 300 bases, which is the length of an 100aa gene. No variation? $\endgroup$ – David Sep 10 '18 at 20:30
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The 95% comparison to chimpanzees is within the animal kingdom. That is, compared to other animals, human and chimpanzee genes have a 95% match. The ratio could be as high as 85% between humans and other mammals such as mice.

On the other hand, within the species, Homo Sapiens, you and your sibling may have only a 50% match of genes compared to other human beings. (That would be true if your parents were totally unalike, say European and African. Often, your parents may have some overlaps or even common ancestors. In that case, you and your sibling will have a better than 50-50 match.)

These percentages represent two different levels of "filter."

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