Another area in which scientists have advanced the study of freckles is through research of the disease xeroderma pigmentosum. Xeroderma pigmentosum is an extreme sensitivity to sunlight -- those who have it are at great risk of developing skin cancer when exposed to the sun [source: MedicineNet]. In people with this disease, the sun's UV rays damage the skin's DNA so that it can't be repaired. Xeroderma pigmentosum patients are usually heavily freckled and have dry skin, and they need to protect themselves from the sun at all times.
What Causes Freckles?
So what exactly causes the appearance of freckles? Freckles appear when the melanin, the dark pigment in skin, isn't distributed evenly -- certain areas of the skin have more than others, and that's why they appear a little darker.
But what does sunlight have to do with the issue? If you're prone to freckles, you're more likely to have them the more you expose yourself to the sun. If you spend outside without sun protection, you'll probably notice the spots on your skin getting darker and multiplying. This happens when ultraviolet (UV) rays from the sun hit your skin. The outermost layer of your skin, the epidermis, thickens a little bit more. This in turn causes melanocytes, the pigment-producing cells in your skin, to produce more melanin, which can darken the freckles on your skin.
Sunlight isn't the only factor that influences freckles. If you have freckles, there's a good chance your parents and siblings have them, too. A combination of both sun exposure and genetics determine how those darker specks will pop up. People with light skin and light or red hair are more likely to have freckles than those with darker skin. Freckles usually appear on children, and they can start to show on people as young as one or two years old. Freckles develop in random places on the body, usually around areas that get the most sun exposure, such as the face and the arms.
Interestingly, research done among sets of twins shows that identical twins have nearly the same number of freckles. Freckles also tend up to show up in similar locations in twins, which gives evidence that freckles pass genetically [source: Alai].
Although freckles are normal, not everyone feels comfortable with them. If you've ever wondered if it's possible to get rid of your freckles, read the next page.