Red-green color blindness in humans is inherited as an X-linked recessive trait. Usually, heterozygous females are not colorblind. However, a doctor identified an unusual case in which a heterozygous female was colorblind in her right eye but had normal color vision in her left eye. If you assume that this phenotype was not due to a new mutation, explain how this might have occurred.

The mentioned characteristics of variability are witnessed because of the phenomenon known as X chromosome inactivation in heterozygous females. As females possess two X chromosomes, they actually should not possess any color blindness.

The phenomenon of X-chromosome inactivation takes place in a random manner at the time of embryo development. During this only one X chromosome is inactivated. If the chromosome comprising the defective gene is inactivated, the individual will possess normal vision.

If the chromosome comprising the usual gene gets inactivated than the individual will be color blind. In the given case of female, X inactivation took place in one eye. The inactivation of normal alleles has taken place and thus she possesses normal vision with one eye and is color blind in the other eye.