Q: (Initially posted on February 14, 2003) FROM MENTEE KUNJAL IN NY
I was wondering, if every gene has 2 alleles, then for skin color, how come some people are albino? Why and how does that happen?

February 17, 2003
A: FROM MENTOR KATHERINE DRENNON IN KY
That is a common misconception. Genes have many different alleles, but each person only carries two. For instance, the gene for blood type has three alleles: A, B, and O. A and B are dominant, meaning they will always be expressed when they are present. O is recessive, so it is only expressed when two copies of this allele are present. Thus people with alleles AA or AO will have A type blood. Persons with BB or BO will have B type blood. Persons with OO will have O type blood. But since A and B are both dominant alleles, when a person has and A and a B allele, both are expressed, giving type AB blood. This is called co-dominance.
With skin pigment, there are many more alleles than just two or three. There are also other factors that play in. Co-dominance is one. With some flowers, for example, both white and red alleles are dominant. When both are present, both are expressed, which is when you get pink flowers. Another complicating factor is Incomplete Dominance. In this case, the flower with both a red and white gene would be spotted red and white. This is all explained in Mendelian genetics, but humans are more complicated than the pea pods and flowers that Mendel studied, and our alleles are a mix of partial dominance and recessiveness that blend into various colors and outcomes. This is why people's skin tones vary so greatly.
On top of the genetic factor, there is also that some people tan in the sun. The radiation from the sun (UV) stimulates the production of melanin, a brown pigment in the skin, which causes people to look tan. In the summer, people spend more time in the sun, thus they are darker skinned than they are in the winter, when it is usually too cold to spend much time in the sun. Tanning beds capitalize on this by using UV lamps to simulate the effect of the sun.
Albinos are a rare case. In their situation, they completely lack any pigment in their skin. They have two copies of the only fully recessive allele in the plethora of skin tone alleles that does not express any pigment. It is also why their eyes applear yellow, a color not normally seen in persons with pigmentation. The yellow is caused by other genes, and is usually masked by the dominant alleles for eye pigment.
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A: FROM MENTOR JOAN LUSK IN RI
In the case of skin color (melanin production), loss of both the
alleles completely eliminates the melanin production and an albino
results. The normal active gene leading to melanin production is
"dominant" and the inactive mutated gene is "recessive", so it takes
two mutations to completely prevent melanin production. You're right
to think that two mutations in the corresponding alleles is less
likely than a single mutation would be - but it can happen. A second
mutation can happen in a person who already has one; or, two parent
who each have one mutated allele can have a child who receives both
mutated allele. In typical notation, where "A" is normal and "a" the
mutant allele, 1/4 of the children of parents who are both Aa will be
AA, 1/2 (=2/4) will be Aa (Aa and aA) and 1/4 will be aa.
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A: FROM MENTEE KATE S. IN NY
Kunjal- I think people are albinos because two recessive genes come up.
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