#9: Twin Me!

9.1  Last week’s challenge was to map out  2 individuals who have a Coefficient of Relationship of 5/8. For the solution, use the fact that when 2 individuals are related to each other in more than one way, their total CR is the sum of the CR’s for each relationship. To get 5/8, one obvious way would be to add ½ or 4/8 to 1/8, giving 5/8. Relationships with these CR’s that spring to mind are father/son (½) and great grandfather/great grandson (1/8). And remember, I suggested that Abner and Zeke be replaced by Atlas and Zeus, who could be horses, swine, or in this case I’m thinking they’re cattle…


9.2  As we can see in Chart 23, Atlas has an offspring with his son’s daughter, his granddaughter. That offspring, Zeus, is thus both Atlas’ son and great grandson. 4/8 + 1/8 = 5/8. In this context BTW, fathers are called sires and mothers are dams. Such a procedure is used in animal husbandry to “concentrate” the favorable genetic traits of a sire.

9.3  And to round out the Eighths, Chart 24 gives examples of the 2 highest such CR’s. On the left, Zeus is both son and grandson of Atlas…2/4 + 1/4 = 3/4 or 6/8. And on the right, Zeus is son, grandson, and great grandson of Atlas…4/8 + 2/8 + 1/8 = 7/8…they call this “line breeding.”  Funny thing, the cattle don’t seem to mind…


9.4  But today’s topic is twins, or I should say identical twins, the result of a single sperm fertilizing a single egg, then that egg splitting in 2, resulting in 2 individuals that have an identical genetic make-up. They have a CR of 1. To the left is a style of headwear called a “cocked hat,” and that’s what identical twins knock the CR’s of their descendants into.

9.5  Well, it isn’t actually that bad…the key point is this: In figuring how closely their descendants are related to one another, identical twins are considered to be the same person. Now obviously, when Identical twin sisters have children, those children are and always will be genealogically 1st cousins. In their families, they are nothing but 1st cousins. But instead of a CR of 1/8, theirs is 1/4, or the equivalent of half siblings. In Chart 25, I have represented the twins as 2 circles touching. You can see the similar “W” pattern in their Parental Tree, the same as that of normal half-siblings.


9.6  The CR between the descendants of identical twins is double what it would be for normal siblings, while the CR between parent and child is remains at ½. But the CR between one twin and her niece, her twin sister’s daughter, is also ½. Strange but true…if you are the child of an identical twin, you are as closely related to your parent as you are to your uncle or aunt, your parent’s twin.

9.7  It’s important to see that the CR between identical twins and their ancestors remains unchanged, as does the CR between the twins (and their descendants) and their collateral relations (and their descendants)…collaterals being other siblings and all numbered cousins. And that makes sense…to the brother of the twin sisters, his nieces may be as closely related to each other as half-siblings, but to their uncle, they are simply nieces, the same as if they really were half-siblings or even full siblings. (Your half-niece is the daughter of your half-brother…but If your full brother has daughters by 2 different women, they are both your full nieces, altho they are half-sisters to one another.)


9.8  Likewise, in Chart 26, when identical twins marry identical twins, the resulting 1st cousins are as closely related as full siblings..with the familiar the “X” pattern above. And I have read several accounts of this happening,with a double wedding naturally, and indeed it was said that the 1st cousins tended to resemble each other physically as much as full siblings would. Which brings us to…The Patty Duke Show.

9.9  And there’s Patty, losing control…apparently there’s a hot dog nearby…or is that Cathy? Or one of each? The premise, as you recall, was that Patty and Cathy were “identical cousins.” And in one episode in the 2nd season, the idea was pushed even further, as a “distant cousin” Betsy from Chattanooga came to visit…she was “identical” too, except for her accent! Now in the never broadcast pilot episode, Cathy and her father, come from England to live with the Lanes in Brooklyn Heights. He’s Martin Lane’s brother, and they were played by different actors….implying that unless this was a momentous coincidence,  it was the mothers who were the indenticals…hence the look-a-like cousins.

9.10  However, during the first season, it was established that their fathers were identical twins. In fact, on an episode that aired on Christmas Day (yup, no re-runs back then on holidays!) Cathy’s father Kenneth Lane is played by William Schallert, who also played Martin, Patty’s father. Now the series ran for 3 seasons, 104 episodes, and I’m pretty sure it wasn’t played out that the mothers were identicals…

9.11 …but had they been, that would certainly increase the chances of a pair of 1st cousins looking very much alike, as full siblings sometimes do. In that case, Patty and Cathy would have been one thing genealogically…1st cousins…but something else genetically…full-siblings. As it is, with identical fathers, they are genetically half-sisters. But other than that, there is no such thing as “Identical cousins.” Altho…with modern fertility techniques, who knows?  Is it possible for a fertilized egg to split in 2 outside the womb, then the 2 embryos be implanted in 2 sisters? The resulting children would technically be 1st cousins, the daughters (“birth daughters”?) of sisters…but by their genetic make-up they would be Identical twin sisters. Well, I’m just sayin’…

9.12  Most twins are not identical twins. Fraternal twins are as related to each other as normal siblings, with a CR of ½. It turns out that about 2% of the word’s population are twins of either kind…with identical twins making up just 8% of all twin births. Now there is some degree to which fraternal twins may resemble each other slightly more closely than normal siblings…due to the fact that as they were together in the womb, they were subject to the same nutritional and other developmental factors…such as the mother’s overall health or lack thereof. But research on this is just beginning…and at any rate, these factors wouldn’t effect the twins’ genetic makeup, which again would be that of normal siblings.

9.13  There is even some preliminary indications that there might, very rarely, be something called semi-identical twins…also called half indentical or polar body twins…and they would indeed have a CR of 3/4…the same as those Super Sister bees back in #5. The biological mechanism is similar, but not identical. Recall that with some species of bees,  a male develops from an unfertilized egg. He has just one set of genetic material, so that his sperm cells are all genetically identical to him and to each other. Thus the daughters of a male bee share all their father’s genes and half their mother’s, for a CR of 1/4 + ½ = 3/4. (And in fact when we say you share half your genes with your sibling, this is due to your sharing 1/4 of your father’s and 1/4 of your mother’s genes with that sibling…full siblings are essentially double half-siblings!) Only female bees can be Super Sisters..there are no other types of bee Super Siblings, due to the unique genetic make-up of the males.


9.14  Human semi-identical twins are different, as seen in Chart 27. The exact mechanism for the production of semi-identicals is not well understood…and some researchers doubt that it is even possible at all. But basically, it would involve 2 different sperm cells fertilizing one single egg…something that due to chemical inhibitors shouldn’t normally happen, but never say never, right?. Either literally, as show in the Chart 27…or an egg fertilized by a single sperm cell begins developing, and one of the normally discarded products of this development, called a polar body, is fertilized by a different sperm cell. This happens early enough in the development of the embryo that the polar body is apparently fully capable of developing into a fetus…in some case, with abnormalities like hermaphroditic features…altho in other cases, the semi-identical twins, both males or both females, would be perfectly normal. For now, the jury is out…and even if it were possible, it would be extremely rare.

9.15  And you’d need extensive DNA testing to reveal semi-identicals…and in fact, whether twins are truly Identical or merely fraternal has up until now always been a matter of “eyeballing” them. One clue people have traditionally used is whether there is one placenta (indenticals) or 2 placentas (fraternals) but there are exceptions to this rule, so it is not foolproof.

9.16 Next week, we’ll “remove the Removeds”…and I’ll answer some readers’ questions. Till then…We’re gonna need more Tinker Toys!


Wicked Ballsy

chart 28

I was flattered to find out that some people didn’t realize these are all my original charts…well, the concepts behind them aren’t always original, but I do draw them myself, trying to find the best way to illustrate those concepts. Here’s a case in point…octuple half 2nd-cousins...yup, Abner and Zeke are half 2nd-cousins in 8 different ways…for an ultimate CR of 1/8, or the equivalent of full 1st cousins. The chart is based on one from this excellent website…which does get a bit dense at times, but check it out if you’re game… Quantitative Genealogy from the UK .


Copyright © 2011 Mark John Astolfi, All Rights Reserved


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