Question

Describe the molecular mechanism for the expression of the two alternate cellular proteins in different tissues of a heterozygote human female. Why do such alternate cellular phenotypes not appear in the female homozygote?

This is the question that was asked by my professor.

Answer 1

Random X-chromosome inactivation is the cause for such a condition in a heterozygote female. In this heterozygous condition one allele of a gene is dominant and encodes for a particular protein, while the recessive copy of the allele encodes for another protein. So if this allele is present in the X-chromosome of female wherein randomly one chromosome undergoes inactivation during embryonic development, it might be so that in few cells the chromosome containing the dominant allele is active and expresses one particular protein while the other X-chromosome is inactive and expresses no protein, while in other cells the other X-chromosome containing the recessive allele is active and expresses another protein which it encodes.

This is however not the case for homozygous females where both the alleles will express the same protein, thus random X-chromosome inactivation does not have an impact on the differential protein expression of those females.