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Chapter 12 WHY DIFFERENT COLORS OF EYE AND HAIR? "I knew she was not her daughter, for her parents had blue eyes and she has brown eyes." That is what the priest-detective Father Dowling said to the resourceful Sister Steve in a recent episode of the television series The Father Dowling Mysteries. Though Father Dowling's deduction led him to find the murderer, it was not scientifically sound. The inheritance of eye color is far more complex than is generally assumed. However, we cannot blame Father Dowling or his script writers for repeating common folk wisdom, which has been taught for years, even in beginning courses on genetics. Eye-color inheritance was considered to be a classic example of a human trait determined by one pair of genes; one gene for the brown color was assumed to be dominant over the gene for blue. Not only was it taught that way, but students were given problems to solve which were based on a simple, but incorrect hypothesis. The hypothesis is contradicted by the fact that there are many documented cases of children with brown eyes whose biological parents have blue eyes. It is indeed an oversimplification leading to misinterpretations of family histories, such as the one that concerned Father Dowling. Sometimes the suspicions may have been justified. But in the great majority of such cases, doubts of parenthood were groundless. That the simple explanation of eye color inheritance is incorrect can be demonstrated very simply by realizing that eye color comes not only in blue or brown but in many other intermediate shades-hazel, green, and gray as well. The color of the eye is due to a number of factors, but variation in them appears to be largely due to the amount and distribution of melanin, the same pigment that gives skin its color. When we talk about variations in eye color, we are restricting our discussion to one part of the eye, the iris. The iris is a round, contractile membrane which controls the amount of light reaching the retina in the back of the eye. Its color is the result of the production of melanin in its tissue, in particular, in the front of the iris. Except for albinos, who have no pigment in their eyes, we all have some. In those of us who have blue eyes, no melanin can be seen in the front of the iris, but it is present behind it; in 97 98 Alain Corcos other words, there is no "blue" pigment. The eyes appear blue because of the same optical effect that causes the sky to appear blue on a clear day. Dust particles in the atmosphere scatter light with short wavelengths, such as blue light, but not light with longer wavelengths, such as red. In blue eyes, the minute protein particles in the iris play the same role as dust particles in the atmosphere. As light traverses the relatively melanin-free front layers of the iris, they scatter the short, blue wavelengths to the surface. In this regard, poets are entirely correct when they liken blue eyes to the blue of the sky. If the front of the iris has a little pigment, the color of the eye will appear blue flecked with brown. If the pigmentation is thinly dispersed, it might form a yellow filter which, with the blue optical effect, would give a greenish tint. A little more pigment combined with the blue optical effect would give a combined shade that is called hazel. With progressively larger amounts of pigment in the front of the iris, hazel grades into dark brown or even very dark brown (black). Obviously, eye color is complex. Not only does it depend on the amount of melanin present but also on the color and brightness of the light striking the eye. In many cases, whether we say a person is blue eyed or brown-eyed depends on how closely we examine his or her eyes and under what conditions of illumination. Eye color also varies with age and sex of the individual. Generally eyes darken, from birth to maturity, and thereafter begin to lighten with aging. Women tend to have darker eyes than men. Most human populations are dark-eyed. Although, as a result of mutations, blue or gray eyes do occasionally occur in every population, light eyes are most common among the peoples of Europe and their descendants. This has suggested to anthropologists that blue eyes conferred some sort of advantage...

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