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View Full Version : twins marry twins, and then...

goenetix
12-18-2004, 03:48 PM
Imagine this (not completely unheard of though) - identical monozygotic male twins marry identical monozygotic female twins and they get their firstborns:
What are the chances that their children will LOOK identical? Will they even have the same DNA?

ultrafilter
12-18-2004, 03:55 PM
Think about it: what's the difference between this scenario and one couple having two children?

Nightime
12-18-2004, 03:57 PM
I guess they would be about as similar as brothers and sisters, even though they would only be cousins.

davenportavenger
12-18-2004, 04:00 PM
They'd be genetic full siblings, so they'd look similar to one another, but I don't think they'd necessarily be identical. Twins don't carry a bunch of cloned egg and sperm cells; the codes are different in every cell, and the probability of two identical egg cells in two different women matching up with two identical sperm cells from two different men would be astronomical. So I think it's safe to say they wouldn't be identical, they just might look a lot alike.

On preview: what ultrafilter said.

Polycarp
12-18-2004, 04:10 PM
Imagine this (not completely unheard of though) - identical monozygotic male twins marry identical monozygotic female twins and they get their firstborns:
What are the chances that their children will LOOK identical? Will they even have the same DNA?

Slender, though higher than two siblings or two strangers. Meiosis selects one gene from each set in each parent. Although the parents have the same two sets of genes, the likelihood that a given gene will be selected is 50% -- and the likelihood that each mother will give the same given gene to her child is also 50% (From gene combo AB, mother 1 may contribute A or B, and mother 2 may contribute A or B -- hence if both ova are A or both are B, each mother has contributed the same gene.) The same holds true for the fathers -- there's a 50% chance that gene A or gene B will appear in each "lucky" spermatozoon that achieves fertilization. Hence there's a 25% chance that both children will get the same gene combination at a given gene site from both mother and father.

But there are thousands, perhaps millions of genes in every human's genetic makeup. Call that number N. Therefore the likelihood of the children having identical genes is .25n -- which rapidly tends toward zero.

There are two alleviating factors:
1. Genes tend to cluster together, being transmitted not singly but in groups. For hypothetical example, trait X and Trait Y are both governed by genes at one end of chromosome 7, which undergoes meiosis as a group. Therefore if the children receive identical or differing genes from a parent for one, they will for the other. This reduces N by an unmeasurable but significant amount.
2. One or both parents may be homozygous at a given gene site, the two alleles being identical (as in blue eyes, for example). In such a case, the factor is not .5 per parent but 1, for that particular gene.

However, not every gene contributes towards appearance -- and nurture has more than a little to do with appearance. Hence, although we can reduce N by a significant amount (e.g., the gene governing whether one can taste substance X, which does not affect appearance), we also need to add in a factor (which I'd hesitate to try to define) governing the extent to which nurture affects appearance.

Nightime
12-18-2004, 04:23 PM
Slender, though higher than two siblings or two strangers.

Why would it be higher than for siblings? I would think it would be lower, since siblings could be twins themselves, which is not possible for the hypothetical cousins of the OP.

Unless the parents of the OP are related... but I read it to mean that the male twins and the female twins come from different families.

Rusalka
12-18-2004, 04:31 PM
Not higher not lower, the chance of these "cousins" being identical is exactly the same as if two separate siblings were identical (which is possible, as a previous poster has stated, but extremely unlikely). This is because, as someone has previously stated, the "cousins" in such a union are like genetic siblings. Since both sets of parents have the same genetic material, you could consider them interchangeable.

goenetix
12-18-2004, 04:35 PM
...but I read it to mean that the male twins and the female twins come from different families.

Yes, you are right - those hypothetical twins come from different families.

Ethilrist
12-18-2004, 04:51 PM
A pity for the sake of science that Cheng and Eng married sisters, (http://engandchang.twinstuff.com/cgi-bin/i/eng_chang_wives.jpg) but not twin sisters.

They did have 21 kids, though.

scr4
12-18-2004, 06:00 PM
Can those "cousins" get married legally? (In states where first-cousin marriage is legal, that is.) The chance of genetic defects is the same as sibling marriage, but I wonder if the law takes that into account.

12-18-2004, 06:12 PM
Firstborns don't have to come from marriage silly.

12-18-2004, 06:13 PM
Oops, that wasn't directed at you scr4, but to the OP.

rocking chair
12-18-2004, 06:45 PM
the most famous identical looking cousins would be george the fifth of england and nikolai the second of russia. it is a def. role of the dice as many of the royals do resemble each other, none were as close as those two.

their mother's were sisters from denmark, and on both sides there was a heavy german royalty interconnection.

http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dgeorge%2Bv%2Bof%2Bengland%26e i%3DUTF-8%26fl%3D0%26qp_p%3Dgeorge%2Bv%2Bengland%26imgsz%3Dall%26fr%3DFP-pull-web-t%26b%3D1&h=689&w=481&imgcurl=puffin.creighton.edu%2Fmuseums%2Fallen%2Fimages%2Fnickandg.jpg&imgurl=puffin.creighton.edu%2Fmuseums%2Fallen%2Fimages%2Fnickandg.jpg&size=82.3kB&name=nickandg.jpg&rcurl=http%3A%2F%2Fpuffin.creighton.edu%2Fmuseums%2Fallen%2Fbignickg.html&rurl=http%3A%2F%2Fpuffin.creighton.edu%2Fmuseums%2Fallen%2Fbignickg.html&p=george+v+of+england&type=jpeg&no=5&tt=208

goenetix
12-19-2004, 02:28 AM
Firstborns don't have to come from marriage silly.

(sigh) Of course. But irrelevant :rolleyes:

Polycarp
12-19-2004, 07:29 AM
I do not know where anybody is getting this "cousins marrying" thing: Floyd and Lloyd Smith, identical twins, marry Jean and Joan Jones, also identical twins (two monogamous heterosexual marriages). Each pair of twins is legal siblings and biological clones within the pair; the two pairs are effectively not related. (Any two persons taken at random are likely to share one or two distant ancestors, twenty generations or so back. This constitutes "effectively not related.")

The two firstborn children, presumed but not specified in the OP to be of the same sex (It would be fairly easy to figure out how Andrew and Beth are not identical), are compared for identity of appearance. There's no question of crossing them. Yes, they would be legal first cousins and genetic siblings. (Why genetic siblings? Because the genes for each parent are interchangeable with the genes for the aunt/uncle of the same sex that is the other half of the identical twin pair. So each child is getting both halves of its gene set from the same two sources -- the gene sets shared by each twin pair.)