Why Is Hemophilia More Common in Males?

Hemophilia is more common in males due to the fact that they have only one copy of the gene on the X chromosome. If any changes occur in that gene, females can compensate for the defect with an additional copy of the gene, but males cannot. In fact, current estimates suggest males are diagnosed with hemophilia at nearly 14 times the rate of females.

It is also the reason why more males get other X-linked disorders like color blindness. X-linked disorders occur due to changes in the gene on the X chromosome. 

Key Takeaways

• Hemophilia is more common in males because they have only one X chromosome.
• Though some providers consider it a “male-only” disease, hemophilia can affect females as well.
• Many health experts think underrecognition and underdiagnosis in females could obscure the real-world scenario. Females often get delayed diagnoses.
• About 1 in 3 hemophilia diagnoses occurs in someone with no family history of the condition — it is not caused by inbreeding, but by a spontaneous gene mutation. 

A Quick Overview of Hemophilia, Causes, Types, and Occurrence

Hemophilia is a genetic disorder in which the blood does not clot as it should. As a result, it can lead to excessive bruising and nosebleeds. Severe hemophilia can cause spontaneous bleeding (bleeding with no obvious cause). 
Hemophilia occurs due to changes in the genes that instruct the body to make blood-clotting proteins called clotting factors. The common types are:

• Hemophilia A: Also known as classic hemophilia, it occurs due to having little or no clotting factor VIII. 
• Hemophilia B: Also known as Christmas disease, it occurs due to having little or no clotting factor IX. 

The more common type — Hemophilia A — affects 1  in 5,617 male live births. According to reports from the Centers for Disease Control and Prevention (CDC), nearly 400 babies are born with this disorder each year in the U.S. Though the exact numbers remain unknown, recent data suggest about 33,000 American males have this condition [1]. About one in three cases occurs in babies with no known family history of hemophilia, the result of a new (de novo) gene mutation.

Why Hemophilia Is More Common in Males: Understanding the Genetics and Inheritance

The fundamental reason why hemophilia is more common in males is that they have only one X chromosome. 

The X chromosome contains the genes for clotting factors VIII and IX. Changes in these genes lead to little or no clotting factor production. Consequently, the blood does not clot properly. 

Because males have only one chromosome, one altered copy of the gene is enough to cause symptoms. On the other hand, females have two copies of these genes, and changes in one copy typically do not cause the condition. 

Here’s how the inheritance breaks down:

• Males (XY): They inherit one X chromosome from their mother and one Y chromosome from their father. A single altered copy of the gene on that X chromosome is enough to cause hemophilia.
• Females (XX): They inherit two X chromosomes, one from each parent. Because the healthy copy usually offsets the altered one, most female carriers do not develop hemophilia themselves.
• Carriers: A female with one altered X chromosome is called a carrier. She has about a 50% chance of passing the altered gene to each child, though the outcome differs depending on the child’s sex.

Moreover, males get an earlier diagnosis than females. Early diagnosis in males is a result of the myth that females cannot get hemophilia. Many females may even ignore the signs of mild hemophilia, such as heavy bleeding. 

According to a 2020 study, females with moderate symptoms receive a diagnosis about 6 months later than males. In severe cases, females face delays of approximately 40 months [2].

Hemophilia is more common in males born to a female carrying the affected gene and an unaffected male. In such cases, there is a 1 in 4 chance of having a baby boy with the condition. On the other hand, sons of a father with hemophilia cannot inherit the condition from him, because fathers pass down a Y chromosome — not the X chromosome that carries the mutation — to every son. Every daughter of a father with hemophilia, however, will inherit his affected X chromosome and become a carrier.

The risk of hemophilia is the same for males and females if both parents have the faulty gene. In such cases, there is a 1 in 4 chance that a baby girl will be a carrier.

In this same scenario, there is also a 1 in 4 chance that a baby girl will be born with hemophilia herself, not just as a carrier.

Why Is Hemophilia Rare in Females?

For females to get hemophilia, both X chromosomes have to be altered. Sometimes, hemophilia may result from having one affected X chromosome and another non-functioning chromosome.  

A female with one altered gene is a “carrier.” She can pass the affected gene on to her children. Female carriers do not usually experience symptoms of hemophilia, but some do. — carriers with lower clotting factor levels can have heavy menstrual bleeding, easy bruising, or joint bleeds, and in some cases their factor levels are low enough for them to be diagnosed with hemophilia themselves.

The prevailing misbelief that hemophilia is a “male-only” disease could be one of the reasons for fewer diagnoses in females.

Frequently Asked Questions

Is hemophilia more common in males or females?

Yes. Hemophilia almost always affects males because it’s passed down on the X chromosome, and males have only one copy of that chromosome. Females typically need two altered copies to develop the condition themselves, which is far less common.

Is hemophilia caused by inbreeding?

No. Hemophilia is caused by a gene mutation on the X chromosome, not by family relation between parents. About one in three cases occurs in babies with no family history of the disorder at all, caused by a spontaneous new mutation.

Can a father with hemophilia pass it to his son?

No. Fathers pass a Y chromosome to their sons, not the X chromosome that carries the hemophilia gene. Every daughter of a father with hemophilia, however, inherits his affected X chromosome and becomes a carrier.

Why are X-linked conditions like color blindness also more common in males?

Because color blindness is also caused by a gene mutation on the X chromosome. Just like hemophilia, males need only one altered copy to be affected, while females usually need two.