The human genome doesn't exist
|René Magritte: This is not a pipe. |
It's a painting. But a painting of 'a'
pipe--but not 'the' pipe, which doesn't
actually exist in the real world. Or does it?
Now what everybody raves on about, the HG, is like that. It is not from one person, not even one copy from one person's two. It's not clear whether this is even from one person or from several donors, or if from one, who that person is in terms of his origins (his, because there's a Y chromosome in the sequence).
In any case, it's not a 'normal' genome, because while we believe the sequenced individual(s) was/were healthy at the time they bled for the cause, they won't be healthy forever. Since we are told every day by the press that every disease without exception must be genetic, and hence we should GWAS it endlessly, the donors will eventually become genetically abnormal.
Worse, 'the' genome keeps changing! We are now in version 19, released in 2009. It's either more details from the same donors, or now has bits that couldn't be sequenced easily from those donors' DNA and so was sequenced from additional donors (we don't know which is the case). There will be future revisions. Not only that, but it is a 'haploid' sequence, with only a single-nucleotide per position reference, whereas any human has two copies (except for X and Y chromosomes). Yet in any one person 1% or more of sites actually vary.
|This is not Van Gogh's |
Probably, we should use a term like 'instance' rather than copy, in the same sense that each chair is an instance of the concept of chair, even if no Platonic actual ideal chair exists, except as a reference concept in our minds.
What is good for the goose is good for the gander. If the HG doesn't actually exist, neither does a given gene say 'the' beta-globin gene, which is expressed in red blood cells and of which some variants are involved in anemia (like sickle cell anemia). Here is hg19's version of a very small part of that sequence (click to see details):
Any one instance of this in an actual person, like you, may or may not have this exact sequence. But unless it's been deleted from your genome, you'll have something very similar, not because it is a copy of some Platonic idea, but because they are descendant copies handed down through the generations since we shared a common ancestor, and that--evolution--is the crucial difference. It's why we should be careful about treating a reference as being real, or how we relate the existing instances of the 'same' (homologous) sequence.
We use 'the' HG sequence as a way of referring to a standardized set of nucleotide locations along the chromosomes in the individuals who were sequenced. Each of us varies from that sequence in millions of places (in each copy that we carry). To find similar functional elements--genes and so on--in my instances of the human genome, we use 'the' HG as a kind of map. If I had your DNA sequence, I could identify which is 'the' beta-globin gene by its similarity to that in the figure, even if you didn't have the exact same sequence.
This is very useful, but it's important to be aware that not only does nobody have the same sequence, but even the structural details--the locations of the 'same' elements in you and us and the donors of 'the' HG--differ among people. So the coordinates (the number at the top line) will not be the same. For example, the beta-globin gene starts at chromosome 11 position 5,246, 696 in draft hg19, but at 5,203, 400 in draft hg18, a difference of around 43,000 nucleotides! Where did they come from? Which draft should we believe--if any? What will hg20 say? What is 'the' HG?
|Robin you'll see in the UK (image: RSPB)|
This leads to interesting questions about how we treat our subjects, biology and evolution. Are there alternatives to type specimens? So, while you nurse your mint julep in suspense, think about this--the whole concept of a reference specimen, be it genetic or physical, because tomorrow we'll give a least a bit of thought to this question.