Like a Virgin by Prasad, Aarathi (recommended reading txt) 📕

whom a group defines as being too close a relation for sexual relations. In many Western cultures, there is an informal taboo around marriages between first cousins or between uncles and nieces. These proscriptions may be fostered partly by religious laws, economic imperatives, or long-standing prejudices, including the socially taught belief that children who grow up together cannot (or should not) develop a sexual attraction to each other. Regardless, most Westerners would not hesitate to say that consanguineous liaisons (those between ‘blood’ relations) are inherently unhealthy, triggering a range of physical and mental deformities – such as the outsized ‘Hapsburg jaw’ you read about in school biology lessons, but also including infertility and early death. The conventional wisdom is so ingrained that, in recent decades, some scientists have begun to argue that the impetus to avoid inbreeding is itself genetic – that there is a gene that discourages inbreeding and promotes a taboo against incest in families that carry it.

In many cultures, however, consanguineous marriages, including between cousins, remain widespread. These marriages are most prevalent in Arab countries, with India, Japan, Brazil, and Israel following them in the rate tables. The liaisons are more common among people with less education (as well as lower socio-economic status), perhaps because higher status groups are more likely to have been influenced by Western beliefs about ‘inbreeding’.

Conventionally, consanguineous marriages are considered to carry social benefits, such as being able to aggregate family wealth and ensure better treatment of the bride, and thus increase stability and security for the whole family. Many arranged marriages occur between ‘blood’ relations. But while those social benefits may hold sway in many families, there can also be biological benefits to marrying within the family: marrying a close relative might save your lineage’s genes from extinction. In fact, there are cases in which inbreeding has actually facilitated a population’s adaptation to an inhospitable environment and parasite threats.

Take, for example, the telling statistic that in many parts of Asia, the Middle East, and Africa, marriages between close biological relatives account for up to sixty percent of all unions. These geographic areas share a long history of exposure to malaria. Indeed, the five hundred million to eight hundred million people who are married to a ‘blood’ relation mostly live in the world’s malarial regions. There are simply more consanguineous marriages in places where there is more Plasmodium falciparum, the protozoan parasite that makes its way into humans through the bite of the Anopheles gambiae mosquito and causes the most lethal form of malaria.

In 1949, Professor Haldane noted an apparent connection between malaria and a high prevalence of thalassaemia, an inherited blood disorder. Thalassaemia gets its name from the Greek for ‘sea’ (thalassa) and ‘blood’ (haema), since one type of the disease is especially prevalent in regions circling the Mediterranean. The condition causes the body to make fewer healthy red blood cells and less haemoglobin, the iron-rich protein in red blood cells that carries oxygen around the body. In people with thalassaemia, this undersupply of red blood cells and haemoglobin leads to an undersupply of oxygen in the bloodstream. In its mild form, thalassaemia may cause tiredness, but when severe the spleen becomes enlarged, and the person may suffer from liver, heart, and bone ailments. Haldane believed that, for some reason, malaria seemed to be ameliorated among patients with thalassaemia. Could it be that the presence of malaria parasites in the environment had promoted the survival of the inherited genes that caused the disorder?

In humans today, the gene that causes the α+ type of thalassaemia is the single most common disorder caused by a mutation on a single gene, what is known as a monogenic disorder. Over five hundred million people carry the gene, and they live primarily in regions where malaria is or was endemic (in the past, malaria was endemic around all of the Mediterranean, including much of Southern Europe). As the Red Queen hypothesis would predict, when malaria emerged ten thousand years ago, humans adapted to the threat. In regions where malaria parasites were present, the population has much higher rates of gene variants – and not just those for inheriting thalassaemia – that decrease the likelihood that you will die from malaria if you are infected.

The variants that protect against malaria tend to change the structure or function of red blood cells. Among them are the genes responsible for sickle-cell anaemia (which changes the shape of red blood cells); the abnormal haemoglobin C (found mostly among Yoruba populations in West Africa); and the Duffy antigen negative blood group (which affects the protein on the blood cells on to which Plasmodium parasites attach). Having the gene for α+ thalassaemia, for instance, does not stop you from being infected with malaria, or from developing the symptoms of the disease. But it does reduce the risk of developing severe malaria, especially malarial anaemia, and therefore the risk of dying. (Although no one is as yet exactly sure why or how, people with thalassaemia also appear to be protected against developing severe anaemia, with or without malaria, and have greater resistance to lower respiratory tract infections.)

The practice of consanguineous marriages appears to map to those geographical regions most hit by malaria, and while that might be due to an assortment of factors, one of them is likely to be because inbreeding accelerates the selection of the α+-thalassaemia gene. Indeed, people with two copies of the protective gene variant have better survival odds against malaria than those with one or no copies. And, of course, to inherit two copies of the gene, both of your parents had to be carrying at least one, and there is more chance of this if they are related, even distantly. On average, twenty percent more of the children from consanguineous unions would survive malaria as compared to those from marriages