
Rare new kind of twins: boy and girl are semi-identical - pseudolus
https://www.cbc.ca/news/technology/semi-identical-twins-1.5036783
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mlacks
> Because of the odd combination of DNA picked up from the two sperm, doctors
> have been concerned that the twins might be vulnerable to cancer of the
> reproductive organs.

Why does 'different DNA' automatically assume cancer?

~~~
jey
That's just the article's handwaving. The case report actually says:

> During follow-up at 3 years of age, routine ovarian surveillance showed Twin
> 2 to have gonadal dysgenesis, and prophylactic oophorectomy was performed.

~~~
jessriedel
Yea, this is highly clarifying. The OP article makes it sound like they
decided to render Twin 2 sterile because of a risk of cancer inferred from
some theorizing about the genetics of a these new sorts of twins. In fact, it
sounds like Twin 2 was already sterile.

> Streak gonads are the defective development of the gonads in an embryo, with
> reproductive tissue replaced with functionless, fibrous tissue, termed
> streak gonads. [It is] a form of aplasia, resulting in hormonal failure that
> manifests as sexual infantism and infertility, with no initiation of puberty
> and secondary sex characteristics.

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scarmig
I guess it might also be possible, then, for semi-identical twins to have
different fathers?

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clairity
this happens all the time in other animals. i'd adopted two cats (boy and
girl) who were littermates but had different fathers. their coat patterning
and eye color were different even though their coloring and (small) size were
similar.

~~~
cimmanom
Animals that commonly produce liters of multiple offspring typically do so by
releasing multiple eggs simultaneously in each period of fertility. This also
happens in humans in the case of fraternal twins (two eggs each fertilized by
a different sperm).

That’s very different from one egg being fertilized by a single sperm and then
splitting (which produces identical twins) or being fertilized by two sperm
(which is what happened in this case).

~~~
clairity
right, that was a bad example (multiple eggs with multiple sperm from
different fathers, not single egg with multiple sperm). i can't remember
where, but i do remember reading about this happening in other animals.

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goodJobWalrus
My question here is: why is this so rare? Given how many spermatozoids are
involved in every sexual contact, why doesn't it happen much more often for
two of them to fertilize one egg?

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JackFr
Race conditions are the toughest to debug. They only show up sporadically.

~~~
thaumasiotes
[https://en.wikipedia.org/wiki/Polyspermy](https://en.wikipedia.org/wiki/Polyspermy)
has some short but interesting discussion of the theoretical background.

A polyploid zygote is completely nonviable, a pure waste of the egg, sperm,
and other mating effort that went into it. It's in the interest of the mother
and the father for the egg to be fertilized by exactly one sperm.

But it's in the interest of an individual sperm to be the one that fertilizes
the egg instead of dying uselessly. So they get faster, more efficient, and
more penetrant over time. When they're too effective, multiple sperm may get
into an egg before the defenses are up. (Which, again, just kills the sperm
that "won" the race, so you don't expect to see incredibly rapid progress in
this area.)

Oceangoing eggs respond by having a fertilized egg's defenses go up _really
quickly_ , because the egg is floating around in the ocean and it's hard for
the mother to influence that environment.

Female mammals have responded with slower defenses, but internal
characteristics that tend to retard or hurt sperm as they make their way
towards the egg, meaning that it's rare for multiple sperm to all get there at
once. This is good for avoiding polyploidy, but bad for fertility -- it is not
necessarily the case that even one sperm will make it.

It's interesting to see competition among sperm hurting the reproductive
chances of the male and female producing and accepting it.

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elil17
Just goes to show how amazingly diverse and complicated sex differentiation is

~~~
qubex
I’ll probably be downvoted to the ground for being so politically incorrect,
but... no.

This isn’t a “celebration of diversity” kind-of-thing. It’s an oh-fuck-look-
how-this-broke kind of tragedy (particularly if you read the references posted
in the commentary that indicate an amputation).

Celebrating diversity has a place. This is not it. Genetic machinery glitched
majorly and these two children will have a difficult time ahead of them. Yes,
we’ll get interesting one-off scientific observations, but that’s hardly a
total reward.

~~~
function_seven
You're reading into that comment a lot more than is warranted I think.

This _does_ show how diverse and complicated sex differentiation is. The
comment does not claim this is a virtue, or that the outcome in the article
here is to be celebrated.

"Diverse" in this context I took to mean "varied", not in the sense of social
diversity.

~~~
qubex
They’re not ‘varied’: one’s genotype and phenotype is female, and the other’s
genotype and phenotype is male.

EDIT: Apologies, the poster below showed I am incorrect.

~~~
theobon
"The girl and boy each have male and female sex chromosomes — in other words,
each twin has some cells that carry an XX pair (female) and some that carry an
XY pair (male)."

They are both genetically ambiguous and present differently due to hormone
generation rates.

~~~
nostrademons
Interesting. This would imply that they're both genetic chimeras, as they
don't carry a uniform genotype throughout their cells.

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thaumasiotes
Wait a minute:

> The twin boy and girl were found to have 100 per cent of their mother's DNA
> in common, but were only 78 per cent identical in the paternal DNA they
> carry.

78% identical seems like a lot more than the expected 50%. How likely is it
for two sperm to be 78% identical?

Also:

> Because of the odd combination of DNA picked up from the two sperm, doctors
> have been concerned that the twins might be vulnerable to cancer of the
> reproductive organs.

> "It turned out that the girl just had some changes in her ovary that people
> weren't comfortable with, so unfortunately she had to have her ovaries out,"
> Gabbett said. "The boy is continuing to have his testes monitored" with
> ultrasound.

> "Otherwise," Gabbett said, "the two twins are beautiful kids, well and
> healthy."

So gee, other than the fact that they're (probably) both sterile, they're well
and healthy. How reassuring.

~~~
mooman219
They mentioned a 3rd embryo not being viable as it only contained sperm DNA.
How the DNA was divided up was probably fairly arbitrary.

~~~
thaumasiotes
Ahh, I see. They're 78% identical in paternal DNA because the same set of
paternal DNA got into both embryos, not because two sperm were exceptionally
similar.

This is underplayed by the terminology "semi-identical twins, with the same
maternal DNA but different paternal DNA". They are significantly more
identical than that. I bet twins actually matching that description wouldn't
have the developmental problems these have apparently already manifested.

I wonder if the boy might have any symptoms of Klinefelter syndrome? (On the
assumption that the two sperm were an X and a Y.)

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b_tterc_p
While technically accurate that headline sure doesn’t anchor feelings in the
right spot. It feels inappropriate to highlight people’s tragedy as a novel
scientific observation, even if true.

~~~
TimTheTinker
Why is this a tragedy? They have 2 beautiful, healthy children.

~~~
duado
The girl has had her arm amputated and her ovaries removed, and she’s 4.
“Healthy” is not the word I would use.

~~~
gowld
She was unhealthy before her arm and ovaries were removed. Now she is healthy,
if partially disabled. We are fortunate to live in a time where arms and
ovaries can be largely compensated for. By the time she is of child-bearing
age, if she chooses, she might well have the option to undergo a semi-cloning
procedure as part of in-vitro fertilization to create an embryo that contains
a blend of her and her mate's chromosomes akin(ha!) to if she had functioning
ovaries.

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aaron695
"In addition, the girl and boy each have male and female sex chromosomes — in
other words, each twin has some cells that carry an XX pair (female) and some
that carry an XY pair (male). Having both male and female pairs of sex
chromosomes throughout various cells in the body is linked to certain
developmental problems in the reproductive organs as well as cancer, Gabbett
said. When the doctors examined the girl twin's ovaries, they found some
changes linked to cancer, and so "the difficult decision was made to remove
them so she didn't develop cancer," he said.

The girl also developed a blood clot shortly after she was born — blood clots
are a common complication for identical twins in general — and the clot cut
off the blood supply to her arm. As a result, doctors also had to amputate her
arm."

"When, in a separate unlikely event, the blastocyst divided into two –
creating twins – more of the cells with genetic material from one sperm (which
had an X chromosome) ended up in one embryo, while the other had a greater
proportion of cells formed by the other, Y chromosome bearing sperm. This is
why one fetus developed female characteristics and the other male, although
both twins have cells from both sperm in their bodies, making them chimaeras"

~~~
SamBam
Oh, interesting. That was totally missing from the original article. I had
assume that the twinning was _because_ the egg had been fertilized by two
sperm cells.

Rather, it seems that it's quite possible for any single-birth pregnancy to
involve two sperm cells, and would result in a chimera. The fact that this one
split into twins was a separate event.

~~~
thaumasiotes
The really simple model for an egg being fertilized by two sperm is that you'd
get one fertilized zygote with three sets of chromosomes, fully triploid. Such
a zygote would miscarry pretty quickly.

To get a less nonviable chimera, you need the egg to divide exactly once, such
that one half gets one of the sperm and the other half gets the other one. If
there is a mechanism that tends to ensure this happens, then it feels like
that mechanism might also be related to the twinning (tripling, apparently) of
the zygote under discussion here.

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jelliclesfarm
Happens all the time with cats!

~~~
jobigoud
Not the same thing. 2 eggs, 2 sperms. Here it's one egg, two sperms.

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jelliclesfarm
Ahh. Yes, you are right.

