#182: 20 Q’s…Half a What Again?

182.1  They say the wise man learns from the mistakes of others…the fool, not even from his own. Let’s see what we can learn from Yahoo! Answers as they tackle the mystifying concept of “half-cousins.” Do they really exist, or are they just an urban myth? Come closer, we’ll find out together…and as last week,  posts are in black italics…my comments in red. OP stands for Original Poster, the question-asker…

182.2  Question: What is a half cousin? Excellent question…ask and you learn…but be careful who you ask! And as the word “cousin” commonly means “1st cousin,” we will take “half-cousin” to mean “half-1st cousin.”

182.3  Best Answer: the cousin of one of your half siblings. Wrong! Fail! But admirably demonstrating the poverty of the whole Yahoo! Answers approach…that is, the one who doesn’t know the right answer gets to decide which answer is right…d’oh!  Still, an interesting idea, one worth exploring a bit further…but first, let’s make sure we’re all on the same page… So, if your mom remarries and has a kid. That kid might have some cousins from his dad. Those cousins could be considered your half cousins. And it’s just as I suspected…we’re on different pages…they are confusing half- with step-. This kid your mom has is your half-brother…but his relations on his father’s side are all step-‘s to you, in the same way as his father is your step-father, and he and his siblings are your step-siblings. Just call them cousins. Why does it matter?  Well, in the first place, the confusion between half- and step- matters when biology matters, since a half- is a blood relative and a step- isn’t (at least not usually…it could be…for example, if your mother’s new husband is also her 4th cousin…this does happen.)

182.4  In the second place, introducing your half-cousin (or 2nd cousin, or half-2nd cousin, or any collateral relative in your generation) as “my cousin” is a completely acceptable everyday simplification…true, they might think you meant “first cousin” but that might not matter, as you say. The trouble begins when such a colloquialism spills over into genealogy…assembling a family tree and considering half-cousin Bob as as full cousin gives Bob one grandparent that he doesn’t actually have…and leaves out one of his actual grandparents. Family trees aren’t of much use if they aren’t accurate. 

chart 639

182.5  But looking at a half-sibling’s 1st cousins…he has 2 sets…those on his father’s (your step-father’s) are not related to you. Those on his mother’s side are the children of his mother’s siblings…and since they are also the children of your  mother’s siblings, they are as much your 1st cousins as they are his. Half or full cousins do not depend on the relationship between the cousins, but the parents…if the parents are full siblings, full cousins…if the parents are half-siblings, half-cousins. 

182.6  Answer #2, “Highest Rated” by others…lol okay i’ve got 2 half sisters and 2 stepsisters. now you obviously know what “steps” are. halfs just means there is blood connection. for e.g.: mother + new man = half whatever. simple as that (-:   This is correct as far as it goes…altho perhaps it goes too far in assuming the OP knows what step-‘s are. I do not fault this person’s lack of capital letters…I don’t blog that way, but I do post on Facebook and generally email in such a casual fashion. I also like that half-relations in the plural is called “hafts” not “halves”…like the Toronto Maple Leafs, not Leaves…LOL me too…

182.7  Answer #3…Half-cousins are more common than most people realize. Sure, and for people who don’t think they exist at all, merely one would come as quite a revelation, nez pah? Here’s one example: ‘My’ brother had a child with a woman, and then he took off. I fell in love with her, we married, and I was raising my nephew as my own. We then had another son. The two boys are half-brother and half-cousin. Sounds like a happy family…God bless and good luck to you…still, if we examine your situation…

chart 640

182.8   …we find that Moe and Joe, while indeed half-brothers, are actually full 1st cousins…assuming your brother is your full brother…if he were your half-brother, then they’d be half-cousins. Notice, you correctly call Joe your “nephew,” not your half-nephew, which is what he would be if his father were your half-brother. Again, it doesn’t depend on them, but on their parents. Altho upon further reflection, perhaps you were talking figuratively…meaning the boys can be thought of as both brothers and cousins, or partly brothers and partly cousins…as for example someone whose mother died when they were very young might consider his much older sister who raised him as “half-sister, half-mother.” Still, genealogically and genetically, Moe and Joe are half-brothers…and full 1st cousins, because their fathers are full brothers, not half-brothers. 

182.9  Answer #4…Half cousins are the children of two half siblings. Direct, to the point, and correct. Sadly, in today’s dumbed-down world, some take this to mean the half-siblings are married to each other…but think about your own 1st cousins…you and they have parents who are siblings…did those siblings marry each other? I rest my case. 

182.10  Answer #5…I think it could also be if, let’s say your Aunt (who is blood related to you), who is married and has children (your cousins)divorces your Uncle. Then remarries and has children with a different man, and those would be your half-cousins?  Nope…if your aunt is your parent’s full sibling, than any children she has with anybody are your full 1st cousins…honest…I wouldn’t kid you about something like that.

182.11  Answer #6… Sorry I honestly am not sure. I know you can have half-siblings (I have them), but I haven’t ever heard of half-cousins…With all due respect, it’s beyond me how your honestly not being sure helps answer the question…at least you’re sorry.

182.12  Answer #7…i dont think there is a such thing  Me, I would be very careful in going around saying things don’t exist. Well, OK…apps don’t exist…they’re actually programs, not apps.  😉 😉  But my point is that taking the world to consist only of those things that you yourself have heard of is a pretty myopic way to look at it…commonplace these days, true, but so limiting. And you’ve got to learn to trust people…so trust me when I say there are half-cousins. i know there is step cousins but if there is two sisters that have diffrent moms or maybe difrent dads but the same of one parent they’re half sisters. You have inadvertently hit upon the “building block” nature of genealogy…say these half-sisters you mention each have a child…those children are half-1st cousins…then they each have a child, the original half-sisters’ grandchildren…those are half-2nd cousins, and on down the line…

182.13  Answer #8…if your mom or dad re marrys, their neices and nephews become your half cousins  Again, step- not half-.

182.14 Answer #9…a half cousin is a person whose related to ur cousin, but ur half cousin isn’t really related to u. Did u get that? Sadly, I did get that…and you’re off your rocker, dear friend… don’t be surprised if nobody listens to you. But to sum up: 1st cousins are the children of siblings…half-1st cousins are the children of half-siblings…2nd cousins are the children of 1st cousins…half-2nd cousins are the children of half-1st cousins…3rd cousins are the children of 2nd cousins…and on and on, like that…


182.15  Question: How much DNA do I share with my “half first cousin”? My mother only has half brothers and sisters. I was wondering how much DNA do I share with my mom’s half sisters’ offspring. Are they genetically more like my second cousins than first cousins since our moms are only half siblings (they share the same mother only)?  In a nutshell, your half-1st cousins (1/16) are exactly half-way between your 1st cousins (1/8) and your 2nd cousins (1/32). All half-relations fill in the missing powers of 2 along the horizontal line of your generational collateral relatives….starting with half-siblings (1/4) being half-way between full siblings (1/2) and 1st-cousins (1/8). But let’s see what others think…

182.16  Best Answer: Dr J is the real expert. It isn’t that simple. You have 46 chromosomes and 44 of them are Autosomes. It is what most of your DNA is. You get it 50-50 from both parents but not necessarily 25% from each of your 4 grandparent. The reason why when your parents passed on the Autosomal they received from their parents to you it went through a process where it was randomly jumbled and recombined. So while you got 50% from your father’s side and 50% from your mother’s there can be a bias in what you inherited from grandmother and grandfather on both sides of the family. How you inherited any bias will not be how your siblings inherited it unless you have an identical twin. So no one can say exactly how much DNA you share with your full siblings if you have some. Autosomal is what determines your “looks” genetically as well as other things. Here is a great link. If you scroll down it discusses the recombining toward the end of the page.  .http://www.dnainheritance.kahikatea.net/…  

182.17  The website cited is fine…and this comment is correct as well. You have 2 complete sets of genes…one comes from your mother’s ovum, the other from your father’s spermatozoa that fertilized it…so yes, exactly 50% of your genetic make-up comes from each parent. But the 1/2 that you share with a sibling…or the 1/4 you share with a grandparent…is indeed an approximation…it could be a little more or a little less. To take a super-simplified example: if one complete set consisted of only 2 genes, there are 4 different ways your father could pass along to you half the genes he got from his parents.

chrat 641

182.18  If we then compare brother to brother, we see there are 16 possible combinations. In 8 of them, you and your brother share half your genes…in 4 of them you share both, in 4 others you share neither. What each of you gets from your father is like flipping a coin…heads you get a blue from your grandfather, tails you get a red from your grandmother…so between brothers, in this scenario, 100%, 50%, and 0% are all possible. But we have around 25,000 genes, not 2. Could 2 brothers actually share all  their genes? It’s possible, but only as possible as each of you flipping a coin 25,000 times…and getting a match every time! In reality, between brothers it averages out to 50%.

chart 642
182.19  Answer #2  “Highest Rated” by others It’s not so simple. It definitely is NOT 1/2. See: http://www.genetic-genealogy.co.uk/Toc11… and look especially at Table 2. This is also a good reference…it’s my favorite site, Prof. Lancaster’s “Genetic and Quantitative Aspects of Genealogy.”  And you’re right, it’s definitely not 1/2…it’s 1/16…but I don’t see where the OP thought it might be 1/2…just that it might be half of something else, which is it…your relationship to a half-1st cousin is half your relationship to a full 1st cousin.

182.20  Answer #3  You share 1/2 of your parents DNA and your first cousin share 1/2 of their parents DNA. The parents who are siblings share 1/2 of their parents  So far this first part is true…AND you and your cousin may not share the same Genealogical DNA, which is compared and correlated differently than Genetic DNA – like paternity testing.  I haven’t a clue what this second part is supposed to mean…I do know that Genealogical refers to where you got it and Genetic refers to what it is you got, so the 2 concepts are different, but they will not give you different numbers in terms of percentage of genes shared.  Source(s): Genealogical researcher 40+ years, Anthropologist & retired Instructor  Maybe in retirement your thinking has gotten a little sloppy? It can happen…no harm done…I caught it for you!

182.21  Answer #4 You and your mother share 50% of your genes. Your mother shares half of that with her mother, so you share 25% with your grandmother. Half of that is shared with her children (your mother’s half siblings), so you share 12.5% with them. And they share half of that with their kids, so you share 6.25% of your genes with them.  This is correct…1/16 is 6.25% because when I went to school, 100 ÷ 16 = 6.25. They are your fourth degree relatives. The term “degree” has several different meanings when applied to kinship, so using it is ambiguous, and I would not.  For most people, first cousins would be third degree relatives and share 12.5% of genes. Because 1/8 is 12.5%…and “for most people” is technically correct. If 2 people are 1st cousins, they are related by 1/8…they may also be related in other ways, say half-siblings or 2nd cousins, in which case they’re closer than 1/8…but what they got solely from that 1st cousin relationship is 1/8. The only exception  is if the parents who are siblings are identical twins, in which case the 1st cousins would be as closely related as half-siblings or 1/4.

182.22  Answer #5  Dr. J is the expert. It’s clear to see that Dr. J holds a lot of sway in this neck of the woods, boy! For swapping stories around the dinner table, without using big words, and knowing it is not really accurate, assume full siblings share 100%. Except…they don’t, they share 50%…why assume what isn’t true? I’ll append the correct percentages in red…

Full siblings = 100%  50%
Their children, 1st cousins, share 50%   25%
Their children, 2nd cousins, share 25%  12.5%
Half siblings = 50%  25%
Their children, half 1st cousins, share 25%  12.5% 
Their children, half 2nd cousins, share 12.5%   6.25%

Trouble is, the numbers for the children are still wrong…when siblings (related by 1/2) have children, those children are 1st cousins, related by 1/8 (dividing by 4) not 1/4 (dividing by 2). This is easy to see using the “If it were 100%” approach. Your uncle is related by 50% to his son, your 1st cousin. If your uncle and your father shared 100% of their genes, your father too would be related to his nephew by 50%. But your father and your uncle aren’t related by 100%, but by only half that. So your father’s relation to his nephew, your 1st cousin, is half of 50%, or 25%.

Now if you had 100% of your father’s genes…then you would related to your 1st cousin by the same amount as your father is, 25%.  But you only have half your father’s genes, so your relation to your 1st cousin is half of 25%, or 12.5% ..which is where the 1/8 between 1st cousins comes from. 

So, again just gross approximations, yes, half first cousins share about the same DNA as full second cousins.  Nope, it’s not the same amount, but half as much, 1/16 as opposed to 1/32. The approximations aren’t that gross! You share exactly half DNA, but the first set of each DNA is the same This statement makes absolutely no sense…seriously, I wouldn’t know where to begin.  Source(s): Biology expert 12 years  Unfortunately, those 12 years were 1965-1968, 1974, 1982-1986, 1993, and 2002.

182.23  Next week, more “experts” whither under our scrutinizing truth-o-scope…be seeing you…


Copyright © 2014 Mark John Astolfi, All Rights Reserved


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