X-Linked Recessive Diseases

Because the disease is inherited by sex chromosomes ( X chromosome), except for rare cases, diseases are determined only in males. Males carry just one X chromosome in their genome, that is why it is not possible for them to be a carrier. Unlike sons of afflicted father, all daughters become carrier, however sons of that father remain unaffected and they even never become carrier, as well.

If any one of the two X chromosomes in females is affected, they do not show the disease but become carrier due to having two X chromosomes in their genetic makeup. The probability of the transmission of the disease to the sons of carrier women is 50 % for every single pregnancy, whereas it is half of that for the daughters.

X-linked recessive diseases can be observed very rarely in females. X inactivation in carrier women occurs randomly due to dosage compensasion that results in inactivation of mutant gene bearing X chromosome and inactivation of wild type X chromosome with equal probabilities.  But in some cases, due to inactivation of X chromosome possessing normal gene, the disease can be observed in phenotype.

In addition to all these, in order to protect the genetic structure in X autosomal translocation, non random translocation of X chromosome is saved and the other X chromosome becomes inactivated. If individual carries the X chromosome on which translocation did happen, this situation may appear in phenotype.

"The essence of evolutionary biology is that, just as our genetic material guides us, we also have the potential to guide it."

Richard Dawkins