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The Spanish Habsburgs’ reign lasted two centuries, until the 38-year-old Charles II, a king whose manifold health woes and infertility scholars often attribute to severe inbreeding, died in...
- Overview
- More on inbreeding:
On November 1st, 1700, an entire dynasty of kings came to a crashing end with the death of Charles II of Spain. Charles had neither a pleasant life nor a successful reign. He was physically disabled, mentally retarded and disfigured. A large tongue made his speech difficult to understand, he was bald by the age of 35, and he died senile and wracked by epileptic seizures. He had two wives but being impotent, he had no children and thus, no heirs. Which is what happens after 16 generations of inbreeding.
Charles II was the final king of the Spanish Habsburg dynasty (see family tree), part of a house that ruled over much of Europe for centuries and which took Spain to the height of its international power. Concerned with corralling their heritage within their bloodlines, the Spanish Habsburgs married heavily between each other. Most of their 11 marriages were between blood relatives, including several matches between first cousins and two between uncles and nieces. Charles’s own mother was the niece of his father, and his grandmother was also his aunt.
Historians have often speculated that this inbreeding was the dynasty’s downfall and contributed to Charles II’s numerous health problems. The more closely related a child’s parents are, the greater the odds that they will be dealt a dud genetic hand. We inherit one copy of almost every gene from our father and one from our mother. Some will be defective, but chances are that a second working copy will compensate for this. But if parents are related, they may already share many of the same genes and they risk of passing down an identical pair of faulty ones to their children. That can lead to genetic disorders or birth defects, like those that afflicted poor Charles.
Through a fascinating piece of historical genetics, Gonzalo Alvarez from the University of Santiago de Compostela has confirmed that inbreeding caused the extinction of this dynasty. He traced the pedigree of the entire line back through 16 generations, including over 3,000 people.
For each person, he calculated a figure called the “inbreeding coefficient”, symbolised by the letter F. It measures the probability that a person with two identical copies of a gene inherited both from the same ancestor. For example, a child born to cousins has an F value of 0.0625, but it becomes much higher if the parents come from a long line of inbred couples. The higher the value, the greater the degree of inbreeding in that lineage.
Alvarez found that the first Spanish Habsburg king, Philip I, had a relatively low F-value of 0.025. But after just five generations, his descendant Charles II had an F-value of 0.254, more than ten times that of his great-great-great-grandfather. This figure is even twice as high as the expected value for the child of an uncle-niece marriage, which reflects just how pervasive inbreeding was in this family tree. It also means that Charles II would have carried identical copies for more than quarter of his genes (his genome was 25% homozygous, in the parlance of geneticists).
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Sidenote: On researching this article, I learned that the Spanish Habsburg dynasty started with a marriage between Philip I, also known as Philip the Fair, and Joanna I, also known as Joanna the Mad. I can only think that either Joanna’s nickname came after the marriage, or that Philip didn’t read the personal ad carefully enough…
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Apr 15, 2009 · The highest values of the inbreeding coefficient in the Spanish Habsburgs correspond to kings Charles II (0.254) and Philip III (0.218), and the crown prince Charles (Don Carlos, 0.211), son of Philip II and his first wife Mary of Portugal.
- Gonzalo Alvarez, Francisco C. Ceballos, Celsa Quinteiro
- 10.1371/journal.pone.0005174
- 2009
- PLoS One. 2009; 4(4): e5174.
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