In human communities, random mating may not occur for sociological or biological reasons. In the first scenario, it is worth highlighting (i) homogamy and (ii) hypergamy. Homogamy would be the union or marriage, with its potential offspring, between individuals with shared sociocultural characteristics such as religion, ethnicity or socioeconomic status (Parkinson and Drislane, 2002). Hypergamy would be marriages made with a counterpart with greater economic power, this being one of the main reasons for its realization. This type of marriage would be more common in societies with a marked social stratification such as colonial societies in America or the caste society in India, still in force today (see the biography of Phoolan Devi ). In the biological scenario, one would speak of inbreeding , the union of two individuals with a kinship relationship. In stratified societies, endogamy can present economic advantages, with marriage between relatives presenting advantages such as the disbursement of a lower dowry or, at higher levels of wealth, that the family patrimony remains in the family (Ceballos and Álvarez, 2011). 
Possibly, inbreeding is a practice that has existed since the origins of the human being as a species, both in small and isolated communities or due to bottlenecks (i.e. marked reduction in the number of total individuals of a species), such as the one that our species could have suffered in the Pleistocene about 100,000 years ago (Gathorne-Hardy and Harcourt-Smith, 2003). A bottleneck could lead to increased inbreeding by chance alone. Since the Ancient Age there is written evidence of their employment in various places. In Egypt, marriage between brothers was a common resource in the royal dynasties, from the time of the pharaohs (Scheidel, 1996) to the Ptolemaic dynasty, being in turn practiced by other dynasties of the Hellenistic Period in a more punctual way (D. Carney , 2010) due to the conception of blood purity and its preservation throughout the generations (Aneni, 2019). Over the centuries in Europe, unions between members of royalty or foreign royalty, mainly for diplomatic purposes, continued to be the order of the day, limiting the number of potential consorts and thus the genetic variability over time. the successive generations. In fact, in different European dynasties, for marriages between two people of unequal social rank (i.e. morganatic marriage) the spouse of lower social status and any offspring of that union were not allowed to inherit title, privilege or property from the higher counterpart. range. This can be seen in article XII of the Pragmatic Sanction of 1776 , sanctioned by Carlos III:
This increased the kinship between different members of European royalty. In addition to the clinical problems associated with consanguinity itself, the transmission of harmful mutations for certain genes was facilitated, as happened with hemophilia until the last century. In other monarchies, such as those in Laos and Thailand, marriages between relatives continued to be used until the first half of the 20th century (Evans, 2010).
Historically these inbred practices also existed in the flat town even between siblings. This has been attested to by official census records during the period of Roman rule in Egypt, or furthered by treatises from various religions such as Zoroastrianism in what would now be present-day Iran (Scheidel, 1996). Although these extreme modalities of inbreeding are currently widely prohibited, not being tolerated by any culture (Harris, 2004), other modalities are still present, such as the union between first cousins. At the current , consanguineous couples and their offspring would represent over 10% of the world population (Bittles and Black, 2010), although their distribution is not homogeneous. In existing studies, consanguineous marriage is considered to be that which occurs between individuals whose degree of kinship is at least second cousins, resulting in the biological effects on the offspring of unions with lesser kinship practically negligible if unions with no relationship were taken as a reference. of kinship (Ceballos and Álvarez, 2011).
Inbreeding and the Spanish Hapsburgs
Bearing in mind the global prevalence of inbreeding unions, whose frequency varies between different countries according to culture, religion, legislation, etc.; it is of interest to address its potential genetic and clinical consequences and the usefulness of the Spanish Austrians as a human laboratory to assess problems that can be found in communities with a high prevalence of inbreeding. Thanks to the precise genealogical information of this lineage (16 consecutive generations spanning 500 years and more than 3000 individuals), it was possible to estimate the high inbreeding coefficients acquired by its members (Álvarez and Ceballos, 2016) and associate these values with historical records on their infant mortality (positively related to consanguinity; see Fig.4 in the article by Álvarez et al., 2009), suffered diseases and anatomical deformities such as prognathism (Vilas et al., 2019), already described by the chroniclers of the time and artistically represented.
As a genetic item, the offspring of a consanguineous union has a higher probability of being homozygous for different regions of the genome (i.e. the same information both paternally and maternally). The problem that arises is the increased probability of being homozygous for deleterious recessive alleles, those that have to be on both homologous chromosomes to have an effect on the phenotype of an individual (e.g. presenting cystic fibrosis). The consanguinity coefficient (F), previously mentioned, is a measure of the degree of consanguinity and varies from 0% to 100%. An individual born from a sibling bond would have a 25% chance of being homozygous for a given gene (F =25%) and would be an estimate of her genome-wide homozygosis. These levels of consanguinity can also be achieved through successive inbreeding unions with more distant kinship relationships (uncle-niece, first cousins, second cousins).
In the case of the Spanish Austrians, the levels of consanguinity reached, due to their marriage policy, their highest values with Carlos II “the Bewitched” with 25.4%, equivalent to what the offspring of a union between blood brothers would obtain. This monarch presented endless ailments throughout his reign, being unable to father offspring despite marrying twice (Álvarez and Ceballos, 2016). His premature death at the age of 39 led to the extinction of the most powerful lineage at that time and to a succession problem that led to the bloody War of the Spanish Succession (1701-1715).
Conclusion
This micro-essay has shown the opening of new fields of research by combining different disciplines. The History , providing genealogical information that allowed the reconstruction of pedigrees; the registry of ailments and diseases that motivated the search for possible genes involved in the diseases suffered by members of this dynasty (see Álvarez and Ceballos, 2016); and as a source of ideas and inspiration for new genetic research that may have positive effects on contemporary society. On the other hand, Biology providing a more holistic vision of History, contributing new causes to consequences already described.
Everything is knowledge, let's use everything to know.
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