My Genetic Replica

I am currently reading this book called "Identical Strangers".  It's about a set of identical twin girls, separated at birth and adopted to different families.  At the age of 35, they find out about each other and meet.  I am loving all the little factoids about twins and the theories of nurture vs. nature.  Being an identical twin, I just kind of thought of my sister as "just" a sibling and never really thought too much about it.  Obviously in college and all my Science & Psychology classes I learned some twin facts, but now as I read this book, I am truly fascinated.  I am realizing I am quite special!  Haha.

First before I delve into my main question to you guys out there...I'd like to tell you a little about my sis and I and how we came to be...

My parents married young, at the ages of 18 & 21.  We were conceived on their honeymoon.  That's their story anyways! =) Well my mother felt as if she was getting way too big...way too fast.  She states the doctors all just kept telling her she was having "one big healthy boy".  In the 70's they did not do routine ultrasounds, so she never had one despite her concerns.  In fact, the doctors told her to stop eating so much and put her on a diet!  And guess how much weight she gained...25 lbs.  With twins!  And they put my petite mother on a diet for being too big.  Anyways, at 36 weeks gestation, my parents were standing in our nursery, admiring the room that was about to be filled by "one big healthy boy", when her water broke.  After a night of labor...I was born at 10:01 am.  So the doctor then tells my mother to bear down to deliver the placenta...and then..."OMG...it's another head!"  My mom said all she could do was scream..."I told you...I told you!"  And just as if out of a movie, my poor dad passed out cold on the floor! So out came my sister at 10:04 am.  We both came into the world weighing and measuring the exact same. 5 lbs. 5 oz. and 17 inches long.  Now since we obviously were not "one big healthy boy", we were two tiny, little girls...we had no names.  We were "Twin A" and "Twin B" for 3 days.  Finally...we became Jessica Kelly and Jennifer Leigh.

The interesting thing about Jen and I is that we are also what they call "mirror- image" twins, who account for 25% of identical twins.  Also known as reversed asymmetrical twins, mirror-image twins occur when the fertilized egg splits later than 7 days after conception.  In most cases, the reverse asymmetry presents itself in small, obvious ways.  Such as I am left handed and Jen is right handed.  However, some identical twins may also show reversals in dental features, fingerprints and facial features.  In the most dramatic form, the internal organs of one twin are reversed, with the heart being on the right side of the body. 

Identical (monozygotic) twins are clones of nature, each descended from a symmetrical splitting of a single fertilized egg into cells that contain the identical sequence of billions of even tinier DNA molecules.  Identical twins share 100% of their genes. 

That being said, my question, and the question of most people I know is...if Jen and I are exact genetic replicas of each other, why is it that I suffer from this debilitating neurological disease and she doesn't???

Now to somewhat anwer my own question...I did some research.  Identical twins are very likely to share Migraine Disease indicating a strong genetic component, however, the disease is clearly not caused by a single genetic mutation; rather a person becomes susceptible by inheriting mutations in a number of genes, each probably making a small contribution.  Nongenetic components operate as well, because even identical twins are "discordant" for the disorder; sometimes one twin will suffer from Migraine, and the other will not.

Interesting.

I am extremely thankful that my best friend, my "clone", does not suffer from the hell of this disease...but...WHY am I the one who got it?!?!?