The role of inbreeding in the extinction of the Spanish
Habsburg dynasty
Wednesday, April 15, 2009
The powerful Habsburg dynasty ruled Spain and its empire from 1516
to 1700 but when King Charles II died in 1700 without any children
from his two marriages, the male line died out and the French
Bourbon dynasty came to power in Spain. Reporting in the
open-access, peer-reviewed journal PLoS ONE, April 15, Gonzalo
Alvarez and colleagues at the University of Santiago de Compostela,
Spain, provide genetic evidence to support the historical evidence
that the high frequency of inbreeding (mating between closely
related individuals) within the dynasty was a major cause for the
extinction of its male line.
Using the genealogical information for Charles II and 3,000 of his
relatives and ancestors across 16 generations, the researchers
calculated the inbreeding coefficient (F) for each individual; this
value indicates the probability that an individual receives, at a
given locus, two genes identical by descent due to the common
ancestry of its parents. They found that F increased considerably
down the generations—from 0.025 for Philip I, the founder of the
dynasty, to 0.254 for Charles II—as the Habsburg kings tended to
marry close relatives more frequently in order to preserve their
heritage. Several members of the dynasty had inbreeding
coefficients higher than 0.20, which means that more than 20% of
the genome is expected to be homozygous in these individuals.
The authors cite three lines of evidence to support the theory that
inbreeding was a major factor in the extinction of the male
Habsburg line, on the death of Charles II.
Firstly, there was a very high level of marriage between biological
relatives (consanguineous marriage) within the Habsburg dynasty:
nine of the 11 marriages over 200 years were consanguineous,
including two uncle-niece marriages, one double-first-cousin
marriage and one first-cousin marriage.
The two individuals with the highest inbreeding coefficient were
Charles II and his grandfather Philip III. Although both were the
sons of uncle-niece marriages, their F values were almost as high
as the expected value for the offspring of an incestuous
(parent-child or brother-sister) marriage. The researchers explain
that this is likely to be due to multiple remote ancestors of these
individuals (remote inbreeding), on top of the high degree of
relatedness of their parents.
Secondly, there was a high rate of infant and child mortality in
the Habsburg families with only half of the children born in the
dynasty during the years studied surviving to age one, compared to
about 80% in Spanish villages of the time. Alvarez and colleagues
calculated that inbreeding at the level of first cousin (F =
0.0625) exerted an adverse effect on the survival to age 10 of
offspring of 17.8 % ± 12.3, which could explain the high levels of
infant and child mortality.
Thirdly, Charles II, dubbed El Hechizado ("The Hexed"), suffered
from many different disorders and illnesses, some of which may
result from the consanguineous marriage of his parents. According
to contemporary writings he was short and weak and suffered from
intestinal problems and sporadic hematuria. Children of closely
consanguineous couples often have an increased incidence of
detrimental health effects due to rare deleterious recessive
alleles inherited from common ancestors, although this will depend
on how inbred their pedigree is already.
Based on this clinical genetic knowledge and on information
gathered by historians on the health of Charles II, Alvarez and
colleagues speculate that the simultaneous occurrence of two
different genetic disorders (combined pituitary hormone deficiency
and distal renal tubular acidosis), determined by recessive alleles
at two unlinked loci, could explain much of the complex clinical
profile of this king, including his impotence/infertility, which
led to the extinction of the dynasty.
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Public Library of Science
Thanks to Public Library of Science for this article.