King Charles the II of Spain

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Born 6 November 1661, Charles was the only surviving son of Philip IV of Spain and his second wife, his niece Mariana of Austria. Marriage within the same extended family was then common among the nobility,[c] but the Spanish and Austrian Habsburgs were unusual in the extent to which they followed this practice. Of eleven marriages contracted by Spanish monarchs between 1450 and 1661, most contained some element of consanguinity, Philip and Mariana being one of two unions between an uncle and his niece.[2][d] This policy may also have been driven by limpieza de sangre or "blood purity" statutes enacted in the early 16th century, which remained in force until the 1860s.[3]
Intramarriage accentuated the so-called "Habsburg jaw", a physical characteristic common in both Spanish and Austrian Habsburgs. One contemporary reported this was so pronounced in Charles that he swallowed his food without thoroughly chewing, leading to frequent stomach problems.[4] A 2019 study based on an analysis of Habsburg portraits concluded this feature was likely due to a recessive trait, but in the absence of genetic material, such claims remain speculative.[5]
The precise causes of Charles' ill-health remain disputed.[6] Based on an analysis of contemporary accounts, some modern researchers argue they may have been due to one or more autosomal recessive disorders,[7][8] while others suggest an herpetic infection incurred as an infant, causing hydrocephalus.[9] Neither his elder sister, Margaret, who married her maternal uncle Leopold I, nor their child and his niece Maria, had similar health issues.[6]
After his birth, he was entrusted to the royal governess Mariana Engracia Álvarez de Toledo Portugal y Alfonso-Pimentel.[10] Under her careful supervision, he survived childhood attacks of measles, chickenpox, rubella and smallpox, any one of which was then potentially fatal.[11] He also had rickets, which left him unable to walk unaided until he was four and required him to wear leg brace