How Hemophilia is Inherited

Inherited bleeding disorders like Hemophilia, von Willebrand Disease, and certain platelet disorders are present at birth, determined by the genes that are contained on chromosomes and passed down in the gametes from our mother’s egg and our father’s sperm. We typically have 46 chromosomes that can be arranged into 23 pairs. The first 22 pairs are known as autosomes and the final pair determines our biological sex and is known as the sex chromosomes. 

Chromosomes contain genes, which are the blueprint or instruction of our bodies. Changes within the coding of genes are no longer called mutations and, instead, referred to as “variants” which are further defined by how we believe the variant will impact the function of a given gene.

Refer to the Genetics section for further details on genes and modes of inheritance.

Occurrence of Hemophilia

In 1974, the Centers for Disease Control and Prevention (CDC) estimated that there were about 17,000 men with Hemophilia in the United States. The current estimate is about 30,000 men and 2000 women. About 80% of them have Hemophilia A and 20% have Hemophilia B. Hemophilia occurs in all races and social groups.

Although women less commonly have factor levels <50%, they can carry the gene for it and may have bleeding symptoms even when levels are as high as 60%. The inheritance is further explained in the Genetics section.

Severity Levels

A person can have mild, moderate, or severe Hemophilia. It depends on how much working clotting factor is in the blood. This is called the clotting factor level. It may also be called the level of deficiency or the level of severity. The clotting factor level can be shown as a percentage (%).

The standard factor level is 50-100%. You can have a factor level between 50% and 200% and still be considered normal. In a person with Hemophilia, the clotting factor level is lower than the standard.

Everyone with Hemophilia should know their clotting factor level!

This chart shows the three degrees of severity based on the factor level. It also shows what to expect with each level if there is no treatment. The kinds of bleeding problems faced by someone with Hemophilia usually depend on their clotting factor level. However, some people bleed more, and some people bleed less than their factor levels would suggest and may indicate defective factor function related to their genetic variant (mutation). For example, some people with moderate Hemophilia bleed just as much as those with severe Hemophilia.

Severe Hemophilia	Moderate Hemophilia	Mild  Hemophilia
Less than 1% factor level	>1% to 5% factor level	6% to 50% factor level
Can bleed without injury	Can bleed with slight injury	Can bleed with bad injury, surgery, injections (shots)
May bleed one to two times per week	May bleed one time per month	May only have minor bleeding
Usually has joint bleeding	May have joint bleeding	Less frequent obvious joint bleeding

 

Severity Does Not Change

Whether a person with Hemophilia is mild, moderate, or severe based on factor level does not change. However, the number and type of bleeds you have might change or evolve. This is because how healthy and active you are can affect your bleeds. All family members with Hemophilia usually have the same clotting factor level.

There is one exception to this: people with a type of factor IX deficiency called Hemophilia B Leyden can be moderate to severe at birth but elevate into the normal range after puberty and Hemophilia carriers’ levels can rise during pregnancy. Genetic testing is needed to confirm the rare genetic variant of Hemophilia B Leyden.