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Was inbreeding responsible for haemophilia in royal families in Europe?

Example claims:

Rationalwiki:

"One general result of this inbreeding in the European royal families was the concentration of the disease hemophilia in the males [3]. So much so that hemophilia was known as the Royal disease until after World War II."

Answer posted in another Stack Exchange:

One of the biggest problems reestablishing humanity with a group that small is a lack of genetic diversity. People make jokes about inbreeding because it can be a serious problem, leading to birth defects and genetic diseases. Read up on haemophilia among European Royalty.

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Yes and no:

  • Haemophilia is heritable
  • Breeding was responsible for it in royal families
  • Not necessarily "inbreeding" though

The relevant fact are stated in Wikipedia's Haemophilia in European royalty

Britain's Queen Victoria, through two of her five daughters (Princess Alice and Princess Beatrice), passed the mutation to various royal houses across the continent, including the royal families of Spain, Germany and Russia. Victoria's son Prince Leopold, Duke of Albany suffered from the disease. For this reason, haemophilia was once popularly called "the royal disease". Tests of the remains of the Romanov imperial family show that the specific form of haemophilia passed down by Queen Victoria was probably the relatively rare Haemophilia B.[1]

The sex-linked X chromosome disorder manifests almost entirely in males, although the gene for the disorder is located on the X chromosome and may be inherited from either mother or father. Expression of the disorder is much more common in males than in females. This is because, although the trait is recessive, males only inherit one X chromosome, from their mothers. Thus if the haemophilia gene is transmitted on it, there is no possibility for the male to inherit a haemophilia-free gene from his father to mask or dilute the symptoms. By contrast, a female who inherits a gene for haemophilia on one of her X chromosomes will also have inherited a second X chromosome from the other parent which is likely to carry a haemophilia-free gene that would prevent full expression of symptoms.

Females who inherit the gene for Haemophilia A or B from both parents would be expected to manifest full symptoms, similar to those seen in affected males, but this is extremely rare. Despite frequent inter-marriage among royalty, no case of such double inheritance is known among Queen Victoria's descendants.

So:

  • Queen Victoria had it
  • Breeding passed it into other royal families
  • Yes, it was because of "inbreeding" that it was passed to several royal families -- if other royal families had intermarried more with commoners there would have been less chance for Queen Victoria's genes to affect every other royal family
  • It expressed in males but, no there was not enough "inbreeding" (i.e. two affected X chromose) for it to ever express in females
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  • If Queen Victoria's descendants had procreated outside the royal families, the number of affected persons had most probably stayed the same, so it's hard to see any relevance to inbreeding here. It should be no surprise that hereditary deceases occur more often within a family. Since the gene defect only survived a few generations despite the relatively small gene pool of the European royal families, this case could just as well be held up as an example that inbreeding is not a problem.
    – Tor-Einar Jarnbjo
    Oct 31, 2014 at 13:37
  • If her descendents had procreated outside royal families then they wouldn't have affected other royal families: so in that sense, the prevalence of hemophilia in (several) royal families was caused by the [in]breeding between the royal families.
    – ChrisW
    Oct 31, 2014 at 13:40
  • That hereditary diseases are passed on by breeding is obviously not disputed. It would be helpful to understand your answer if you decide for breeding, inbreeding or [in]breeding (whatever that is supposed to mean).
    – Tor-Einar Jarnbjo
    Oct 31, 2014 at 13:46
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    @Tor-EinarJarnbjo I'm saying that it's because royal families exclusively inter-breed that a single rare abnormality appears in several of them. But, that abnormality is not caused by inbreeding, i.e. it is not an autosomal recessive disorder.
    – ChrisW
    Oct 31, 2014 at 13:52

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