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Dear G4BB: Thanksgiving has come and gone, and once again at our family gathering, the traditional arguments raged on…including the one about whether it’s really possible to have “identical cousins” like Patty and Cathy on “The Patty Duke Show.” Can you settle this, so I can be on the winning side for sure this Christmas? …from Lambsie, in Pie City
97.1 Dear Lambsie: Be happy to…and BTW, the tomato is a vegetable, not a fruit, got that? But listen…did you take a Biology class in high school? Hey, there’s my old teacher…she was double-jointed and had a habit of leaning on her desk with her arms bending at all the wrong angles. Anyway, she taught us about phenotypes and genotypes…sound familiar? A phenotype consists of all the physically observable characteristics or traits of an organism. A genotype is the genetic make-up that makes the phenotype possible.
97.2 From the point of view of phenotype, is it possible for 2 people, not identical twins, to still be so similar in appearance that they could be mistaken for identical twins? Absolutely…and I’m sure we’ve all seen examples of that at one time or another. I once worked with a women whose teenage sons, separated in age by a couple of years, were to me indistinguishable. Then you have the Olsen girls, Mary-Kate and Ashley…the family says they’re fraternal twins, but they resemble each other to the point where they were both cast as Michelle Tanner on “Full House” at age 9 months…such a shared role is usually filled by identicals. And in researching this topic, I came across a blogger who remembers 2 unrelated boys from high school who were able to convincingly play identical twins in a school play.
97.3 So between siblings on the one hand, and completely unrelated individuals on the other, it’s certainly possible for 1st cousins to look identical. On the show, Patty’s father Martin Lane and Cathy’s father Kenneth Lane were identical twins. For the record, in the unaired pilot, it was their mothers who were identical…this pilot was never broadcast because a different actor, Mark Miller, played Martin. On the show, it was William Schallert, and he also played his twin brother in several episodes, with a mustache. He even once played Martin impersonating Kenneth.
97.4 And in a second season episode (the show ran for 3 seasons), they took it one step further as Duke played yet a 3rd cousin, Betsy, from Chattanooga, with a Southern accent of course. And just for the heck of it, here’s a shout-out to the “4th cousin”…then teen actress Rita McLaughlin…she had been one of the kiddie helpers on “Watch Mr. Wizard,” and stood in for Patty Duke when one of the cousins was seen from the back. Under her married name of Rita Walker, she went on to a career in soaps on “The Secret Storm” and “As the World Turns.”
97.5 But I guess the producers of the show thought identical twin fathers was enough to justify the concept…identical mothers as well would have been even better. With identicals on both sides, the resulting girls would genealogically be double 1st cousins, who are normally are as closely related as half-siblings….but genetically they’d be as close as full siblings. Even so, the way they did it, with identical fathers and unrelated mothers, Patty and Cathy are as closely related as half-siblings…since genetically, their fathers can be considered the same person. So in the sense of appearance alone, “identical [1st] cousins” is a plausible idea…very unusual, but certainly possible in real life. Now in terms of genotype or DNA, it’s a different story altogether.
97.6 Since their mothers are unrelated, Patty and Cathy cannot have identical an genetic makeup, as would be the case with identical twins. Identical twins start at conception as one single fertilized egg…with one single set of genes. The egg then splits into 2, and both halves develop into separate individuals. These 2 identical twins have identical genes because there’s no way that they couldn’t have…the only source of their DNA was that one single fertilized egg and its genetic content. There is simply nowhere else “non-identical” genes could have come from.
97.7 Now to examine this a little further, the reason full siblings are different, both in appearance and in genes, is twofold. First of all…of the approximately 20,000 genes we have, we actually have a pair of each…one from each parent….for a total of 40,000. And to take the father as an example…of the paternal set of 20,000 genes that 2 brothers get, 10,000 will be the same, and 10,000 will be different…that’s because when the sperm cell that formed each brother was produced, its 20,000 genes were a random mix, half from the father’s father, and half from the father’s mother…because the father himself has 2 sets of 20,000 to start with. This “shuffling” of genes is the whole point behind sexual reproduction…each time the father has a child, he gives it a different combination of his genes.
97.8 When I said 10,000 match, 10,000 don’t, it’s never that exact…it’s always more or less, but it’s close, due to random probability. Could all 20,000 match? Well, imagine you and a friend flip coins 20,000 times and every time what you get a match, heads or tails. Ridiculously unlikely, but not absolutely impossible. And since Patty and Cathy’s fathers’ have the same genes…both pairs, all 40,000…let’s assume both girls get exactly the same 20,000 from their fathers. Huge assumption, I know, but play along…
97.9 Now the second way genes control what an individual will be is by the process of dominance. Remember, you have 2 complete sets of genes, one from each parent. So which genes will make you, you? Well, of every pair, one will be expressed, called the dominant gene…and one will be repressed, called recessive gene. Again, it will normally be 50/50, father’s genes versus mother’s genes. But it is not absolutely impossible that all 20,000 of the Lane genes could be dominant…so that effectively Patty and Cathy’s genetic makeup could functioning as if they were truly identical twins. Just imagine you and your friend flipped coins 20,000 times and got identical results…and did that twice!…once to get identical genes from the fathers, and again so that only the fathers’ genes are expressed, not the mothers’. That’s how likely it is…which is to say, not very…but that should keep the Yuletide argument going for a while, huh? 😉 😉
97.10 Couple more from the wiseGeek “Cousins” page…and first-up sounds pretty confident…and confidence is a good thing, generally. In this case, they happen to be wrong, but I’m here…no harm done!
97.11 The trouble is, you reckon cousins from the CLOSEST common ancestor, not from ANY common ancestor. As shown in Chart 343, YOU and your cousin are indeed half-4th cousins…you have a 3G grandfather in common, but your 2G grandfathers are half-brothers, not full brothers, since they have different mothers…and thus you have different 3G grandmothers. All that you got right. But that’s as far as it goes…certainly, your 3G grandfather’s parents are 4G grandparents to both you and your cousin…but they are not your CLOSEST common ancestor…we’ve already established that your 3G grandfather is.
97.12 Using your line of reasoning, that cousins can be figured from ANY common ancestor…2 full siblings, with the same mother and father, would also be 1st cousins, since they have grandparents in common. They would also be 2nd cousins, with great grandparents in common, and 3rd, 4th, 5th cousins, on up the tree. That obviously isn’t right. Again, it’s the CLOSEST common ancestor that determines it…in the case of full siblings, that would be their parents, period…it stops right there.
97.13 And this next one is not a question but a declaration…it needs no response, except to say: whatever floats your boats, twin cousins. Funny how that ties in with Patty and Cathy…but also notice that you are “irregular double cousins,” different on each side…full 1st cousins since your mothers are sisters, but half-1st cousins since your fathers are only half-brothers. Double 1st-cousins…that is, the same on both sides…have a CR of 1/4 = 4/16, equivalent to half-siblings. Yours is 3/16, just a bit less. But that’s pretty cool you 2 cousins and your dad being born on the same day. Do you look anything alike, I wonder? Next week…gee whiz, who forgot to close the barn door??? See yez then.
Copyright © 2012 Mark John, All Rights Reserved
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