Hemophilia is sex-linked. Both major forms of the disorder are caused by mutations in genes located on the X chromosome, which is why the condition overwhelmingly affects males. But the full picture is more interesting than the textbook summary suggests: women can develop hemophilia too, new mutations arise constantly from surprising origins, and the severity of the disease depends on far more than the single gene involved.
Why the X Chromosome Matters
Hemophilia A results from mutations in the gene for clotting factor VIII, and hemophilia B results from mutations in the gene for clotting factor IX. Both of these genes sit on the X chromosome.1Europe PMC / Haematologica. Hemophilia A and B: molecular and clinical similarities and differences Males have one X chromosome and one Y. If a male inherits an X chromosome carrying a hemophilia-causing mutation, he has no second X to compensate, so he will have the disorder. Females have two X chromosomes, which means they typically have a backup copy of the clotting factor gene on their other X. This is the core reason hemophilia shows up far more often in boys and men.
The pattern was recognized centuries before anyone understood chromosomes. Families noticed that bleeding disorders passed from mothers to sons while daughters seemed unaffected. The genetics eventually confirmed what those observations suggested: a mother who carries one mutated copy has a fifty-fifty chance of passing it to each son, who would then have hemophilia. Daughters who inherit the mutation from their mother usually have enough clotting factor from their other X chromosome to avoid serious problems. Although hemophilia was long believed to affect only men, it is now universally acknowledged that women and girls can also be affected.2PubMed. The history of women and hemophilia: a narrative review of evolving beliefs and testing practices
Hemophilia A and Hemophilia B
Although both types share the same X-linked inheritance pattern and produce similar symptoms, they involve different genes and differ in their molecular details. Hemophilia A, sometimes called “classic hemophilia,” is the more common form, accounting for roughly four out of five cases. It centers on factor VIII. Hemophilia B, also known as Christmas disease, involves factor IX. Both cause bleeding into joints and muscles, and both range from mild to severe depending on how much functional clotting factor a person produces.
The types differ in their mutation landscapes. The most frequent cause of severe hemophilia A is a particular rearrangement called an intron 22 inversion, which disrupts the factor VIII gene. Severe hemophilia B, by contrast, is more commonly caused by missense mutations, which are single-letter changes in the genetic code of the factor IX gene.1Europe PMC / Haematologica. Hemophilia A and B: molecular and clinical similarities and differences These molecular differences matter for treatment decisions and for predicting the risk of complications like inhibitor development, where the immune system attacks the replacement clotting factor given during treatment.
How Women and Girls Develop Hemophilia
The idea that females are merely carriers and never truly affected is one of the most persistent misconceptions about hemophilia. In reality, women and girls can have clinically significant hemophilia through several genetic routes. A review of genetic causes classified affected females into distinct categories: those who are homozygous (carrying two identical hemophilia-causing mutations), compound heterozygous (two different mutations, one on each X), hemizygous (one mutation with no normal copy, as in Turner syndrome where a female has only one X), or heterozygous (one mutation and one normal copy).3PubMed Central. Genetic causes of haemophilia in women and girls Women and girls who are homozygous, compound heterozygous, or hemizygous clearly have hemophilia because they lack a normal copy of the gene entirely.
Heterozygous women, who carry one mutated copy and one normal copy, can still develop symptoms through a phenomenon called skewed X-chromosome inactivation. Early in embryonic development, each cell randomly shuts down one of its two X chromosomes. Usually this is roughly balanced, so about half of a carrier’s cells use the normal X and half use the mutated one, producing enough clotting factor overall. But sometimes the inactivation is lopsided. In one documented case, a woman had 97 percent inactivation of one X chromosome and presented with factor VIII activity of just 2.3 percent, putting her squarely in the moderate-to-severe range.4Journal of Thrombosis and Haemostasis. Molecular mechanisms underlying hemophilia A phenotype in seven females A study of severe and moderate hemophilia in U.S. females confirmed that mutations in the factor VIII or IX gene, combined with extremely skewed X-inactivation patterns, were the primary drivers of severity in affected women.5PubMed. Severe and moderate haemophilia A and B in US females
A large survey of hemophilia centers identified 28 females with very low levels of factor VIII or IX activity, including 10 with hemophilia A and 6 with hemophilia B. The remaining 12 had severe von Willebrand’s disease, a related but distinct bleeding disorder. Those 16 women with isolated factor VIII or IX deficiency illustrated the range of genetic explanations for hemophilia in females.6The American Journal of Medicine. Severe factor VIII and factor IX deficiency in females These cases are rare, but they are real, and recognizing them matters for getting the right diagnosis and treatment.
Carriers Bleed More Than Previously Thought
Even women who do not meet the threshold for a hemophilia diagnosis often experience more bleeding than women without the mutation. A study of hemophilia carriers found that clotting factor levels from 0.60 down to 0.05 IU/mL were progressively associated with prolonged bleeding from small wounds and after procedures like tooth extraction, tonsillectomy, and surgery. The findings showed that carriers bleed more than other women, especially after medical interventions, and that even mildly reduced clotting factor levels between 0.41 and 0.60 IU/mL were associated with bleeding problems.7PubMed. Bleeding in carriers of hemophilia
This is clinically significant because many carriers have historically been told they are unaffected and require no special precautions. In practice, a woman planning surgery, dental work, or childbirth should know her factor levels and discuss them with her care team. The old binary of “affected male” versus “unaffected female carrier” understates the range of bleeding risk that X-linked inheritance actually produces.
Where New Mutations Come From
About a third of hemophilia cases arise from new, or de novo, mutations, meaning the child has hemophilia even though neither parent carried the mutation in their own blood cells.8Egyptian Journal of Medical Human Genetics. Molecular genetics of hemophilia A: Clinical perspectives This has important implications for families with no prior history of the disorder. A boy diagnosed with hemophilia does not necessarily have a carrier mother, and genetic counseling needs to account for that possibility.
Interestingly, the type of mutation determines where it is more likely to originate. A detailed study of 147 patients with sporadic hemophilia A found that point mutations showed a five-to-tenfold higher mutation rate in male germ cells (sperm) compared to female germ cells (eggs), and inversions showed an even greater skew toward male origin. Deletions, on the other hand, arose more than five times as often in female germ cells.9PubMed Central. Characterization of the factor VIII defect in 147 patients with sporadic hemophilia A: family studies indicate a mutation type-dependent sex ratio of mutation frequencies This means that the overall sex ratio of new mutations for a given X-linked disorder depends heavily on which types of mutations are most common in that gene. For hemophilia A, where inversions and point mutations dominate, new mutations originate disproportionately during sperm production.
One question researchers have explored is whether the modern chemical environment has changed mutation rates. A comparison of disease-causing mutations in the factor IX gene found no significant difference between the pattern of mutations arising in the past 150 years and the pattern inferred from ancient mutations. This suggests that endogenous biological processes, rather than man-made chemicals, dominate the mutation process in human germ cells.10PubMed. Mutations in the factor IX gene (F9) during the past 150 years have relative rates similar to ancient mutations
Mosaicism Complicates Genetic Counseling
When genetic testing of a hemophilia patient’s mother comes back negative for the mutation, the usual assumption is that the mutation arose fresh in the child. But this assumption can be wrong. In roughly 30 percent of apparently non-carrier mothers of boys with sporadic hemophilia, the mutation can actually be detected at low levels as somatic or gonosomal mosaicism.11PubMed. Genetic mosaicism in haemophilia: A practical review to help evaluate the risk of transmitting the disease Mosaicism means the mutation is present in some of the mother’s cells but not others, so standard genetic tests may miss it.
A study that used sensitive techniques to look for low-level mosaicism in 61 families with sporadic severe hemophilia A found mosaicism in 13 percent of families overall. The rate was much higher in certain mutation types: among families with point mutations, a quarter showed mosaicism, and among families with a specific subtype of point mutation at CpG sites, half were mosaic. No mosaicism was detected in families with deletions or intron 22 inversions.12PubMed Central. Somatic mosaicism in hemophilia A: a fairly common event The practical implication is that a mother who tests negative on a standard carrier test still has a meaningful chance of carrying the mutation in some of her egg cells and could pass it to another child. Genetic counselors now factor mosaicism into risk calculations for these families.
Why Severity Varies Even with the Same Mutation
Two people with the exact same mutation in the factor VIII or IX gene can bleed very differently. The genotype is the main determinant of how much clotting factor the body produces, but clinical severity is shaped by additional factors. Research has found that patients with so-called non-null mutations, which allow some residual protein to be made, were more likely to have a milder bleeding pattern than patients with null mutations, which eliminate the protein entirely.13PubMed. Severe hemophilia with mild bleeding phenotype: molecular characterization and global coagulation profile
Beyond the hemophilia gene itself, mutations in other parts of the clotting system can push symptoms in either direction. One well-studied example involves factor V Leiden, a common genetic variant that makes blood clot more readily. When hemophilia A patients also carry this variant, it partially offsets their clotting deficiency. Research has suggested that coinheritance of this variant may be an important determinant of clinical phenotype in hemophilia A.14Blood. Moderation of hemophilia A phenotype by the factor V R506Q mutation More broadly, variations in other coagulation proteins and in the body’s clot-dissolving (fibrinolytic) system can all influence how severely a person with hemophilia actually bleeds.15PubMed. The phenotypic heterogeneity of severe hemophilia This is why two brothers with the same mutation can have different experiences, and why genotype alone does not tell the whole clinical story.
Acquired Hemophilia Has Nothing to Do with Inheritance
There is a completely different condition called acquired hemophilia A that is not inherited and is not sex-linked at all. It occurs when a person’s immune system spontaneously produces antibodies that attack factor VIII. This can happen in anyone, at any age, regardless of sex or family history.16PubMed Central. Acquired hemophilia A as a disease of the elderly: A comprehensive review of epidemiology, pathogenesis, and novel therapy It is most common in older adults, and about half of cases have no identifiable trigger. The other half may be associated with pregnancy, autoimmune diseases, cancer, infections, or certain medications.17PubMed. Acquired hemophilia a: diagnosis, aetiology, clinical spectrum and treatment options
Acquired hemophilia looks different from the congenital form. Instead of the joint bleeds that characterize congenital hemophilia, acquired hemophilia typically causes bleeding into the skin, muscles, and soft tissues. It can be life-threatening and requires urgent treatment aimed at controlling bleeding and suppressing the antibodies. The condition is rare, but it is worth knowing about because it can be confused with congenital hemophilia if a clinician is not aware of the distinction, and because its treatment strategy is fundamentally different.
The Royal Disease Finally Identified
The most famous pedigree of hemophilia runs through the descendants of Queen Victoria of England. For more than a century, the specific type of hemophilia in the royal family was debated. DNA analysis of remains from the Romanov branch of the family finally settled the question. Researchers identified a mutation in the F9 gene on the X chromosome of Czar Alexei, the son of the last Russian tsar. The mutation disrupted RNA splicing and produced a truncated, nonfunctional form of factor IX.18PubMed. Genotype analysis identifies the cause of the “royal disease” The royal disease was hemophilia B, not hemophilia A as many had assumed.19PubMed. The ‘royal disease’–haemophilia A or B? A haematological mystery is finally solved
Victoria herself was a carrier. She passed the mutation to at least three of her children, who married into royal families across Europe, spreading the gene into the Spanish, German, and Russian dynasties. The pattern of inheritance in the royal pedigree, with affected males and carrier females transmitting the disorder, became a classic illustration of X-linked recessive inheritance. The identification of the specific mutation a century later also demonstrated how far molecular genetics had come: researchers extracted usable DNA from century-old bone fragments and applied modern sequencing to resolve a historical puzzle.
Hemophilia in Animals
Hemophilia is not unique to humans. Dogs, in particular, develop both hemophilia A and hemophilia B through X-linked mutations that closely mirror the human disease. Researchers have used dog colonies carrying these mutations to study inheritance and treatment. Breeding experiments with dogs segregating for both hemophilia A and B showed that the two genes recombine freely, meaning they sit far apart on the X chromosome, at least 50 map units apart. The study also produced males with both hemophilia A and B simultaneously, and found evidence that carrying both mutations in the same configuration may increase the risk of death before or shortly after birth.20Blood. Expression and Linkage of Genes for X-linked Hemophilias A and B in the Dog The genetic parallels between human and canine hemophilia have been critical for testing new treatments, including gene therapy, before they reach human trials.
Gene Therapy and the Prospect of a Single Treatment
Because hemophilia is caused by a single gene defect and the missing protein circulates in the blood, it has long been considered an ideal target for gene therapy. The goal is straightforward: deliver a working copy of the factor VIII or IX gene to cells (usually liver cells) so the body can produce its own clotting factor. After decades of research, this approach has reached regulatory approval. Three gene therapy products have been approved: two for hemophilia B and one for hemophilia A, all based on adeno-associated viral vectors that carry the corrective gene into liver cells. Ideally, a single administration provides therapeutic factor levels over a period of years, without the need for frequent injections.21PubMed Central. Gene therapy for hemophilia – From basic science to first approvals of “one-and-done” therapies
Gene therapy does not change a person’s DNA across their whole body, and it does not alter the X-linked inheritance pattern. A man treated with gene therapy for hemophilia B still carries the mutation on his X chromosome. All of his daughters would still inherit that X and be carriers. The therapy corrects the protein deficiency in treated cells, but it does not rewrite the genetic blueprint that gets passed to the next generation. This distinction matters for families making reproductive decisions: gene therapy treats the individual, not the lineage.
Genetic Testing and Carrier Detection
Identifying carriers and diagnosing hemophilia before birth has become routine in settings with modern laboratory resources. Molecular testing can pinpoint the exact mutation responsible for a family’s hemophilia, determine whether a woman is a carrier, and enable prenatal diagnosis in subsequent pregnancies. In countries with advanced molecular facilities, the preferred approach is direct mutation analysis, which sequences the relevant gene to find the causative change. In settings with fewer resources, an indirect approach using genetic markers linked to the hemophilia gene can still provide carrier and prenatal testing at lower cost.22PubMed. Genetic diagnosis of haemophilia and other inherited bleeding disorders
Carrier testing is most valuable when paired with an understanding of the nuances already discussed: that a negative standard test does not rule out mosaicism, that carrier status carries its own bleeding risk, and that skewed X-inactivation can push some carriers into clinically significant territory. The combination of factor level measurement and genetic testing gives the most complete picture. Factor levels tell you how the person is doing right now; genetic testing tells you what they might pass to their children and whether deeper investigation for mosaicism is warranted.