# double first-cousin marriage

’cause you can never have too much of a good thing, right? (~_^)

double first-cousins are just that — first-cousins twice over:

“‘Double first cousins’ arise when two siblings of one family reproduce with two siblings of another family. The resulting children are related to each other through both parents’ families. Double first cousins share both sets of grandparents in common and have double the degree of consanguinity of ordinary first cousins. Genetically, they are as related as half-siblings.”

double first-cousin (d1c) marriage looks (or can look) like this:

in the parts of the world where father’s brother’s daughter (fbd) marriage is practiced, d1c happens at not a small frequency. in afghanistan, for example, 6.9% of consanguineous marriages are to double-first cousins (46.2% of marriages are consanguineous).

d1c marriage obviously changes the degree of relatedness within the d1c family, compared to other cousin-mating families that is. broadly speaking, double-first cousins are like half-siblings in terms of genetic relatedness.

if you use my system for calculating new-and-improved coefficients of relationship/relatedness, which is based on the differential inheritance of the x- and y-chromosomes, then the relatedness of two brothers from a father’s brother’s daughter double first-cousin mating (fbd d1c mating) looks like this (if i’ve done the maths right!**):

B-B (FBD D1C marriage)
**a man and his FBD share 1/8 of their autosomal dna; a woman and her MZS share 1/8 of their autosomal dna + 1/4 of an x-chromosome + another 1/8 of an x-chromsome.
**therefore, two brothers from an FBD D1C marriage share: 1/2 autosome + 1/16 autosome + 1/16 autosome + 1/2 x-chromosome + 1/8 x-chromosome + 1/16 x-chromosome + y-chromos
ome
(96.42% x 0.5) + (96.42% x 0.0625) + (96.42% x 0.0625) + (2.60% x 0.5) + (2.60% x 0.125) + (2.60% x 0.0625) + 0.99% = 0.5701

so, an fbd d1c mating raises the genetic relatedness of two brothers to the same level as that of two brothers from an mzd marriage:

what would be interesting to know is if fbd d1c marriage was more prevelant in the past when transport wasn’t so good and people prolly didn’t marry over such long distances. in other words, there might’ve been more cousin marraige, including double first-cousin marriage, in general in the past.

_____
**have i got 1/16 autosomal dna too much there with the two brothers (B-B)?

(note: comments do not require an email. i’m my own grandpa!)

1. “in afghanistan, for example, 6.9% of consanguineous marriages are to double-first cousins (46.2% of marriages are consanguineous).”

I believe the rates are about *double* this in some parts of the modern UK – but I only have newspaper reports linkable to back this – I read it in the work of my esteemed friend Jon Gower Davies

Anecdotally the number for UK sub-populations seems likely to be broadly accurate. At least, nobody has denied it.

And this is consistent with the extremely high rates of recessive genetic disorders observed by medical services – which have been serious enough to cause official UK government comment, despite breaking numerous PC taboos.

Of course, in the past – and in most parts of the world at present – children with serious genetic disorders obvious at birth or early infancy would not survive – one way or another… :-(

– which means that very high incidence of double first cousin marriage might well be ‘genetically’ sustainable almost indefinitely, assuming it offers other compensatory survival and reproductive advantages; and especially in a system of polygyny with infusions of extra wives from outside the gene pool (eg. from conquest and slavery)

2. @bruce – “I believe the rates are about *double* this in some parts of the modern UK….”

yes, i believe you’re right. i’ve read about some of the problems with the south asian community in the u.k. — particularly bradford. not good. i even posted about it some time back, pointing out that not only are there the medical costs to think about (i.e. a drain on british society), but also the social factors. already by importing quite unrelated peoples to britain, one is setting the whole society up for balkanization — but then to import a cousin-marrying population?! then you’ve got to add in all the clannish/tribal behavior that goes along with that. assimilation is just NOT going to happen.

i’m a big anglophile, btw. (^_^) been to the u.k. several times and so it’s sad for me, personally, to see what’s happening to dear old blighty. not to mention how brits must feel about it all.

3. […] in oxford in the 1980s were to first- through-third cousins (a couple of those first-cousins were double-first-cousins, so the genetic relatedness is even closer) — and another 11% were to someone in the same […]

4. Can you have answer me the below question.
My parents are cousins and now i want to marry to cousin( my mom younger sister daughter).
If there is this, what will happen to our baby?

5. @anonymous – hi, anonymous. i’m not a doctor, so i don’t want to give you any advice. the best thing to do is to check with a doctor if you’re concerned. a doctor might suggest some genetic screening (tests) before you marry to check for any possible problems.

best wishes to you!

6. Incidentally, North Carolina is the one state in the US that bans double first cousin marriages, but not first cousin marriages in general.

7. @john – “Incidentally, North Carolina is the one state in the US that bans double first cousin marriages, but not first cousin marriages in general.”

how interesting! thanks! i wonder how long that law has been on the books there?

8. i want to marry a double first cousin, do you guys think it will be that bad if we do get married and have children??

9. @isabel – “i want to marry a double first cousin, do you guys think it will be that bad if we do get married and have children??”

hi, isabel. the best thing to do is to consult a doctor and, perhaps, get some genetic testing done before you guys get married (especially if you happen to know of any genetic conditions/diseases or birth defects running in your family). this is particularly important to do in cultures where cousin marriage is common and has been practiced for many generations. your doctor can probably advise you on genetic testing.

best wishes! (^_^)

10. What if u dont have a gentic counslor or doctor in ur country(trust me I have checked)
And a genetic condition would be partial deafness, of whom my grandfather (on my mothers side) has. Luclky his children aren’t deaf and my generation in the family is not deaf. I could really use some help, ive been trying to contact genetic counselors and it nothing has happened.
Let me know if u can atleast help me out on something
Your amazing andThank u for this post ! :)

11. @isabel – “Let me know if u can atleast help me out on something”

i don’t want to give you any direct advice on this, because i’m not a doctor or a geneticist or anything like that, so i really just don’t know.

but, yes, there’s probably a higher risk for your kid(s) getting the partial deafness condition. that’s a risk, of course, and not a guarantee. that’s why actually getting genetic tests done would be the best thing — so you could know for sure one way or the other. (obviously, if such tests are not available, you can’t.)

let me see if i can find someone who could offer you better advice than i can. check back here tomorrow (tuesday).

all the best! (^_^)

12. Hi ,what would be the best possible option to defy any gentic disorder if I marry my double cousin.I would like to inform that both of our parents have no genetic disorder.

13. My grandfather and grandmother were double first cousins.. Among their children at Leaost five died as infants. In my generation, bipolarity is rampant, so is dyslexpia

14. @anonymous – “My grandfather and grandmother were double first cousins.. Among their children at Leaost five died as infants. In my generation, bipolarity is rampant, so is dyslexpia”

that’s rough. i’m sorry to hear all that. =/

i’m not certain if all those problems are *definitely* connected to your grandparents’ inbreeding; but certainly, going forward, if anyone in your family does plan on marrying a close relative, i’d *highly* recommend genetic testing beforehand to find out about any possible risks.

15. My father’s grandparents are first cousins. His parents are double first cousins being his father and his father’s brother are married to sisters. The sister half of his relationsship are the children of the first cousin marriage I mentioned. This resulted in my 2nd great grandfather’s being brothers. How does this effect my percentage of DNA from that line.