Talk:Double first cousin

Consanguinity, algo'rhythm and the fourth cousins rules
What if two random persons, albeit the improbability of so occuring, share all of their great great great grandparents but none of their great great grandparents? (As in double cousins who share all of their grandparents but none of their parents) What would their consanguinity be? Is there a way to calculate this?

Elsewhere in wikipedia it is said that if any two people taken at random are not siblings, cousins (first, second or third, an undefined but posibly finite number of times removed at most) or ancestors-descendant their consanguinity is genetically the equivalent (not necessarily the same) as if they were fourth cousins. But in this case they are {doub[to the x]}le fourth cousins and if double cousins are taken as full siblings, half siblings or something in between depending on their twinness or lack thereoff these would certainly have a closer relationship, yet, not as close as that of full or even half siblings... Would they? If X generations behind all ancestors are the same but X-1 generations behind all ancestors are different (with an algorithm, if possible, to determine this as identical, non-identical twins occur more and less in numbers), which value must X have so their consanguinity is, as exactly as it can be, the equivalent to that of fourth cousins? (I know the question might be bizarre, but it would illustrate better the genetics and genealogy of inheritance to the uninitiated) Undead Herle King (talk) 15:25, 27 March 2008 (UTC)

PS: What if at X-1 generations behind some (Varying numbers) ancestors are the same? What if some ancestors at X-1 generations or later behind are anything once, twice or more removed to anyone at X or more generations behind? How's consanguinity calculated for his varations? Undead Herle King (talk) 15:30, 27 March 2008 (UTC)

What is a third cousin once removed. —Preceding unsigned comment added by 67.80.106.105 (talk) 21:06, 24 May 2008 (UTC)

Siblings marrying cousins
What happens if two men who are brothers marry two women who are cousins? I had some friends whose fathers married women who were second cousins (IIRC). The children were not double first cousins, but they were both first and third cousins. Is there a technical term for this? 169.199.113.163 02:38, 18 July 2007 (UTC)


 * No, there is not a technical term for this. Most often, the closest relationship would be used. In this instance, it would be first cousins. Charles 04:32, 18 July 2007 (UTC)

???
I'm confused. It seems to me that if identical twins married identical twins, like mentioned in the article, it would not be inbreedng, assuming the sets of twins are not related. Thus, the resulting offspring would only be first cousins, not double first cousins. How common is this phenomenon anyway, and does what I said make any sense to anyone? Thomasiscool 00:39, 23 September 2007 (UTC)


 * Hi Thomas;
 * When one set of identical twin brothers each pair up with a set of identical twin brothers sisters, the resulting children will all have the same four grandparents, making them double first cousins, as (single) first cousins only have two grandparents in common. Double first cousins have no more to do with inbreeding that having a first cousin does, although there have been double first cousins who later marry or whose parents were already related to each other (refer to the Spanish House of Habsburg in the 1500-1700s). The difference between double first cousins whose mothers and fathers are identical twins and those who have non-twin parents is that the ones with twin parents are genetically full siblings, since their mothers are genetically identical and their fathers are genetically identical. Hope this helps and if you need clarification, let me know by posting here. Charles 00:59, 23 September 2007 (UTC)


 * Thank you, it makes more sense now :) Thomasiscool 20:35, 1 October 2007 (UTC)


 * Not problem, glad to have been of help. Charles 06:14, 2 October 2007 (UTC)


 * You're confused? Well, when one set of identical twin brothers each pair up with a set of identical twin brothers, there would be no resulting children. See the birds and bees articles for clarification. --carlb (talk) 01:13, 8 February 2008 (UTC)


 * Everyone makes mistakes! At least there are no "mistakes" between men. Charles 03:24, 8 February 2008 (UTC)

Two sets of cousins marry two other sets of cousins
What would happen if two female first cousins married two male second cousins.If both couples have children what would the relation be between the children. —Preceding unsigned comment added by 67.80.106.105 (talk • contribs)

Charles 00:56, 2 February 2008 (UTC)
 * The children would be maternal second cousins & paternal third cousins. GoodDay (talk) 02:51, 2 February 2008 (UTC)
 * Ooh, I missed the "second" in the latter. Thank you for catching that! GoodDay is correct. However, in terms of describing genealogical relationships, the closest one is used. In practice, the order of precedence is relationships of descent, ones of varying degrees of aunt/uncle/niece/nephew and finally those of just cousins, with the closest relationship being used. The children of these first cousin wives and second cousin husbands would more often than not be described as second cousins to one another. Charles 03:21, 2 February 2008 (UTC)

Well what if the children on there mothers side only shared one great grand mother but two different great grand fathers. —Preceding unsigned comment added by 67.80.106.105 (talk) 21:47, 12 March 2008 (UTC)

Double Cousins are the same genetically as half-siblings?
How is it - as this article states - that double cousins (who share two sets of grandparents) are the same genetically as half-siblings (who share only one set of grandparents)? —Preceding unsigned comment added by 98.220.196.139 (talk) 15:38, 2 June 2008 (UTC)


 * Because every person has a total of four grandparents. So when we're talking about two different people, there are a total of 8 grandparents to take into account - Person A's four grandparents, and Person B's four grandparents.  In the case of half-siblings, two of those grandparents overlap.  That is, there are four common grandparents (two individuals).   In the case of double-first cousins, all four of those grandparents overlap.  That is, there are still four common grandparents.  Just in this case, there are also four individuals.   When computing genetic proximity, it's the number of ancestors in question that matter, not the number of actual individual persons.
 * 204.97.183.31 (talk) 18:45, 10 June 2008 (UTC)


 * They have the same coefficient relationship, so they'll have approximately the same amount of raw genes the same. However they are distributed differently http://www.reddit.com/r/askscience/comments/2fewhl/my_brother_married_my_wifes_sister_how_similar/ck8x2qt --88.111.129.157 (talk) 21:01, 17 May 2015 (UTC)
 * Agree This is correct.--88.104.132.1 (talk) 18:00, 30 May 2015 (UTC)

Citation for calculation of coancestry coefficient?
Can anyone provide a citation for this? I haven't been able to find a site yet that actually shows how the coefficient is calculated. However, from what I can find - the dictionary definition - the figures in the article are wrong. The definition is "the probability that two homogenous genes from two different people came from the same ancestor". For first cousins, that probabilty is 1/8. Let GP1 and GP2 be the first cousin's non-shared grandparents, GP3 and GP4 the second cousin's non-shared grandparents, and GP5 and GP6 the shared grandparents. The possible genetic sources are then:

GP1 and GP3, GP1 and GP4, GP1 and GP5, GP1 and GP6

GP2 and GP3, GP2 and GP4, GP2 and GP5, GP2 and GP6

GP5 and GP3, GP5 and GP4, GP5 and GP5, GP5 and GP6

GP6 and GP3, GP6 and GP4, GP6 and GP5, GP6 and GP6

As you can see, out of the 16 possible combinations, 2 of them result in a match. 2/16 = 1/8 = 0.125.

Similarly, half-siblings and double-first cousins are both 1/4. Siblings and double-first-cousins of identical twins are both 1/2.

If I've made a mistake in this mathetmatics, please provide a reference that instructs how the coefficient of coancestry is calculated.

MrItty (talk) 19:16, 10 June 2008 (UTC)

Calculating inbreeding coefficient?
There are six people. Persons A, B, C, D, E, F. Persons A, C, and E are full siblings. Persons A and B marry and have a child: person G. Persons C and D marry and have two children: persons H and I. Persons E and F marry and have a child: person J. Persons G and H marry and have a child: person K. Persons I and J marry and have a child: person L. Persons K and L marry and have a child: Person M. What is the degree of consanguinity between persons K and L? What is the coefficient of inbreeding of person M?

In another case, there are four people. Persons 1, 2, 3, 4. Persons 1 and 3 are full siblings. Persons 1 and 2 marry and have two children: persons 5 and 6. Persons 3 and 4 marry and have two children: persons 7 and 8. Persons 5 and 7 marry and have a child: person 9. Persons 6 and 8 marry and have a child: person 10. Persons 9 and 10 marry and have a child: person 11. What is the degree of consanguinity between persons 9 and 10? What is the coefficient of inbreeding of person 11? — Preceding unsigned comment added by 188.221.174.97 (talk) 03:13, 4 September 2016 (UTC)