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Physical Anthropology : Inbreeding To Forward

Physical Anthropology : Inbreeding

Due to anthropology's various fields of application, we can also address that certain phenomena of the subject can be measured scientifically. In the previous article, the definition of inbreeding was given, and now the consequences of inbreeding are going to be reviewed - in relation to the inbreeding that occurred within the Charles Darwin family tree.

Charles Darwin was a pioneer in the study of the effects of inbreeding within living things. In 1876 he published 'The Effects of Cross and Self Fertilization in the Vegetable Kingdom'; a work resulting from his experiments on the self-fertilization of 57 species of plants, including the relationship between family crosses and the reduction in stem length, weight, and number of seeds produced. It is possible that Charles Darwin's interest in inbreeding was related to his own life experience, as Darwin married his first cousin Emma Wedgwood, as was common practice in the upper echelons of Victorian societies. Three of Emma Wedgewood's brothers also married relatives: Josiah Wedgwood III married his first cousin Caroline Darwin (the sister of Charles Darwin), Hensleigh Wedgwood married his first cousin Frances MacKintosh, and Henry Wedgwood married his double first cousin Jessie Wedgwood. These consanguineous marriages accounted for up to 25 of the marriages within the Darwin and Wedgwood lineage. The numerous family crosses had a large impact on the Darwin lineage's infant mortality rates, with 30% of the children dying before they reached the age of 10. From this case and many other cases of consanguineous marriages, the popular questions that arise, surrounding the subject, are often related to the effects of inbreeding in humans and the effects of consanguineous marriage upon consequent descendants.

Charles Darwin family tree. Source

An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. A deleterious gene is an allele of a gene whose effects on the phenotype are likely to result in reduced genetic fitness. A deleterious gene can cause various diseases, but in some cases, deleterious genes can have benefits, as is the case of malaria resistance: the gene that causes sickle cell anaemia is deleterious if the person carries two copies of it. However, if a person has only one copy of the sickle cell deleterious gene, they will have resistance to the malaria disease.

So, it can therefore be said that, commonly, inbreeding increases the probability of homozygous alleles, which in deleterious cases can involve the manifestation of various diseases, and the decline in fertility. In union with environmental stress, issues related to inbreeding could lead to the extinction of a closed consanguineous group. However, it should not be forgotten that there are two main problems between inbreeding and the expression of certain diseases, infant mortality, pre-reproductive mortality and/or early mortality, including:

1. The non-standardization of studies between the various research centers dedicated to the subject.

2. The lack of control over sociodemographic variables: since, in populations where the majority of consanguineous bonds occur, it is usually involving people with low or no resources, basic access to health and sex education, or a healthy lifestyle and diet more generally. Therefore, early mortality or the outbreak of certain diseases could be more related to a way of life rather than because a pair are related to one another.

Map of consanguineous marriages around the world

Consanguineous unions frequent history, and continue to this day, representing 10.4% of relationships in the world. The reasons for these consanguineous links have been varied, whether because of geographic isolation, religion, for the maintenance of royal lineages, or for the sake of preserving the assets of high-society families or landowners. It should also be noted that many cases of consanguinity occur in undeveloped countries, isolated populations, or within groups of people with limited resources - implying poor or deficient access to health-related or educational materials. A 2003 study showed that maternal education was recorded as negatively correlating with consanguineous unions; if women have access to better education, consanguine relations decrease. It has been suggested that both excessive inbreeding and excessive outbreeding can lead to the expression of deleterious recessive genes, or to poor health conditions in general. As a result of the variables affecting the research into the positive and negative effects of inbreeding on humans, there is not yet a formula to know whether inbreeding is decisively more or less favourable.


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- Nabulsi, M. M., Tamim, H., Sabbagh, M., Obeid, M. Y., Yunis, K. A., & Bitar, F. F. (2003). Parental consanguinity and congenital heart malformations in a developing country. American Journal of Medical Genetics, 116A(4), 342-347.

- Ober, C., Hyslop, T., & Hauck, W. W. (1999). Inbreeding Effects on Fertility in Humans: Evidence for Reproductive Compensation. The American Journal of Human Genetics, 64(1), 225-231.

- Pekkala, N., Knott, K. E., Kotiaho, J. S., Nissinen, K., & Puurtinen, M. (2014). The effect of inbreeding rate on fitness, inbreeding depression and heterosis over a range of inbreeding coefficients. Evolutionary Applications, 7(9), 1107-1119.

- Wright, L. I., Tregenza, T., & Hosken, D. J. (2008). Inbreeding, inbreeding depression and extinction. Conservation Genetics, 9(4), 833-843.

- Yu, J., Pressoir, G., Briggs, W. H., Vroh Bi, I., Yamasaki, M., Doebley, J. F., … Buckler, E. S. (2006). A unified mixed-model method for association mapping that accounts for multiple levels of relatedness. Nature Genetics, 38(2), 203-208.

Media sources:

- Darwin–Wedgwood family—Wikipedia. (n.d.). Retrieved 5 February 2022, from

- Global rates of Consanguineous marriages (also known as Cousin Marriage)—Vivid Maps. (n.d.). Retrieved 5 February 2022, from



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Melisa Silva

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