Dear Stolf: I keep reading that humans and chimpanzees share 98% of their genes…and all humans share 99.5% of their genes with each other…yet I only share 50% with my daughter. Can you explain how this can be? …from Puzzled, in Platypus City
64.1 Dear Puzzled: Yes, yes I can. You are confusing genetics and genealogy. Genetics tells you what you have…genealogy tells you where it came from. Confusing the 2 yields the following paradox: genetically, everyone is virtually identical to everyone else…and yet everyone is almost completely unrelated to everyone else, even most of their relatives.
64.2 You hear the word paradox thrown around a lot these days, intended to mean something that is confusing or mysterious. Strictly speaking, a paradox is a situation which on the surface seems to make no sense…seems impossible…but when investigated further, turns out to be correct and completely true. A paradox indicates that people don’t really know as much about something as they think they do…their innate grasp of a subject, and hence their erroneous gut reaction, is not all it could be.
64.3 A perfect example is the Lottery Paradox. Suppose you buy a ticket for a lottery in which a million people have entered. Your chance of winning is one in a million…it’s extremely likely that you’re not going to win. Trouble is, this is true of everyone entered in the lottery…none of them has any real chance of winning. And yet…guess what?…somebody has to win…and somebody does win…despite the million-to-one odds. This paradox illustrates that our instinctive understanding of odds and probability, at least in this case, isn’t very trustworthy.
64.4 Now our DNA contains 23 pairs of chromosomes…each pair consists of 2 duplicates. Each chromosome is made up of a series of genes, and between the 23, we have between 20,000 and 30,000 genes. Technically we have twice that, but only one of the 2 duplicates will be active or as they say “expressed”…that is, only one of each pair will determine our physical makeup. Before the Human Genome Project, the number was estimated to be as high as 100,000. Of each pair of chromosomes, one came from your father, one from your mother…so Chart 213 shows a simplified closeup of one pair of chromosomes, father’s on left, mother’s on right.
64.5 I have indicated, in an imaginary and somewhat whimsical fashion, the purpose of each gene…and as you can see, 9 of the 10 genes are identical. And that’s what makes us all human beings, and not chimps, dogs, flounders, or banana trees. Where there exist more than one variety of the same gene, these different variations are called “alleles.” 9 of these 10 genes have no alleles…only the one for eye color does. But again, these alleles don’t somehow blend together to give you your eye color…instead, one of each pair of genes will be “expressed”…called the “dominant” gene…the other will have no effect, the “recessive” gene. Thus in this case, your eyes are brown…
64.6 On the far right of Chart 213, I have checked off which of each gene pair is dominant, your father’s or your mother’s. In a sample as small as this, it might be 7/3 or 10/0 instead of 5/5, but overall it averages out to 50-50. And as you can see, only 1 of these 10 gene pairs will make any physical difference in your makeup…and of course it’s not limited to physical appearance…the genes for the various blood types for example are also alleles. Assuming you have as many as 30,000 genes…altho it’s likely closer to 20,000…99.5% of them have no alleles…thus only .5% of them can make a difference between 2 individuals…that’s a mere 150 genes. This is why we say that everyone on Earth is virtually identical genetically.
64.7 Now the degree to which we are related genealogically…as opposed to genetically…is another thing entirely. It depends on receiving the same gene, whether allele or not, by descent from a common ancestor. Chart 214 shows how many direct ancestors you have going back each generation. As you can see, by the time you get to 13G grandparents, you have 32,768 of them…this is based on every person having 2 parents…in reality, all those 13G slots may not be filled by a different person…and indeed at some point they cannot be, since there couldn’t have been that many humans alive on Earth at the time.
64.8 But theoretically, since you don’t have 32,768 genes, you couldn’t have received one gene from each of your 13G grandparents…it is numerically impossible…there are too many ancestors and not enough genes. And given 25 years per generation, this is only on average 400 years ago! In terms of the genes that make you you, the ones that have alleles and thus can differ, you cannot have received one from each of your 256 6G grandparents, since again there are only 150 such genes. This phenomenon is called “flushing”…the point at which genetic material from any one specific ancestor is for all intents and purposes no longer present in your DNA by direct descent…of course you could still have that gene, but from someone else. However there are 2 important points to bear in mind.
64.9 First, the probability of inheritance by descent of any genes at all, let alone any individual gene specifically, is never mathematically 0 for any of your ancestors. That’s because, as we saw in the case of the Lottery Paradox, you have to get those genes from someone…for every one of your genes, one of your ancestors in every generation is going to be a “winner”…even tho the chances of any of them individually are practically nonexistent. So genetically, you have to be related to some of your ancestors, just not most of them. And secondly, you are still genealogically 100% related to every one of your ancestors, by reason of the basic parent/child link. This is a social connection that is absolute, regardless of how little you might be biologically related by descent. And remember, considered in this way, it’s thought that everyone on Earth is related by at most a distance of 50th cousins…and many of course much closer.
64.10 Thus, given any random person alive today, you have practically the same genetic profile as they do. On the other hand, the odds that both of you got any of those genes from the same person, by direct descent, are practically, but never absolutely, 0. Which is why you shouldn’t confuse genetic similarity…what we have…with genealogical kinship…where we got it, or might have.
64.11 I might also add that the DNA tests you hear so much about these days can reveal important medical information…and certainly confirm parentage…but they can’t tell you who your ancestors are. How could they, unless we had 50 billion DNA profiles on file…one for every person who ever lived. At best, they can trace you back to, say, Hungarians who migrated to Finland at some point in the past…or indicate in what part of Africa your roots are planted, that sort of thing. In fact, in this sense, genealogy helps genetics more than genetics helps genealogy…regardless of the sales pitch you might hear.
Dear Stolf: Well, they’re at it again: “OMG! KEVIN BACON MARRIED TO HIS COUSIN!” This was recently revealed on “Finding Your Roots with Henry Louis Gates Jr.” on PBS. What’s the story, morning glory? …from Ida, in Idiotsville
64.12 Yeah, and altho you’ll hear it has to do with DNA analysis, since that’s a hot topic these days, he actually does it with good old-fashioned family trees and parental pedigrees…and for famous people to boot, since theirs are more completely researched and readily available. But once again, the level of ignorance and mean-spiritedness, egged on by the lunatic Media, is rather sad. On-line commentaries use words like “creepy,” “queasy,” and “weird”…and come up with feeble quips about “degrees of separation” and “shaking up the family tree.” And yes, some of the more conscientious ones will add in “marginally” or “distant”…because sure enough, Kevin Bacon and his wife of 23 years Kyra Sedgwick are 10th cousins once removed. She’s also related to Richard Nixon and Marilyn Monroe.
64.13 But she herself was quoted as saying: “It was a little unsettling, I’m not going to lie.” Well, with all due respect to an talented actress, she’s also a moron. As you can see from the right half of Chart 214, any common genes by descent have been flushed out between 2 individuals at the stage of 7th cousins…and as for those all-important alleles that determine individual differences, they’re all but gone by 4th cousins…which is to say, in ways that matter, you’re as related to your 4th cousin as you are to a random person off the street. But then what can you expect from PBS, when the History Channel is doing shows about Nostradamus and the Mayans, for gosh sakes…
64.14 And by way of review, if somebody says they’re your Xth cousin Y times removed, this means that one of you is the Xth cousin of a direct ancestor of the other. Remember, Xth cousins share a pair of (X-1)G grandparents if they are full cousins, only one if they are half cousins. And the Y tells you how far back that ancestor is…1 for parents, 2 for grandparents, 3 for great grandparents…beyond that, Y for (Y-2)G grandparents. So in Chart 215, I have a 50/50 chance of being right…either Kevin is 10th cousin to one of Kyra’s parents, or Kyra is to one of Kevin’s. In short, it’s possible you’re more closely related to Kevin Bacon that Kyra is, even if you aren’t an actor…I’m just sayin’…
Dear Stolf: wise/dumbGeek query alert! Eghh! Eghh! Eghh! …from Albert Einstein III, in Bolsa Chica, CA
64.15 Thanx, Al…I’m on it! And you know, it’s gratifying to think that before I started seriously getting into this stuff, maybe a year and a half ago, I would have had to do some serious brainwork on this question…today, it comes automatically, as I hope it is starting to for some of you, dear friends…next week, more choice missives…adiós…
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