Thursday, September 17, 2009

Follow-Up on Hapsburg mandibular prognathism (or 'the protuberant jaw')

Bob was right-- the end of the Spanish Hapsburg line was not caused by inbreeding alone. However, the legendary "Habsburg Jaw" certainly was. A testament to the effect of a large proportion of homozygous alleles, the heavy jaw was present from the time of Maximilian I's grandmother on. In its most severe form, it could inhibit talking or eating, and it was the stuff of many monikers (Leopold "the Hogmouth").

As you may recall from my presentation, Maximilian's grandson Charles V split the line into two branches, and the deformity continued through both royal families. There have been few studies on hereditary prognathism, but most experts agree that the level of intermarriage among close relatives was likely to exacerbate the degree of the deformity, but not necessarily its prevalence among family members.

The degree of intermarriage among the Hapsburgs can be illustrated by Philip II, who married an two cousins, and a niece (at different times). His final marriage to his niece was the first of three such marriages that brought forth Charles II, the final Hapsburg king of Spain. Philip II's great-grandmother was Juana 'the Mad' of Castille, who brought mental instability to an already impressive litany of familial problems including asthma, epilepsy, and, apparently, as Bob indicated, a weak resistance to syphilis, which may have been spread prenatally. The gene (or gene combination, it is unclear) was estimated to effect 50% of the Hapsburgs, and thus was likely a recessive trait. Now that the Hapsburg blood has been sufficiently diluted by a broader spectrum of marriages, it is no longer in evidence on a consistent scale today.

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