It just didn鈥檛 make sense.
I was working in a genetics laboratory, and we were trying to find the mutated gene that caused a particular disease that was common in a few families my supervisor had seen in the clinic. But in one family, it looked like the oldest child had an entire stretch of chromosome that neither the mother nor the father had. We were only looking at that stretch, but it was quite possible that half of that child鈥檚 genetic material came from someone who was not, on the paper I was looking at, their father or mother. I had found my first case of non-paternity.
Anyone who works in genetics will be familiar with these cases. What was particularly interesting here was the story that dates revealed. The parents were cousins, and they had married each other around the time this child was born. It seemed likely that the mother had become pregnant out of wedlock, which in this tightly knit community was frowned upon, and she was quickly paired up with her cousin to preserve appearances.
These two married cousins went on to have children of their own, many of whom were affected by this disease.
Hollywood often presents inbreeding as proxy for body malformations and, subsequently, as an indication of evil. Movies like听The Hills Have Eyes, The Texas Chain Saw Massacre,听and the听Wrong Turn听series show us antagonists that are either explicitly or implicitly inbred鈥攁nd living in rural areas, notably the American South. This clich茅 is part of a larger problem movie studios have of portraying scars and handicaps as evidence of degeneracy and wickedness.
But what do we actually know about the consequences of inbreeding?
One in ten
There is a big myth to address first: that inbreeding is rare. It鈥檚 actually quite common.
The scientific term we use is 鈥渃onsanguinity,鈥� which comes from the Latin for 鈥渇rom the same blood.鈥� A consanguineous marriage is the union between two people who share at least one common ancestor, although in practice the definition is limited to people who are second cousins or closer. A second cousin is someone whose grandparent was a brother or sister to one of your grandparents, meaning that their grandparent is your great aunt or great uncle.
Shockingly, consanguineous couples and the children they have make up听听human beings on planet Earth. Far from being limited to Appalachia and to Amish communities, consanguinity is common, especially in North Africa, the Middle East, and South Asia, and there are听听for this.
In many communities, marrying a cousin can be seen as strengthening family ties; as avoiding the health or financial uncertainties of tying the knot with a stranger; and as reducing the risk of your spouse abusing or deserting you. Consanguinity can even be encouraged by religion: it is sanctioned by Judaism and Buddhism, with Islam remaining ambiguous and Christianity having conflicting attitudes toward it. (We shouldn鈥檛 overgeneralize: religions are known for splitting into sects, and religious leaders can interpret their sacred texts in many different ways.) And before we indulge in the stereotype of the inbred redneck, let鈥檚 remember that ruling classes often favoured consanguinity to corral power.
It鈥檚 easy to think that most people would avoid inbreeding, but worldwide that is not the case; even animals听, as was shown through scientific research.
But if we are to understand the reason why many fear consanguinity, we have to talk about genetics.
The chances of getting a pair go up听
To many, I suspect that something being 鈥済enetic鈥� means that one of the parents has it, therefore their child has it as well. Your father is tall and you are tall too? It鈥檚 genetic!听
But genetic inheritance takes many forms. One of the simplest is what happens with Huntington鈥檚 disease. Typically, when a parent has it, their child has a 50% chance of having it. The disease is due to a mutation in a specific gene, and most affected individuals only have a single copy of this mutated gene. The other copy is normal. (We have two copies of most genes because we get one from our mother and another from our father.) When sperm and egg are made, they only retain half of the genetic material, which means it鈥檚 a lottery: will the child receive the copy with the mutated gene鈥攁nd thus the disease鈥攐r will they be lucky enough to get the normal copy?
Other diseases are transmitted differently. For some, you need to have听both copies听of the same gene mutated in order to have the disease, because a normal copy can compensate for the other. It鈥檚 a bit like losing a kidney: the other鈥檚 function is often good enough for living. Parents are carriers (having one functional kidney, in this analogy), and the unlucky child inherits both mutated copies of the gene (no functioning kidney).
The problems seen in children of consanguineous unions seem to be caused mainly by an increase in听these types听of听diseases鈥攖he ones described in the previous paragraph. The parents don鈥檛 know they are carriers because they don鈥檛 have the condition. Genetic diversity in the child is reduced because the parents are related to each other, and so the odds of getting a normal copy of the gene go down. Imagine playing a card game where you鈥檙e only given two cards and where receiving a pair means you lose. If you鈥檙e getting your cards from a standard deck, your chance of getting a pair is low enough. But if the deck only contains cards that have a value of 2, 3, 4, or 5, your chance of losing has gone up.
Second cousins share听. First cousins once removed鈥攚here one of them is the child of the other鈥檚 cousin鈥攕hare 1/16th听of their genes. That fraction changes to 1/8th听for first cousins and to 1/4th听for half-siblings, uncle-niece pairings and aunt-nephew pairings. A parent and their child or any pair of siblings will share听half听of their genes.听If听diseases run in the family that require both copies of a gene to be mutated, a child鈥檚 chance of inheriting these diseases goes up when their parents are consanguineous.
Scientists have attempted to study the real-life consequences on disease and fertility of consanguinity among humans. The risk is real, but how bad is it really?
Stigmatizing real issues
There is a听听of birth defects that have been linked to consanguinity in the parents, and it includes sensory issues like hearing loss and visual impairment, as well as neural tube defects and heart disease. The problem is that many of the studies that have reported on these associations have failed to look for other explanations.
If I focus on separating the children of the world based on whether or not their parents are consanguineous, I can look to see which group has the most birth defects or any other disease, like an immune problem predisposing them to infections. And if the answer is the consanguineous group, it鈥檚 easy to conclude that consanguinity itself caused this increase. But communities in which consanguineous marriages are common may differ in other ways. Typically, they tend to听, which can contribute to making them look worse off health-wise.
The inverse can also be true when we try to find out if consanguinity reduces someone鈥檚 fertility, a question that has been asked time and time again. Some studies show no such reduction in fertility, meaning that the average number of children a consanguineous couple has is the same as that of an unrelated couple. But a phenomenon known as听听can obscure what鈥檚 really happening: that an infant dying at an early age due to consanguinity can be 鈥渞eplaced鈥濃�攖o use a cold-hearted word I鈥檝e read in the scientific literature鈥攂y another one, especially when the culture encourages women to have children at a young age and to have many of them. It turns out that studying the specific consequences of consanguinity is hard.
I think it鈥檚 fair to say that children born of a consanguineous union are more likely to inherit a medical condition, especially if their parents are very close relatives, but that the problem is not as bad as Hollywood pretends it is for shock value. In animals, given enough time and enough consanguinity, the situation can turn into 鈥渋nbreeding depression鈥濃�攚hich has nothing to do with the mental health of mice but rather announces a drop in survival and fertility within an animal population when too much of the progeny is inbred.
I am also reminded of the concept of a 鈥渇ounder effect,鈥� which is when a community is founded by very few people, and everyone ends up descended from them. If one of the founders had a genetic condition, it will become common among their lineage. It鈥檚 not because everyone is encouraged to marry their cousin, but simply because the starting population is small, so genetic diversity is limited.
Inbreeding is useful in research, believe it or not. Laboratory mice are often inbred to guarantee a consistent genetic background, so that tests performed in one group of mice can be compared to those performed on another of the same type. In humans, consanguineous families have been key to identifying the genetic causes of certain rare diseases. The family I mentioned in the beginning, where the oldest sibling was a non-paternity case? Its members were participating in a research project to identify the cause of a disease that was common in their family and in others in Quebec. Consanguinity is helpful here because there is much less genetic variability within the family: we can thus compare relatives who have the disease to those who don鈥檛 and more easily find the stretch of DNA that is mutated in the one but not in the other. Knowing the mutated gene allows genetic counsellors to better advise family members considering having children, and it helps us understand the function that this gene normally plays in the body.
I鈥檓 certainly not trying to encourage people to look at their cousin through a different lens. In fact, consanguinity also includes unions between underage children and their older relatives, which I would argue are ethically indefensible. But there is stigma and dramatization around the issue of consanguinity, which can be harmful.听
In听, Alan Bittles, a research expert on consanguinity, denounced the fact that many doctors will simply blame inbreeding when immigrant parents show up with health problems or when their pregnancy doesn鈥檛 turn out the way it was hoped for. A mother鈥檚 advanced age has similar risks but may not be as stigmatized. We are quick to be repulsed by inbreeding but not so much by older women choosing to have biological children. And because of this judgment, consanguineous couples may be less likely to see a doctor for fear of being maligned.
Consanguinity is听听than it used to be thousands of years ago, as societies have industrialized and migrated toward large cities, but it does remain quite common, especially in certain parts of the world. Because specific genetic disorders run in certain populations, diagnostic tests are available (and in some places mandatory as part of听) to help inform couples looking to have kids. By destigmatizing the reality of consanguineous unions, we鈥檒l be in a better place to help a large chunk of the world鈥檚 population.
Take-home message:
- Consanguineous marriages are unions between two people who are second cousins or closer
- They are not rare: consanguineous couples and their children make up about one in every ten human beings on the planet
- Children born of consanguineous parents are at an increased risk for certain genetic diseases because of a more limited genetic diversity, but studies on these risks tend to be imperfect, and Hollywood has grossly exaggerated the consequences
