Twin Type + Dr. Segal + Colorblindness

 Recently my mom was watching an episode of The Doctors that had identical mirror twins on the show.  Dr. Sears explained to viewers that mirror twins typically split between 9 and 10 days post fertilization and that they are the last split before conjoined twins, which never fully separate.

I had been under the impression previously that my twins' zygote split between day 3 and 4 because they were diamniotic, dichorionic twins.  (Twins with their own sacs and their own placentas.)

But I also felt certain they were mirror twins.  Jack is right handed, his hair whorl goes clockwise, his front cowlick goes to the right, and his prominent dimple is in his right cheek.  Logan is the complete opposite: He is left handed, his hair whorl on the back of his head goes counter clockwise, his front cowlick goes to the left, and his prominent dimple is in his left cheek.

After hearing this new information about when mirror twins split and comparing it with my knowledge of the boys' twin type, I knew I needed more information.  We know for certain that Logan & Jack are identical (we had their DNA tested for their first birthday to be sure), but I couldn't be sure my memory was correct about them being di-di twins.  I just didn't have the answers!  I ended up ordering our medical records (mine and theirs) from the hospital and our old health insurance company.

Reading through the mountain of paperwork I received was fascinating.  

I confirmed (both via ultrasound reports and an actual pathology report) that the twins had separate sacs and placentas, and were monozygotic, diamniotic, dichorionic twins.  But I was still confused.  How are they mirror twins if they split soon enough to have their own sacs & placentas? It just didn't make any sense.

I read and read and read, every online article I could get my hands on, about mirror twins; when what splits occur; and what twin types are created, and couldn't find anything that made sense for my particular situation.

Finally I found world renowned twin expert (and fraternal twin herself) Dr. Nancy Segal's website and read a few more articles on twin genetics.  This lead me to a fascinating occurrence in identical twins called epigenetics {amazing video explaining epigenetics in identical twins} where certain environmental factors can activate or deactivate genes, explaining the differences identical twins often display, especially as they age.

On Dr. Segal's website I found (!!!) her contact information (email and phone number). I was beside myself! I decided to email her and hope for the best.  

Hi Dr. Segal,

I am the mother of a set of dichorionic/diamniotic identical twin boys.  (We did a DNA test on their first birthday to determine zygosity, and I recently received copies of their medical records confirming they were di/di twins.)  They were born June 6, 2008 and will turn six shortly.

I had assumed pretty much from birth that if they were identical, they were mirror twins based on the hair whorls that went opposite directions on the back of their heads and the cowlicks they have on mirrored sides of their foreheads.  They have since displayed their handedness (Baby A, Logan, being left handed; and Baby B, Jack, being right handed.)

The question I have is this:
How is it that Logan & Jack appear to be mirror twins (which split fairly late in development) but were di/di twins, which are said to split before Day 4?  Is it possible that epigentics is playing a role in their handedness?  

They were discordant twins, with Baby A weighing 4lbs. 6oz. and Baby B weighing 6lbs. 13oz.  And they differ in size currently, by about ten pounds in weight and nearly 2 inches in height.

I would be so fascinated to hear your take on this, or be directed to someone (anyone!) who could shed some light on it.

Thank you so much,
for reading this, and also for the work you do with twins.
It's all so fascinating!
Sincerely,
Rochelle Cunningham

She emailed me two days later with a response.  I was elated!

Rochelle,

The day of division is an estimate--it may be that if your boys split at about day 4-5 they could still show some reversals--a fascinating pair!

Dr. Segal

---

In my research on twins, twin type, and epigenetics, I came across quite a few interesting twin sites.  Perhaps the most informative was this one with 17 facts about twins.  Number 10 is especially disturbing, but also explains why raising Wyatt has seemed so easy in comparison to his brothers.  

Another favorite is this one by NBC.  It set out originally to prove that being the mother of twins would be really burdensome and perhaps result in a shorter lifespan.  Instead they found that mothers of twins "lived longer, had longer reproductive life spans, needed less time to recover between pregnancies, and had more children overall."  It says that having twins doesn't make us live longer, it says that we are stronger in the first place!  Fascinating!

The last topic I wanted to touch on before signing off for the evening is colorblindness.  I have wondered for a while if the twins were color blind.  As Wyatt's ability to correctly label colors has improved/increased, I have been increasingly concerned that Logan and Jack are colorblind.

Last weekend I gave them two tests.

This one, seen above, for small children of just about any age.

And this one where the child needs to be able to recognize numbers.

On the one pictured above, the twins did not see the squirrel or bear AT ALL.  The first thing Wyatt pointed out was the bear.  On the other test, The Ishihara 38 Plate CVD Test, they both scored as having Moderate red-green deficiency.  From what I can tell thus far, they struggle to differentiate between brown & green, yellow & light green, and purple & blue.

 Immediately when I got the results I posted them to Facebook, seeking the advice of my dad, who is also colorblind.  He said, "It's not the worst thing, but it sucks."  I appreciated his point of view. He has managed to live with it for 50+ years, so my outlook for both Jack & Logan is good, however, I must admit, I am sad for them. I am sad that they can't see the bear or the squirrel. I am sad that at Saturday Social they can't always tell the lady behind the counter which color Slushee they want.  I am sad that when they are trying really hard to follow the directions on a coloring sheet, the green is brown, the purple is blue and the yellow is light green.  

The discussion on Facebook got really interesting really quick thanks to some of my relatives.  

Colorblindness is an X linked chromosome, which means that it is mostly carried by women (who are XX) and displayed by men (who are XY).  My dad is colorblind, and when he donated an X to make me, it had colorblindness on it.  However, since I also had my mom's X to choose from, I am not colorblind.  When Josh and I went to have babies, though, I had two X's to choose from, and the poor twins got my colorblind X, which I inherited from my dad.

What is amazing about this is that my dad got his colorblindness the same way.  My great grandpa Arthur was color blind, and passed his colorblind X to my Grandma Pansy, who then passed it on to my dad.  We are five generations deep in this thing!

I was also fascinated to learn that among Grandma Pansy's three sisters, every single one has a descendent who is colorblind (two have colorblind sons, one has a colorblind grandson), but her two brothers are not color blind and none of their offspring are colorblind.

I am bummed for Logan and Jack that they are colorblind, relieved for Wyatt that he's not, and anxious for our appointment with our eye doctor this summer to confirm my "home testing" and see what, if anything, I can do to help them with this.