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It is often cited that humans share 99% (or 98%) of their DNA with chimpanzees.

On the other hand it is stated that siblings share only half of their genes.

What (if any) is the difference between DNA and genes; or, how can the above two claims be reconciled? (I am assuming of course that chimps and humans are more different from each other than two siblings are from each other.)

(I am asking this question as a layperson, who had a little biology in school but not much more.)

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Note that siblings sharing half their genes only reflects the approximation that they get one of each pair from their father and one from their mother. If the father and mother have the same genes at that location, the siblings will match 100% –  Ross Millikan Jul 29 at 14:01
    
Human DNA and chimp DNA is very similar(depending on how you measure, 98% or more). Siblings are even more similar, and they are guaranteed to share at least 50% of their DNA. People say siblings 'share' 50% of their genes because that is the fraction that is always identical, even though in practice way more than that is identical. The two statements are contradictory because they use two different meanings of 'share': 'Happens to be the same' and 'Is enforced by genetic machinery to be identical'. –  Resonating Jul 29 at 15:14
    
It is not a duplicate of the marked question; that question only considers genes vs. DNA - this question asks about the conflict between 50% of genes being shared with siblings and 99% of DNA with chimps. –  Jack Aidley Jul 30 at 8:03

4 Answers 4

Here's a language metaphor. In a single language, words may be said or written across a spectrum of accents or spellings but have the same meaning. Likewise in a single species, genes (words) may exist in a spectrum of diversity (DNA sequence) but maintain the same meaning. So, when 2 species share "most of their DNA" they share most of the same genetic meaning (like 2 languages which share word origins). While siblings who share "half their DNA" share half sequence identity (same accent or spelling).

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This has nothing to do with the proportion of the genome that is coding as suggested in another answers.

The reason the figures are so different is because they are measuring different things. The chimp-human figure is measuring sequence similarity whilst the figure for kin are measuring gene similarity by descent.

This gives two sources of difference in the two numbers: firstly, there is the distinction between sequence similarity and gene similarity. Let us suppose you have a 1000bp gene, which contains or does not contain a single nucleotide polymorphism. These two variants are different alleles so if you're considering gene similarity then you would say these are 100% different but if you're comparing sequence similarity then you would say that there is just a 1bp difference so these are 99.9% similar. This is how chimps can be both so similar to humans and so different - it is not a matter of how many base pairs are different but where those differences are. A single base pair change in a 1000 bp gene can have a dramatic effect.

Secondly, the figure for relatedness to your parents/siblings is talking about similarity by descent. In fact, your actual gene similarity to your parents is likely to be substantially higher than 50% because your parents are likely to have the same version of many genes so which parent you inherit the gene from makes no difference. Additionally, they will also be homozygous for a number of alleles so that you always inherit the same version of this gene from that parent and are thus more related to your siblings than a crude calculation of relatedness would suggest. The 50% figure comes from only considering two genes the same if they come from the same source, i.e. similarity by descent.

Another, perhaps, better way of thinking of this would be: among those genes that exist in different forms in your parents you will share 50% of them with your parents and 50% of them with each of your siblings on average.

As a bit of an aside: the figure for similarity between human and chimp DNA you've given is based on crude annealing. So if a highly similar piece of DNA occurs in both human and chimp DNA they will match up but this does not consider how the DNA is arranged within the genome so it actually misses a considerable amount of the variation between organisms.

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The human genome is composed of roughly 3 billion DNA bases called "nucleotides", typically represented as A, T, C, G.
Genes are sequences of nucleotides that encode a protein; but they only comprise about 2% of the full genome. (http://en.wikipedia.org/wiki/Human_genome). Most components of cells are made of proteins. The rest of the genome is made up of nucleotide sequences that:

  • have regulatory functions in controlling when and to what degree genes are expressed to create the proteins they encode
  • do not have any known function
  • are likely remnants from certain viral infections we or our ancestors contracted

The DNA that makes it into our genome is not necessarily there because we need it or because it confers some kind of reproductive advantage. A lot of our DNA is in our genome just because our ancestors had it and they happened to be reproductively successful.

So basically: humans and chimpanzees can share 99% of their DNA and have quite different genes because genes make up only a very small part of our genome.

That said, having similar nucleotide sequences in our genes does not guarantee that we will be similar because we still need to consider things like:

  • how similar are the genes really? Some differences in a gene's sequence mean more than others
  • how similar are regulatory sequences affecting the gene? Having the same gene as another species is not the same as expressing it the same
  • how many copies of the gene are present? There's more than one way to take this into consideration

For more information about comparing genes, refer to the other answers offered which go into greater detail.

Here's a genetics learning resource that is pretty accessible to laypeople: http://learn.genetics.utah.edu/content/basics/

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Also, different alleles are often very similar when comparing nucleotide sequence. The difference may be as small as a single nucleotide polymorphism. –  canadianer Jul 29 at 4:36
    
The proportion of the genome that is coding has absolutely nothing to do with the answer to this question. The same conditions would hold for, say, yeast which has a much higher C-value. –  Jack Aidley Jul 29 at 13:42
    
The question asks about the difference between DNA and genes; pointing out that genes make up only a small fraction of the genome is an important distinction for lay people. –  Slavatron Jul 29 at 14:23
    
Sorry, I should have specified: the bolded bit of the answer is completely and utterly wrong. In fact the sequence similarity between chimps and humans is higher for genes than for non-coding DNA as you should expect. The rest of the answer is fine. –  Jack Aidley Jul 29 at 14:40
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I see what you mean; you're right - that is wrong. The proportion of coding/non-coding DNA has nothing to do with the similarity of either between individuals. –  Slavatron Jul 29 at 15:02

When people say that siblings share half of their genes, they're talking about alleles, which are different versions of the same gene and generally differ slightly in nucleotide sequence. In essence, all humans share 100% of their genes (almost), but the number of shared alleles varies.

Example:

geneA, allele1: atgccc
geneA, allele2: atgccg
geneA, allele3: atgcac

Indeed, we share about 99% of our DNA and consequently genes with chimps and bonobos. There are alleles of these genes that are unique to humans, unique to chimps and unique to bonobos. Of course, there are also many alleles that are shared between all three species. There are more shared alleles still between chimps and bonobos, since their most recent common ancestor lived after the most recent common ancestor of humans, chimps and bonobos. However, looking at the above example, you can nonetheless have a situation where Joe Sixpack and Charlie Chimp both have allele1 of geneA, whereas Joel Sixpack has allele2 of geneA.

Also, the conclusion of Slavatron's answer is complete nonsense.

Somebody made the same question here.

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Curious as to what you think about @Slavatron's answer is nonsensical. You both look to have given correct answers to me, approaching this complex question from different angles. –  Matt Thrower Jul 29 at 11:30
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The fact that humans and chimps share about 99% of their DNA means that they also share about 99% of their genes. Humans on the other hand share practically 100% of their genes with one another. There might be a tiny difference due to "horizontal gene transfer" from Neanderthals, Denisovans and other possible archaic humans to just certain populations, and I guess that it's also possible that some bottle-necked populations have lost something, but in practice, humans share all their genes with each other. It's the alleles that vary within families, populations and such. –  5heikki Jul 29 at 13:12
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@5hekki Ah, I see. It's confusion over genes/alleles that's caused that, I suspect. Thanks for clearing it up. –  Matt Thrower Jul 29 at 13:28

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