Question

3. In humans Red-Green colorblindness is due to an X-linked recessive allele. A woman was diagnosed with colorblindness in her right eye, but nommal vision in her left eye. It is known that her father was colorblind and that her mother had normal vision. Explain how the phenomena of dosage compensation could explain this woman's condition.

Answer 1

As mentioned in the question, Red-Green color blindness is a X linked recessive disorder. The woman who was colorblind in her right eye, but not her left one, must be heterozygote for the trait, as different alleles are expressed in different eyes. As her father was color blind as well, she must have inherited the recessive allele from her father and the dominant allele from her normal visioned mother.

But, considering the fact that color blindness is a recessive trait, the woman who was heterozygote for the trait must have produced fully functioning protein from the dominant allele, thereby masking any deleterious effect of the recessive allele. But here it is seen that she has alternative phenotypes in each of her eye, as if only one allele is expressed in each case.

This condition can be explained by the phenomenon of dosage compensation. Dosage expression occurs in organisms which have one biological sex as homogametic and the other as heterogametic. In such cases, one sex has twice the amount of gene from one chromosome than the other. To equalize gene expression across sexes, in the homogametic individuals, expression of one of the chromosome is randomly silenced.

For example, humans follow XX/XY sex differentiation, where females are XX and males are XY. Here, females would have twice the amount of X chromosomes as males. Therefore, in order to equalize the amount of gene actually expressed, in the diploid somatic cells of females, only one of the two present X chromosomes is actually expressed. One of the X chromosome undergoes extreme condensation to form a darkly stained structure called the Barr body. Thus, in diploid somatic cells of females, one of the X chromosome is randomly inactivated by formation of Barr body to ensure dosage compensation.

Thus, in the woman with color blindness in only one eye can explained by stating that, in her left eye the X chromosome she inherited from her mother with the dominant allele for the trait is expressed and the X chromosome from her father with the recessive trait is inactivated to form Barr body. Hence she has normal vision in her left eye.

However, in her right eye, the X chromosome inherited from her mother is inactivated and the one from her father with the recessive allele is expressed. Thus, the woman is color blind in her right eye.

As her father, only had one X chromosome with the recessive allele it was expressed in both eyes and hence he was color blind in both of his eyes.