Traits and Multiple Genes
Please, could you give me an example of how human traits are
controlled by more than one pair of alleles?
Your question is just a bit vague, there are different answers depending on
just what your question is. I will answer it in terms of polygenic traits
also known as additive alleles. When you think of traits such as skin color,
hair color and eye color, or traits where there is a wide range of phenotypes
they are usually under the control of more than one pair of alleles. These
alleles can even be on different chromosomes! Each pair of additive alleles
adds to the phenotype. For instance in the case of skin color, scientists
now believe that 3 genes control skin color. You then get 3 sets from your
mother and 3 from your father for 6 possibilities. If all 6 of the alleles
are for dark skin, you will have the darkest possible skin. If you have 5
dark alleles and one light, you will have very dark skin. If you have all 6
light alleles then you will have the lightest skin possible. Is it possible
to have a child that is light skinned when both parents are dark-skinned?
Well, not if both have all 6 dark alleles, but if they have some light
alleles and the child inherits all of the possible light alleles available,
then yes, the child could have lighter skin than either parent. It is now
believed that eye color is not simply brown being dominant over blue because
how many people do you know that have the same shade of brown or blue eyes?
Eye color must also be polygenic.
Albino trait is one of these. No matter what color your genes have for
skin, hair or eye color, two recessive genes at the location that determine
albinism will result in no color for any of the other traits.
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Update: June 2012