Greetings. Can someone please explain to me what it really means when I read that something like 98% of a gorilla’s genes and 99% of a chimpanzee’s genes are identical to those of a human being? Similarly when it is said that a banana plant (I know, the banana is not a tree) and human beings share 60% of their genetic material, what exactly does that really entail? I realize all life on Earth is related and that there is no separate, independent sort of life that isn’t a cousin to everything else, but how can something as different from a man as a banana plant be 60% the same? The great apes are clearly close relations to us, but have the genes that make us different been identified? If so, what is it that they do? Are they genes for hairlessness, for more finely tuned brains? Or what? Excuse me if my percentages are a bit off, and my terminology is wanting, but I hope you get what I mean to ask.
What this means is that by strict genetic sequencing, these genomes are 98 or 99% identical. So a given sequence will be identical at 98 nucleotides out of 100 on average. There are something like 90 MB (90,000,000 bases) of changes between the two including deletions and insertions (example ref: http://genome.cshlp.org/content/15/12/1746.full). (I expect there are a few other comparative sequence reads, all with varying redundancy checks (4x is common). Not completely my area.) This doesn't mean that sequence divergences necessarily occur in functional areas: most of the genome of most species is just 'filler'. Something to fill the spaces between 'functional' genes. Now, as far as what are the functional changes that make a human of a chimpanzee, there are about 50 different coding genes that are missing in humans. There are also a number of gene duplications specific to humans, and that could be important also. But the situation is more complicated because some of the changes even in non-coding sequence might affect expression: sequence changes in non-coding regions might, say, change the stability or 3D organisation of DNA near a gene, resulting in less access of various enzymes to the location and thus a change in activity. So there are other things beside sequence: turning off certain genes via methylation also wouldn't show up in sequence analysis but still might result in such a change. Also, the products of some genes might be regulated above the level of DNA in concert with any of the above. All that being said, this 90 MB the authors quote isn't a trivial distance: that's a lot of material. And there are inversion events also, where little sequence change exists, but where the contents of part of a chromosome have been pulled out as a complete segment and reinserted in reverse, or on a different chromosome - inversion and transposition events are serious interspecies business. And finally, all these changes are in simultaneous existence. That is, they're fixed - so far as we know - between humans and chimps, presumably because of some functional importance. They occur simultaneously. So which one makes the human? Or is it all together, so that 'human' is a kind of state? We don't have any 'partials' with chimp-specific sequence at these 50 genes, above, so that there are intermediates between the two species at the 90 MB which we know is different. There are techniques to investigate, such as inserting human genes in chimp embryos, etc, but that kind of thing is staggeringly unethical. Hope that helps.
