Question

Describe how a child with one blue eye and one brown eye can be explained through X inactivation. Include the mechanisms involved.

Answer 1

X chromosome inactivation is the process of inactivation of one of the chromosomes in human male in order to compensate the dosage of protein by X chromosomes in females and in males.

Human females have two chromosomes and males have only one chromosome. Because of this, the amount of proteins formed by X chromosome in females would have been double the amount of proteins by X chromosome in male. Therefore to equate the amount of proteins in males and females, the phenomena of X chromosome inactivation occurs.

One of the X chromosomes in human females is inactivated so that it does not undergo transcription and translation thereby, equating the amount of protein is formed by this chromosome in both males and females. The mechanism of an activation of X chromosome is called as lyonization.

The choice of the chromosome which is to be inactivated is completely random. In some of cells, one X chromosome inactivated and in some others as, the other chromosome maybe in activated. But the descendants of that cell will have the same X chromosome inactivated. This inactivated X chromosome appears as darkly stained barr body inside the nucleus of the cell.

So, human beings having one blue and one brown eye is because of the random inactivation of X chromosome in both the eyes. One of the X chromosome contains an allele for brown eyes and the other contains allele for blue eyes. In one of the eyes, X chromosome carrying blue allele is inactivated resulting in brown eye and in other eye, Xchromosome carrying brown allele is inactivated resulting in blue eye.

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