Hemophilia A and B are transmitted in a sex-related way, as the defective genes are located on the X chromosome. Women have two X chromosomes and practically never have ailments. Although they inherited a hemophilic chromosome from a parent, they are protected by the normal chromosome, which manages to produce a sufficient factor VIII factor for normal hemostasis. In women, hemophilia is very rare and can occur in the case of daughters born to a haemophilic father and a carrier mother, or in other exceptional conditions.

Males have only one Y chromosome and, when they inherit a hemophilic X chromosome from the carrier mother, they may have a more or less serious clinical disease.

Hemophilic father and healthy mother:

At each conception, the males will always be healthy (they receive the Y chromosome from the father and a normal X chromosome from the mother); daughters will always be carriers (always receive a normal X chromosome from the mother and the other X chromosome, haemophilic, from the father.

Healthy father and carrier mother:

Male children are 50% likely to be haemophilic (they receive the Y chromosome from their father and the normal X chromosome or the one affected by the mother); daughters will have a 50% chance of being carriers, since they always receive the father’s X chromosome and the normal or hemophilic chromosome from the mother. Even in subsequent pregnancies the risk does not change, since the possibility of receiving an abnormal chromosome from the mother is always 50%, regardless of whether healthy or carrier daughters were born from previous pregnancies.

Hemophilic father and carrier mother:

In this case, women would also be ill, but it is a truly exceptional case.

The four modes of gene transmission

There are four channels for transmitting genetic information from generation to generation, which Portugália Genética will look into. Which are? The Y chromosome, X, mitochondrial DNA and the autosomal pathway. The Y chromosome information only exists in the male individuals who inherited it from the father, that is, it is only transmitted from the father to the child. The X chromosome can be transmitted by the mother and, eventually, the father to daughters, with women having two XX and men an XY. 

There is also the information contained in mitochondrial DNA, one of the components of our cells that only mothers transmit to their children. Finally, there is the autosomal pathway, in which father and mother contribute equally, through genes from chromosomes other than sexual ones. Through comparisons in genetic material, it is possible to know how populations evolved, their migrations, separations and unions. The fact that, for example, the Y chromosome reflects the history of men and the mitochondrial DNA of women, being independent from each other, is a precious way to investigate our origins and to understand our evolution. ACF