Readers have asked, wondered, emailed, chatted, tweeted questions for Alex. For the next couple months, Alexandra Hargreaves will guest post on this blog to answer your questions. If you have a question you'd like Alex to answer, feel free to email her at: AlexandraHargreaves at gmail dot com.
Identical male-female twins or Single Zygotic male-female twins
Can you imagine saying, 'I'm a single zygotic male-female twin' when you're ten years old? Now imagine being a military kid who just moved to Denver, Colorado (where the people are not all that friendly) from Fort Bragg, NC (a military base where Dad was boss)? I had a thick Southern accent, a bad haircut, and an identical looking male twin.
Trust me. We didn't go over well in Catholic School.
If you were me or Max, _you'd drop the whole single zygote thing and just go with being identical twins._
Not that it made any difference. We had no friends. We didn't meet Zack until we were almost eleven. No girl wanted to be my friend. And Max never really cared about anyone but me. So we were the weird Hargreaves twins.
You know how that goes: "*That Samantha, she's the cutest little thing. And Colin, such an athlete. And little Erin is smart and darling. She's going to grow up to be a man killer. Oh, and those twins? Is there medication for them*?"
So when people ask me why I say that Max and I are 'Identical Twins' instead of single zygotic male-female twins, I want to punch them in the face.
Because Max says that people have a right to ask a legitimate question, I asked our twin expert Wyatt Klassen, MD, PhD to explain the science of how Max and I can be identical twins or single zygotic male-female twins. You probably remember him from *Who I Am*. But we don't talk about him much. You'll get a better idea of him in Lean on Me (coming February 29, 2012). Because we're fascinating, Wyatt's not going anywhere.
So take it away, Wyatt.
Wyatt Klaussen, MD, PhD
We're talking today about how Alex and Max can be single zygotic male-female twins. In order to discuss twins, let's go over the basics of conception. I will provide links in case you'd like more information. Because this is a general conversation for the general public, we're going to use generalized terms such as 'conception,' 'chromosomes,' etc.
Human beings have 23 pairs chromosomes - 22 autosomes and 1 sex chromosome. For the purpose of our conversation, we are going to focus primarily on the sex chromosome. Every human cell has one X sex chromosome. A male human has an X + a Y sex chromosome where as a female human has an X + another X sex chromosome.
Male = XY
Female = XX
A normal egg carries 22 autosomes and 1 X sex chromosome. Sperm, on the other hand, carry the same total number of chromosomes. However normal sperm carry either an X sex chromosome or a Y sex chromosome.
The egg is confronted with [sperm](http://en.wikipedia.org/wiki/Sperm). ![Usual Single Zygotic Conception](/assets/img/alextalks/UsualConceptionSingleZygote02-fixed.jpg)
There are a lot of theories and research into the dynamics of eggs and sperm. While the conversation is fascinating, it does not relate to our topic. We have a lot to cover, so let's stay focused.
The end result of a healthy egg confronted with healthy sperm is usually the creation of a single zygote.
Single zygotes can grow into entire human beings. A single zygote has a single [amniotic sac](http://en.wikipedia.org/wiki/Amniotic_sac).
**Incidence of twins:**
![Incidence of twins](/assets/img/alextalks/Identical-fraternal-sperm-egg-fixed.png) The diagram to the left was created by ChristinaT3 for wikipedia and edited by Trkly. It's better than anything I created so I thought I'd add it.
Multiple births are relatively rare. Even with the rise of [in-vitro fertilization](http://en.wikipedia.org/wiki/IVF), the chances of an individual having [dizygotic twins](http://en.wikipedia.org/wiki/Dizygotic#Fraternal_.28dizygotic.29_twins), non-identical or fraternal twins, is 1:80 in North America. [Identical twins](http://en.wikipedia.org/wiki/Dizygotic#Monozygotic_.28.22identical.22.29_twins) happen at a rate around 3:1000 or .03% of all births are identical twins.
So far, we've covered normal eggs and normal sperm. Most fertilization events happen with normal sperm and normal eggs. However, Alex and Max weren't created by normal eggs and possibly normal sperm.
**What causes male-female single zygotic twins?**
The first step in the creation of male-female single zygotic twins is a __post fertilized egg that has the combination of XXY sex chromosome__. ![Creation of XXY egg](/assets/img/alextalks/howaneggbecomexxy-fixed.jpg)
This zygote is created two ways:
An egg contains two X sex chromosomes and is fertilized by a Y chromosome.
A sperm contains two sex chromosomes XY and fertilizes a normal egg.
The incidence of an XXY zygote is shockingly high - 1:500-650 male births. Thus it's more likely to have an XXY zygote than it is to have an identical twin.
An XXY human being is considered to have [Klinefelter's syndrome](http://en.wikipedia.org/wiki/Klinefelter's_syndrome). Please follow the link to learn more about this syndrome and the challenges these individuals struggle with. It should be noted that most of these zygotes are miscarried. Less than 40% of these zygotes develop into full term babies. In 2010, 100 such zygotes were altered using in-vitro technology to correct the zygote to a male XY zygote.
Max and Alex started out as a Klinefelter's syndrome zygote. For reasons we don't quite understand, even in same sex single zygotic twins, Max and Alex's zygote split into two zygotes.
Alex and Max have a story that they like to tell about how they became two separate people. The story was created by their father Patrick after they were kicked out of military school. Patrick told them that, like their favorite movie Pinocchio, Max wanted to be a boy so badly, he threw himself into being his own person. Of course, the moment he separated, he regretted it deeply and wanted back. But the damage was done. He could only cling to Alex which is what Alex and Max did the entire time they were in utero. While this is a great story, it is not scientifically accurate.
**What we know about Alex and Max:**
Alex and Max's entire family has been incredibly generous with their time and bodies. They have withstood every test I've asked for including extensive genetic work. This is what I now believe was the genesis of Max and Alex.
1. __Rebecca had double XX eggs__. We also found these eggs in Alex and Max's sisters Samantha and Erin. Further, the child Alex called "Simon" was an XXY zygote. We know the potential for XX eggs is passed mother to daughter. We can assume that Rebecca's mother had the same difficulty. Thus, we believe that Alex and Max were created with a double X egg and a Y sperm.
2. Twins run in the Doucet family. While the incidence of twins is not specifically linked in families, it is correlated equally to both the mother and the father's family. We can assume that zygote that was Max and Alex __split due to the influence of their biological father, Benjamin Doucet__.
3. While most twin splits happen late, thus causing chromosomal damage and serious side effects, __Alex and Max's zygote split almost upon conception__. The sex chromosomes were able to remanufacture what they lost in the split with ease. In the Hargreaves family, there is an idea that the Hargreaves heal very quickly. We have followed this healing capacity through a number of incidents with Alex, Max, as well as their siblings. For example, Colin Hargreaves was experiencing beginning stages of alcoholic hepatitis from his excessive drinking. One year later, his liver is almost to normal functioning.
We postulate __this capacity to heal as the primary reason Alex and Max experience limited effects from this chromosome splitting__. The truth is that unless we are able to monitor an XXY zygote split, we will never know exactly how this happened.
When I originally presented the material about Alex and Max, a number of researchers came up to speak with me about a few of their patients. This led me to believe that single zygotic male-female twins are much more common than we think.
**Are Max and Alex identical?**
No. Primarily because Max has a Y chromosome. However, outside of the obvious Y chromosome, they are identical.
It's important to note that looking identical is vital to Max and Alex. They work hard to look exactly a like.
For example, Max's hair is a little more red than Alex's hair. They both dye their hair chestnut brown to be exactly the same. Alex gets white tips in her hair to cover the fact that she's died her lighter hair so dark. They are both very thin. While Max is bigger and stronger than Alex, their faces are remarkably similar because they are so thin. They both wore braces and had their teeth modified to be exactly the same. Individually, these changes are minute and subtle. However, their efforts to look identical makes the biggest difference in them seeming identical.
At the same time, in our testing, we found that they have the same mannerisms, vocal range, and facial expressions. Their brains are the same size, shape, and share near identical neuroelectric patterns. We attribute this to the sheer amount of time they spent together when they were children. However, the fact that their genetics are more than 98% identical cannot be discounted.
If you have questions for myself or Alex, please leave them in the comments.
*>>>>>Alexandra Hargreaves is a fictional character created by Claudia Hall Christian for the [Alex the Fey](http://alexthefey.com) thriller series. Opinions expressed are that of Alexandra Hargreaves and may not reflect the opinions of the author or Cook Street Publishing.*