365 Days of Twinipedia - Day 49

Day Forty-nine of Twinipedia: 

Fact #49

Yes, it is true that identical twins come from the same egg that then splits to form two eggs but we also know that identical twins, although called identical, do tend to have some physical and personality differences that definitely prove that not all things about them are truly 100% identical.
We found a great explanation of why this is the case at ask.com.  A molecular biologist explained in simple terms that "The reason that "identical" twins can still have some significant physical differences is because of which genes get expressed. 100% of your DNA is not expressed. Far from it actually. Humans have 46 chromosomes, however, they occur in homologous pairs, which basically means there are only 23 chromosomes that carry all our different traits, and we have copies of each one of those 23, one from mom and one from dad. So twins can express slightly different parts from each pair. Especially identical twin girls who have 2 X chromosomes, one of them gets deactivated completely."