People ask me and my twin sister all the time, "are you two identical?" I'm so used to this question that I never think twice about it. I didn't realize the biological significance of being an identical twin until I took AP Biology. After learning that identical twins have the same exact DNA, I began to think, why are my sister and I so different? I mean, we're pretty similar, don't get me wrong, but we do have some fundamental differences. I have ADHD, and Annabelle (my twin sister), doesn't. I am half an inch taller than her. We both have different interests, I want to go into the healthcare industry and she wants to go into the business industry. She has a slightly crooked nose (called the Svenson nose) and I have a straight nose (called the Kemp nose- my mom's last name). We aren't even interested in the same type of colleges. There are many more differences, but the most important is that I prefer dogs and Annabelle prefers cats. Why aren't me and Annabelle the same?
Before I get into phenotypic plasticity, I should probably explain how twins even exist. You've probably heard of fraternal twins and identical twins. The difference is fraternal twins are the product of two different eggs being fertilized by sperm, and this results in around a 50% difference in genome, which is the average difference in genes of normal (non-twin) siblings. Identical twins are when one egg was fertilized by the sperm and then that fertilized egg, also called a zygote, splits into two zygotes, that contain identical genomes.
Difference between fraternal and identical twins
Some people might believe that until the moment they are born, identical twins are exactly the same. But this is not true, I was born at 5.5 lbs and Annabelle was born at 5 lbs. Differences in identical twins can arise right after fertilization and the splitting of the egg. I was born at 5.5 lbs because I got more nutrients in my mother's womb and was able to grow a little bit heavier then my womb-partner. Studies show that a heavier birth weight can directly affect the rate at which infants grow all the way up to toddlers at age 2.5, but after that there isn't much of a correlation between birth weight and growing rate. This might explain why I am half an inch taller than my sister, I was heavier, I had more available nutrients, so I grew at a faster rate, resulting in being .5 of an inch taller at the end of puberty and the overall growth period. This effect is called phenotypic plasticity, which is when your environment and its conditions shape your phenotype. Another hypothesis for our different heights is Annabelle used to be pigeon toed when we were growing up- that's just how she learned to walk- and she still is ever so slightly, which could affect her perceived height. But again, this is phenotypic plasticity! Not to say that we have no difference in our genes, twins' genes are 99.9% the same, and this is mostly due to replication errors after fertilization. So maybe that 0.1% difference gave me an extra 0.5 inch, and straight nose and Annabelle a crooked nose, or maybe my parents dropped her on the head when she was a baby that gave her nose a little divot.
I was diagnosed with moderate ADHD with an emphasis on compulsiveness when I was 15. Both of my parents and my little brother also have ADHD, so it wasn't really a surprise when I was diagnosed, but my sister never showed any symptoms that suggested she should get tested. And because the testing process in long and expensive, my parents never bothered to test her. With this information, I can make a lot of assumptions. Maybe both Annabelle and I have mild ADHD, but Tik Tok (I should note that Annabelle never used Tik Tok) shortened my attention span and somehow caused my ADHD to become moderate. I don't really know, but all I can assume is the real reason why Annabelle likes cats(ugh) and I prefer dogs, is because our environment influenced us enough to differ in thought, as well as why she has a divot in her nose and I don't, why I am 0.5 of an inch taller than her and our differences in preference of college and post-college plans.
I guess the point of this article is to say that I don't really know exactly why Annabelle and I have these differences, all I can say is it was probably our environment that caused these differences in our appearance, preference and way of thought. Now it should be easy for you to answer, why do I have more freckles than Annabelle?
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