Cool soundbites about genetics can confuse more than they clarify. An illustration. "You share 98.8% of your genes with a chimpanzee!" Is this true? What does it actually mean? Let's unpack this often quoted pearl of wisdom in a thread all about sharing...1/13
...Statements about sharing genes are notoriously slippery. Here's another. "Identical twins share 100% of their genes, but non-identical twins share 50%, same as regular siblings." So, I share more genes with a chimpanzee (98.8%) than with my own sister? Something's up
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...First, what's the origin of the 98.8% estimate of chimp/human sharing? It relates to matching sections of the genomes of the two species, where we can align them & count how many DNA bases are identical. On average, for every thousand aligned bases, twelve don't match...3/13
...This 1.2% difference can be interpreted in many ways. Some cite it as proof that, from a genetic perspective, we are just another ape. Others marvel at how tiny changes in genomes can, by altering gene regulation, yield key differences in physiology, anatomy & cognition...4/13
...Yet others point out that, actually for genomes of well over 3 *billion* bases, 1.2% corresponds to at least thirty-five million differences in DNA sequence, which is a huge catalogue to sift through if you are searching for genomic changes that might make us human...5/13
...But that's not even half the story. When the first draft chimpanzee genome was assembled, large parts couldn't be aligned with the human sequence at all. There were many places in which stretches of DNA had been inserted or deleted during the evolution of each species...6/13
...Taking into account insertions/deletions, human-chimp genomic identity drops to <96%. (Is that high, medium or low? Your call.) Various types of chromosomal rearrangement also distinguish the species. PLUS those comparisons were based on *incomplete* reference genomes...7/13
...Ape genomes have parts that we struggle to decipher with routine sequencing (e.g. due to complex repeats). Prior announcements of complete genomes ignored such gaps. Recent studies used novel methods to produce high-quality ape genome assemblies with enhanced coverage...8/13
...Comparisons of new ape assemblies from this year find genomic differences that are complex & diverse e.g. >17,000 human-specific insertions/deletions, including hundreds with potential functional impact. Bottom line, we can't reliably capture it all with one neat metric...9/13
...Back to the other stat from the top of the thread: "siblings share 50% of their genes". Here "share" means something quite different. A person has two copies of every gene, one from mum, one from dad. (Ignoring sex chromosomes here, that's something for another day.)...10/13
...Gene copies get shuffled every generation (recombination). Your genome is a patchwork quilt of gene copies from four grandparents. If we trace origins of each copy of each of your genes, & do the same for a sib, about half (on average) will come from the same source...11/13
...We can trace the distinct (grand)parental origins by tracking DNA variations; a tiny proportion of the genome, but scattered throughout it. If we align matching parts of your DNA with that of a sibling (as we did in the human-chimp example) 99.9% of bases are identical...12/13
...There's a bigger issue here, given the rise of direct-to-consumer genetic tests. Fun facts about genetic sharing (what % Neanderthal are you?) must be handled with care. Focus too much on percentages, taken out of context, & we lose sight of the biology that lies beneath.13/13
And another thing. For an especially lucid, entertaining & accessible account of genes you share with other humans across the world, past, present & (perhaps) future, get yourself a copy of @AdamRutherford's brilliant 'A Brief History of Everyone Who Ever Lived'. Message ends.
