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Name: Jo P.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
What is it called when a person has two different colored eyes? (one blue and one hazel)


Replies:
This condition is usually the result of what is termed a genetic mosaic. A mosaic is something which occurs in females for traits which are controlled by genes on the X chromosome. In organisms with the double XX genotype female, one of the X chromosomes gets turned off early in embryonic development, apparently at random. So half of the cells have an active paternal X and half have an active maternal X. Although there are genes on other chromosomes which are related to eye color, there may also be a gene on the X chromosome which has an effect.

Because the process of turning off one of the X chromosomes occurs when the embryo is still very small, and because during the further development all of the cells derived from one cell usually stay close together, we should expect that it is usually true that all of the cells in the irises, which give color to the eyes, have the same active X chromosome. However, it may sometimes happen that some of the cells in one of the irises have the other X activated. This can cause these cells to be a different color from the others.

The condition of genetic mosaic can happen in males, however, the mechanism is not clearly understood and the occurrence is even more infrequent than the occasional mosaic female.

Paraphrased and submitted by Cherie Breffeilh


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