John Hawks has an interesting post on what it means to be human in which he argues that our “human-ness” (humanity?) is our shared evolutionary history. I like it. But Hawks also writes the following:
It is our history that connects us to our distant relatives, not our genes. Even with a close relative like a twentieth cousin, there is a decent likelihood that you will share no genes at all because of your shared kinship from your most recent common ancestor. By the fiftieth generation, it is a virtual certainty. You are a genetic stranger to your ancestors.
I could share no genes with my twentieth cousin? This kind of sloppy use of terminology is not what I’ve come to expect from John. He’s usually at his best when writing about human genetics. You see, the quoted statement could vary from true to wildly inaccurate depending on the definition of gene we’re using.
Let’s start with the molecular genetics definition of “gene”. In this case, a gene is a locus on a chromosome that performs some function. Recent work has shown that individuals vary in the particular genes they carry (as a result of copy number polymorphisms, CNPs). That means I might have a gene you don’t have, or you might carry a gene that I’m missing. But CNPs do not lead to totally different genes between two individuals. In fact, a large number of our genes are shared with our most distant mammalian relatives, indicating they’re probably found in all humans as well. So, using this definition of “gene”, Hawks is dead wrong. But that’s not the definition he intended.
Next, let’s consider a population genetics definition, where “gene” actually means “allele“. In this case, “sharing a gene” with someone actually means having the same allele as another person. If we limit ourselves to the 20,000 or so protein coding genes in the human genome, what’s the likelihood that two individuals have no genes with the same sequence in common? An analysis of coding sequence polymorphisms within humans and differences from chimpanzee genes found that 92.6% of the >10,000 genes analyzed had at least one protein coding polymorphism or difference (doi:10.1038/nature04240). That means, conservatively, a few percent of all human genes do not vary within human populations (if anyone can cite data on synonymous variation, I’d greatly appreciate it). If those genes don’t vary within the human species, you’re guaranteed to have at least some shared genes with everyone. But I that’s not what John Hawks meant either.
Finally, we’ll consider a genealogical definition of gene. In this case, we’re interested in whether the genes two people have are identical by descent because they were inherited from one of the most recent common ancestors of the two people. For every gene, full siblings have a 75% chance of sharing the same gene, according to this definition of a gene. Because we have two copies of each gene (one from the mother and one from the father), we can also determine the probability full siblings will share zero, one, or two genes. There’s a 25% chance full siblings share zero genes, a 50% chance they share one gene, and a 25% chance they share both genes. First cousins will have 25% chance of sharing a gene. The probabilities of sharing genes by descent continue to decrease as you begin to deal with more distant relatives. This is what John Hawks meant when he wrote that you probably don’t share many genes with most of your relatives.
Despite the low probability of having the same genes passed on to you as your cousins had passed on to them, you still share an evolutionary history with them. In fact, all humans share an evolutionary history — a tighter history than we share with our closest living relatives, the chimpanzees. And that’s what Hawks says makes us human. As I said before, I like that definition, but I don’t think it’s the only one we can use.
Bustamante et al. 2005. Natural selection on protein-coding genes in the human genome. Nature 437: 1153-1157 doi:10.1038/nature04240
Gillespie 2004. Population Genetics: A Concise Guide. Johns Hopkins University Press, Baltimore.