Expert: John Locke - 8/10/2009

Question
QUESTION: Hello! I have a quick question about freckles. Is it possible for one fraternal twin to have her freckles fade away more than the other twin by the time they're young adults?

ANSWER: Thanks for using AllExperts. The answer is undoubtedly yes. As you may be aware, fraternal twins are produced by two separate sperm cells that fertilize two separate eggs. These two embryos then develop simultaneously but independently of each other. This means that the resulting twins will have very similar but not identical DNA--similar because they both have the same parents, but nonidentical because any two cells always contain slight variations in their DNA sequence.

These variations can certainly include the melanocortin 1 receptor gene (MC1R), which controls the expression of melanin (the dominant skin pigment); melanin expression is what controls the development of freckles. Variations in the MC1R gene will control how much melanin is made in response to sunlight exposure and how long that melanin resides in the skin before it is degraded. Hence, you can have fraternal twins in which one of them produces more melanin at an earlier age in response to sunlight, while the other produces less melanin and/or has that pigmentation fade away more readily as they age. Whether any two twins will wind up looking like that, however, is an entirely different question, and it's not one that I'm sure we are even able to answer at this point.

---------- FOLLOW-UP ----------

QUESTION: Thanks so much! Your answer definitely helped. Do you know if it's possible for identical twins to ddevelop differently when it comes to freckles? I know they'd have the same DNA but I don't know if they're freckles would be the same too.

Answer
Thanks for using AllExperts. Yes, it would not be unexpected for identical twins to develop different freckle patterns, for the simple reason that even identical twins will not be exposed to identical amounts of UV light. Ultraviolet light--which we receive mostly from the sun, but also can be exposed to from other sources--is responsible for stimulating the melanocytes in our skin to produce the pigment melanin. If you receive more UV light, you will produce more melanin, and your skin will tan as a result (it may tan evenly or in a freckled pattern; this is controlled by the MCR gene described in the first answer). People simply receive different amounts of UV light, no matter how closely related they are or if they share a common environment, meaning that their freckles will develop differently. They may be quite similar, but the pattern is certain to be unique.