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Twins and Chimaeras
Posted: Sat Mar 31, 2007 12:26 pm
Most people consider genetic paternity tests definitive. Two recent findings add some nuances to this issue and broaden the question (and the potential) of paternal genetic contribution.
One is the recent discovery of semi-identical human twins, arising from a rare event: the fertilization of an egg by two sperm. In a singleton zygote, the co-existence of three sets of unpaired chromosomes results in failed sorting and spontaneous miscarriage, but twinning allows sorting and hence survival. The paternal genes become a mosaic -- which means it could also happen with two different fathers contributing.
An even more diverse chromosomal mixing and matching happens in marmosets, distant primate cousins. As a result, a mother can have children with distinct paternal contributions and families reflect awareness of this: marmosets raise their children in extended cooperatives, in which fathers and other first-degree relatives participate in child rearing as actively as mothers.
The findings highlight a fact that I've often witnessed as a biologist: nothing that we have done so far in genetic engineering does not already happen "naturally", from gene splicing to creating chimaeras. Nature is still the most adventurous engineer.
Here are the links:
New type of twinning
Who's your daddy?
Posted: Sun Apr 01, 2007 9:35 pm
The links are for you lucky few with Nature subscriptions...
But there have been a number of non-pay stories covering the issue. Interesting stuff for sure. A couple questions from the peanut gallery:
1) This appears to be an extraordinarily rare occurrance based on the press it's getting, etc. Is this from lack of looking or is there a good reason to think it's extremely rare?
2) It seems to me that given our ability to modify primate eggs' DNA and so on, eventually we should be able to replace egg DNA (I'm sure there are many technical hurdles - not trying to make it sound easy). This would allow for example, a gay couple to have a child that shared their genes. This seems an ethical use of technology to me - is it likely to be possible?
Posted: Sun Apr 01, 2007 9:58 pm
Oh, dear. I didn't realize that Nature was a closed site. Here is the BBC version:Atypical twins
. I think this outcome is rare, but I suspect the estimates of occurrence will increase when people look more carefully.
In terms of modification, the problem of creating a zygote from a male gay couple will be the missing X chromosome. X and Y chromosomes are very, very unequal and a zygote without an X chromosome is not viable. In women, every cell randomly inactivates one of the two X chromosomes (not completely, which is an important nuance -- XO people suffer from Turner syndrome). This is true for all female mammals -- which is why, for example, calico and tortoiseshell cats are all female. Because of this asymmetry, lesbian couples would have an easier time having children derived genetically from both partners than gay men. The hue and cry of moralists would be another matter, of course.
Posted: Sun Apr 01, 2007 11:14 pm
When I said "gay" I was referring to either gender, as I think you gathered. I find your answer counterintuitive (like many things that are accureate) and would appreciate a little more information. If men have both X and Y chromosomes, it would seem simpler to get an X and Y from men than from women that have two X?
Posted: Sun Apr 01, 2007 11:42 pm
You know, I was thinking of meiotic mixing of already formed gametes, which is in fact impossible because of the egg/sperm morphological dichotomies. In principle, you are right: if our technology progresses to the point that we can pair individual chromosomes in vitro, then the field is level as far as nuclear DNA goes.
Because of the obligatory non-nuclear contributions of the egg, the most obvious way would be to add the genetic material to an enucleated egg. For female couples, one of the partners could provide the egg, whereas for male couples this is de facto impossible. As a result, a child of two men would have some genetic contribution from the woman who donated the egg, because of imprinting, mitochondria, etc. On the other hand, women could only have daughters whereas men could have either daughters or sons.
At the next stage, if we don't need eggs at all to provide context, all limitations are removed.
Posted: Wed Apr 04, 2007 11:12 pm
You know, I believe I read a discussion very much like this a while ago and it was abandoned after it was declared sexist. But as you both said; moral considerations aside - is this not one of the possible paths for genetically engineering beings capable of living on extra-terrestrial (or non-terrestrial) planets?
Posted: Wed Apr 04, 2007 11:16 pm
On further consideration it seems a bit like reinventing the wheel to abandon the ovum. I mean after all, the material or catalyst is already there. A same sex male couple could use genetic material from both partners, correct?
Posted: Wed Apr 04, 2007 11:24 pm
rocketscientist wrote:Is this not one of the possible paths for genetically engineering beings capable of living on extra-terrestrial (or non-terrestrial) planets?
Yes, very much so -- that will be the next installment on my Making Aliens blog entry.
rocketscientist wrote:On further consideration it seems a bit like reinventing the wheel to abandon the ovum. I mean after all, the material or catalyst is already there. A same sex male couple could use genetic material from both partners, correct?
I agree. Biological engineering already going on in our cells is incredibly intricate and has solved problems we didn't even know existed until we delved into them! Adjusting them to our specific needs (as we have just started doing) is probably the most sophisticated way to address many issues -- but it requires courage, acceptance of limitations and the willingness to face possible unintended consequences.