Lyonization

I like to make the point that identical twins are not really "identical" -- hopefully most people realize that every individual has traits that are not determined by genetics.  But did you know that some identical twins have differences that are determined by genetics?

According to the McGraw Hill Encyclopedia of Science & Technology,

For example, the random “shutting down” of one X chromosome in every female cell soon after conception (a process called lyonization) can cause monozygotic female twins to differ in X-linked traits, such as color blindness.