Opposite sex fraternal twins - studies

[Jamie D]

Just a question thrown out for discussion and education.

Preface:
Opposite sex fraternal (dizygotic) twins (or supertwins, larger multiple births) occur when the mother hyperovulates multiple eggs that become fertilized.  The result can be same sex boy twins, same sex girl twin, or opposite sex twins.  These twins generally share aout 50% of their DNA, lke any other siblings.  In rare case, the twin can be half-siblings, who have different fathers.  In very rare cases, they can even be conceived at different times and be, for instance, once month apart in pre-natal development.

Question:
These twins, in utero, have separate placentas.  This seemingly would shield the other twin from the biochemical processes affecting the developing sibling.  But has this ever been studied?

Do we know for certain that the unborn brother does not exert a masculinizing effect of his unborn sister, and visa versa?

This interests me because I have opposite sex fraternal twins, now adults, and have wondered if this played a role in their subsequent physical and emotional development.

[spacial]

It's interesting. Hopefully we can hear from people with a twin and some, trained in this type of medicine. I like to know as well.

Sadly, while like most people, I grew up, believing, (hoping, wishing) I had a twin, the reality is, I arrived on my lonesum.

[Cindy]

There  are a few studies. The male homosexuality study between dizygotic twins produced , I think, 10-20%  incident of co-homosexuality. Which again is statistically high compared to non-dizygotic brothers but quite low to fit a hypothesis that maternal hormonal influence determined homosexual 'behaviour'. Obviously the numbers are ridiculously small so they become statistically non-meaningful. When you move into TG the numbers are even worse. BTW I am not equating homosexuality with being TG, I was using it as a reference for power calculations. The numbers are far too small to verify anything. There is some data that there are five twin dizygotic twin XY females on the International twin registry , how good that data is, is open to many questions and even the definition of TG has changed so often that it is difficult to understand the reasons for classification. I have never found any meaningful data for FtM twins, I don't think it exists.

There is no way that I know of to distinguish between biological influence, in its broadest terms, compared to behavioural factors, in those broadest terms, in these studies.

As you know we also have to compare these stats against a normal distributed male population, (lets try and keep it simple), and the test group. I think it will be a long time before we have data that is of any use. 

Cindy

[Cindy]

Sorry additional post. I use the twin registry and they and the twins are incredible people. They are of course totally anonymous. But I have been to a few of the international parties and i can assure while some ID twins are id, there are a number who are not. Genetics, as we talk about it, is not everything. You need to think about epigenetics. And it is a barrow load of fun

Cindy

[Jamie D]

Good information, cindy.  I know the numbers are quite limited.  For instance, look back studies on identical twins separated at birth (to help eliminate the effects of "nurture" rather than "nature"), are few are far between.

[Jamie D]

In cattle the blood flow between twins in utero usually merges, and when one is male and one is female, the female is masculinized and sterile because of the male hormones she gets -- she is called a freemartin.

It doesn't happen in people.  I think people used to think it did happen to humans, but I'm pretty sure science has quashed that notion.

That's all I know on the subject.

I am familiar with the concept of freemartinism.  It is an interesting concept that has been used in some science fiction.  But can the physical process be completely disregarded in humans?  I don't know.

I did read in one study a reference to chimerism in callitrichid primates.  That's getting close to home.

[Cindy]

I think many people think that these studies are easy. They are very complex, as I think you know. It is so easy to put in bias and many people are not aware of that. And no one understands control groups, unless they are a good scientist, and they are extremely difficult to even contemplate, and getting  ethics is a nightmare, and to be honest most of the studies we are discussing are a waste of time, as the funding bodies just laugh. We have X mill and you want to look at queer people, what's wrong with looking at a sick mouse? Sorry being cynical

[Cindy]

There are human chimera's, very rare because it is bred out by NS. In bovines it can be an advantage until the farmer realises stock loss.

I  read a study of human chimera's because of  an person who had problems with blood doping , he had a considerable a mount of  circulating blood that matched his ABO system but did not match 'minor' blood antigens. The argument was if he had been blood doped, against his knowledge, or he was a chimera to a dead twin at birth whose blood group lasted  to the time of testing. He was a cyclist who are a group who may benefit from blood doping in a physical sense.


I've had one foetal chimera in my life. We ran to the ward to see a beautiful baby and her Mum. We never told them.

Brings back some good thoughts


Cindy.

Oh if anyone is worried about these concepts or information please contact me. My opinions are mine and mine alone.