semi identical twins? please explain

This article doesn’t make sense to me. How can two sperm fertilize one egg and the egg split into two embryos? The chromosome math doesn’t work. Can anyone explain this?

The term I always heard was Polar Twins and they hadn’t been proven to exist the last time I checked a few years ago. Mary Kate and Ashley Olsen are suspected to be polar twins because they look so much alike but they aren’t identical twins.

When I read about it a while ago, the egg split first and then got fertilized by two sperm. That makes more sense to me but I can’t say for sure.

Interesting. But how can an egg split when it is haploid?

The article uses a different definition of semi-identical twinning than I was previously aware of, so they may be positing a different process: “Two sperm cells fertilized one egg—an event assumed to be very rare—then split into two embryos.” The theory of polar twins is usually explained as “an unfertilized ovum splits into two equal parts and is subsequently fertilized by two separate sperm.” cite.

When a sperm penetrates the outer layer of the egg, usually, instantaneously (or close enough to it), an impenetrable barrier springs up and no other sperm can get in. Obviously in reality, it can’t be literally instant, so theoretically at some time or another two sperm must get through at once. My guess is that most of the time, the resulting theoretical “triploid” zygote would simply be spontaneously aborted. This article seems to suggest that for some reason, the nucleus of the egg may double as well, there could be two haploid nuclei in the cell. This could only really happen in an egg which had not yet disposed of its second polar body.

The polar body is usually a tiny non-procreative cell that is also haploid but contains very little cytoplasm. It’s basically how the ovum “gets rid” of half its DNA to become haploid and ready for the sperm. The lack of cytoplasm is what makes it usually just die off, and what makes it not an ovum itself - it doesn’t have enough cellular guts to survive, to grow, and to split.

So I guess, if the timing was perfect, the egg could be about to pinch off its second polar body, meaning there are two haploid nuclei, and just at the right moment, two sperm make it through the ovum’s membrane, each one works its way to one haploid nucleus, and then the polar body, with sperm nucleus inserted, finishes pinching off the ovum.

The usual theory - misdivision of the egg into two viable haploid cells and subsequent fertilization of both of them by different sperm - seems more likely to me. If what should have been the polar body gets enough cytoplasm and cellular guts in the division to keep it alive, there’s no reason it wouldn’t fuse with the sperm’s nucleus, grow, divide and turn into a zygote.

Did this help, or was it more confusing?

Or they could just be siblings who happen to be the same age. I’ve certainly known pairs of siblings who look so much alike that the only way I can tell them apart is by seeing them side-by-side so I can see who’s older. Take away the age cue, and yes, they could have been mistaken for identical.

Who says the Olsens are not identical twins? The only evidence I could find is in Wikipedia pointing to a site that says they are fraternal because one is an inch shorter than another and one has a birthmark above their upper lip, and they are opposite-handed. If those facts are the basis of a conclusion that they are fraternal twins, it is simpy a wrong conclusion. Height is not entirely based on genetics, and specific birthmarks are never based on genetics. Opposite handedness could mean they are mirror-image identical twins.

The fact that one of them is left handed would make me think it’s more likely they are mirror-image identical twins, since left handedness is especially rare in females.

The trouble with WhyNot’s explanation is that such a pair of twins would be less closely related than ordinary fraternal twins, not more. The two eggs would be complementary, so less closely related than a random pair of eggs, while the sperm would be as close as any other random pair of sperm.

Are you familiar with the technical term for “siblings who happen to be the same age”? :stuck_out_tongue:

Seriously, this is a good point. Many younger people (using that “get off my lawn!” term to encompass infants, toddlers, children, youth, early adulthood) look virtually (but not precisely) identical to the appearance of a sibling as he/she hit the same development point. They are, after all, the products of a very restricted gene pool (two complete diploid sets) so similarity is expected; near-precise similarity is called for as an outlier that will occur, simply by the laws of chance. There is no reason that a set of strictly fraternal twins, much less the “polar twins” of this thread, should not have that same resemblance.

Are you thinking of The Tale of the Twins Who Weren’t? If you are looking at that sort of diploid-complement twins, you’d be correct. As the author notes, they’re effectively unrelated.

But what I’m hearing here is the product of mitosis, not meiosis – having gone haploid in a meiotic split, the egg then undergoes a mitotic split, resulting in two eggs with identical genes – but which may be fertilized by sperm with significantly different genomes.

I’m pretty sure that the girls parents and doctors made the determination as to them being fraternal twins when they were born. It probably had something to do with counting placentas and afterbirth or somesuch.

Oh I understand what you said. WhyNot proposed that the discarded half of the genome that was packaged in a “polar body” had been regenerated and somehow two sperm entered simultaneously and one fertilized each half. I just wanted to point out that could not explain near twins. Your mechanism, if it actually occurred, does.

I should say that I am not a biologist and I never heard of polar bodies before. At first, I thought they were the “Barr bodies” that were formed when one of the two X chromosomes in each female shrivels up and becomes dormant. But I believe this happens randomly in each cell (so that all women are slightly chimeric). Presumably this prevents women from expressing two copies of each gene on the X since one clearly suffices for men.

It’s my understanding that a lot of research based on monozygotic twins conducted until recently is suspect because there are plenty of non-monozygotic twins who appear identical but aren’t, and that the criterion of “looking identical” is not sufficient.

Here is the original news article from Nature, with this relevant quote:

Maybe someone with more of a genetics background can explain this, but I assume that one sperm fertilized the egg first, then lingering DNA from another sperm gets incorporated once the fertilized egg begins to split (in the second scenario). Note that both individuals are chimeras, with the ratio of cells containing the different DNA varying between the two.

An egg (or any other cell) can divide through the usual process of mitosis. The ploidy level has nothing to do with it.

Counting placenta isn’t reliable, as was explained* to us when our twins were born. It is possible, although rare, for two placenta to merge, or for identical twins to have separate placenta. So, while it might be indicative, it’s not conclusive.

  • This was an extra pre-natal course sponsored by the Parents of Multiple Births Association, designed to supplement the City-offered pre-natal classes. I presume that their information was correct, but I can hardly claim it’s an authoritative cite.

I think that, before true genetic testing, the standard way to determine whether twins were identical was to compare fingerprints. The details are somewhat random and therefore vary, but the broad pattern of arches, swoops, and whirls is genetic. So if both twins have the same pattern on all their fingers, they’re probably identical.

In the BBC article online:

The twins:

Since both have genes from both of the sperm, I think the egg had to have split after fertilization.

I recommend an article in today’s Science Times (NY Times) on marmoset twins (normally they come as twins) which come from two eggs and two sperm, but nonetheless the pairs of twins share each other’s hereditary. In fact, they are chimeric with each one having cells from each fertilized egg. Bizarre.

Now that I think about it, I guess this is true.