|MadSci Network: Genetics|
Hello there, Skin colour is a bit different. You see, being white is only the absence of colour so to be white you need to have low expression of melanin. This is more about expression levels of genes. A person is white if they have no melanin so both parents must be white or one needs to have had a white parent (so has one gene which is not for black skin) has been passed on. If one parent is half white and half black and the other is white then the child could either be white or black (thought probably not as black as the parent). If both parents are light brown (meaning they have one gene for melanin and one for without) then the child could be white (if it gets both genes for light), light brown (if it gets one gene for white and one for dark) or black (darker than the parents) if it gets both genes for dark skin. It isn't really that the black is more dominant than the white gene. It is just that being white is like having a blank canvas and any gene which instructs to make dark pigment is going to show up over the white. The most simple way to explain it is an example of goldfish. Goldfish were a kind of dark colour but in some types this pigment was lost. The underlying gold then showed up but in some types this was lost and the underlying white (lack of pigment) showed up. There is a disorder where people with black skin lose pigmentatioin and areas are then white (see Vitiligo society) It is just all about leves of expression rather than dominance. White people have freckles so they have very low expression of melanin except in these areas. They have the ability to produce melanin but generally the genes they have cause it to be expressed at very low levels. I suspect that your student has dark skin because her dad had two genes for high melanin expression and she has one (or two, depending on what her natural mother was like). Hope that helped, Yvonne
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