The Habsburg Curse - Recessive Genes and the Doom of Royal Inbreeding
Carlos II was born into the Habsburg family, a dynasty that had ruled Spain for centuries. He suffered from numerous severe physical and mental health issues, including developmental delays, infertility, premature aging, frequent convulsions, and chronic gastrointestinal problems.
Notably, his oversized jaw, a hallmark of the Habsburg lineage, made eating and speaking particularly difficult for him.
These weren’t just random bad luck; it is said that they were due to the Habsburgs’ habit of inter-family breeding. This was supposed to keep their royal bloodline pure, but it ended up causing its downfall. Carlos II was the last of the Spanish Habsburgs, marking the end of their rule.
It’s common sense that inter-family breeding, also known as consanguineous marriage, is a bad idea. But why?
Consanguineous marriage can increase the risk of certain genetic disorders due to the inheritance of recessive genes. Genes can be either “strong” (dominant) or “weak” (recessive). If you have a dominant version, that’s the one that gets expressed, even if you also have a recessive version. The recessive gene will only be expressed if you inherit two copies of it—one from each parent.
For example, think of eye color. If brown eyes (dominant) and blue eyes (recessive) are options, you’ll have brown eyes if you inherit one brown-eye gene because brown is stronger. To have blue eyes, you need to inherit the blue-eye gene from both parents, because two recessive genes are needed for it to show up.
In consanguineous marriages, because the parents are related, they are more likely to both carry the same recessive genes, so the chances of their children inheriting two recessive versions are higher.
Inheriting two recessive versions of a gene isn’t always a problem. However, some recessive genes carry instructions that don’t work properly, and if a child inherits two copies of a recessive gene mutation, it can lead to health issues.
For example, Cystic fibrosis (CF) and sickle cell disease are genetic conditions caused by inheriting two faulty copies of recessive genes.
Although not all of Carlos II’s health conditions may not come from detrimental recessive genes, scientists from Spain’s University of Santiago de Compostela argue that his conditions such as muscular weakness, willpower deficiency, infertility/impotence are likely to come from his genetic disorder.