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We are currently learning about the Human Genome Project in high school biology. Wouldn't the human genome that was mapped be different from, say, mine if it was mapped? How much variation is there between individual humans in our genetic sequencing? I'm assuming there is not that much difference since multiple volunteers were used in the project. I did see another question that talked about mutation rate, so I'm not sure if that's the same thing.

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  • $\begingroup$ Welcome to Biology.SE! This is a very basic question and thus appears to meet this sites criteria for a homework question. In these situations we expect you to do some research on your own and then ask questions informed by what you have learned (ideally with references to reliable sources). For instance the "Related" links led me to this question. ——— Please also take the time to go through the tour and the help pages starting with How to Ask questions effectively on this site. Thanks! 😊 $\endgroup$ – tyersome Nov 5 at 23:03
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    $\begingroup$ Possible duplicate of Why are the genomes of Humans 99.5% the same? $\endgroup$ – David Nov 6 at 10:52
  • $\begingroup$ Hi Sarah, and welcome to Bio.SE! @tyersome I disagree that this is a "basic" or trivial question. So do the 500K genome project, gnomAD, ClinVar, Genomics England & BioBank, and countless other massive projects. Even post-sequencing, calculating the rate of variation is incredibly challenging. This question clearly is a fundamental question about genetics that has massive ramifications in rare disease, cancers, developmental disorders etc. $\endgroup$ – James Nov 15 at 14:37
  • $\begingroup$ @David I think that I side with Sarah on this. Mutability (i.e conservation) and variation are fundementally different principles. Conservation (evolutionary constraint/mutability) are measuring changes of genetic regions over huge timescales by comparing homologous sequences in other species. Variation, however, is snapshot of what is currently varied in the population of a single species. Of course, there is a lot of overlap between constrained genes in gnomAD, for example, and highly conserved regions. Practically I'm not sure if this will be reflected in the answers... $\endgroup$ – James Nov 15 at 14:51
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    $\begingroup$ @James — Perhaps we could do with a community wiki (whatever that is) on this whole topic. I think the current dispersed nature of questions relating to it makes it very hard to get a coherent view. $\endgroup$ – David Nov 15 at 16:11
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You are correct in saying that the human genome sequenced in the project was contributed by a dozen anonymous volunteers. However, the final sequence that is used as a standard reference is a somewhat arbitrary construct that represents a collection of consecutive segments taken from one or other of the contributors. It is a haploid sequence, plus a Y and an X chromosome.

Now you ask how different are the genomes of any two subjects, taken at random from a population. Imagine for simplicity looking at your own genome. For each of the 22 autosomes, you know you have two copies, one transmitted from your father, the other from your mother (for the X chromosome you have two copies, if you are a girl). Now imagine comparing the sequence of each of your pair of homologous chromosomes side by side; remember that they are derived from very different individuals (unless your parents are close relatives), so that they will not be identical, as they represent independent evolutionary histories.

Then, the question becomes: how much variation is there between the sequences of your paired chromosomes? And the answer is known: approximately every 1,000 base pairs your chromosomes show a base pair in one chromosome and a different base pair in the other; otherwise they are identical. The sites where this variation occurs are called SNP, for Single Nucleotide Polymorphisms (many other kinds of variation exist, but SNPs are the more common). We can generalize this observation, as it remains true for any pair of chromosomes chosen at random from any pair of subjects. Therefore, we can say that humans are identical for 99.9% of their genomes, whereas the genetic variation responsible for the differences that allows us to recognize ourselves as unique are limited to 0.1% of our genome.

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    $\begingroup$ I think this answer gives a good basic overview to the question, for further detailed reading it would be helpful to include links about the already mentioned SNPs and also the 1000-genomes project (which is essentially about exactly this question) $\endgroup$ – Nicolai Nov 6 at 15:33

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