Why Is Marriage Between Siblings or Close Relatives Medically Condemned?

Have you ever wondered why marrying a sibling or close cousin is such a big no-no in most cultures and from a medical standpoint?

It’s not just about tradition or religion – there’s solid science behind it. This practice, called inbreeding or consanguineous marriage, used to be common in ancient times for keeping wealth in the family or royal bloodlines pure, like in the case of Egyptian pharaohs or European royalty. But over the centuries, as we learned more about genetics, major religions and societies started banning it. Today, only a few isolated communities still do it.

So, What Exactly Is Inbreeding?

Inbreeding means marriage or reproduction between people who are closely related by blood. This includes siblings, first cousins, uncles/aunts with nieces/nephews, or even half-siblings. The closer the relation, the higher the risks. Inbreeding is not about creating new bad genes out of nowhere – that’s a myth a lot of people believe. But in actuality, what it does is that, it amps up the chances of harmful traits showing up in kids.

You see, every human carries some “faulty” genes, but when parents are unrelated, those faulty genes usually stay hidden. But with close relatives, things can change really fast.

In many parts of the world, including Nigeria, inbreeding was once accepted in some communities for social or economic reasons as stated earlier. But now, health experts condemn it because of the clear medical dangers associated with iut. And no, the dangers aren’t just limited to rare disorders alone – it can lead to everyday health issues too.

The Genetic Science Behind the Risks

Let’s get to the heart of it. We all have about 20,000 genes, half of which was inherited from our mother and half from our father. Many diseases have been found to come from recessive genes – these only cause problems if you get two faulty copies, one from each parent.

Normally, unrelated parents have different gene sets. If one has a faulty gene, the other’s healthy version usually covers it up, and the kid stays fine. But siblings or cousins share a lot of the same DNA – up to 50% for siblings, 12.5% for first cousins. So, they likely carry the same faulty genes from common ancestors.

When they have a child:

The kid might get two faulty copies.
Boom – the disorder appears.

For example, imagine a gene for a rare condition like cystic fibrosis; normally, carriers of this gene look normal. But when both parents are carriers of it and related, the risk skyrockets.

Studies show consanguineous marriages double the chance of birth defects compared to unrelated couples, and mortality in kids from first-cousin marriages is about 3.5% higher.

Transitioning to real examples, think of it like this: Unrelated parents’ genes are like mixing two different decks of cards, which creates lots of variety. But related parents are like shuffling the same deck twice which makes duplication easy to occur.

Common Health Problems from Inbreeding

Inbreeding has been found to increases the risks for:

Genetic disorders: Like thalassemia, sickle cell anemia (common in Nigeria), cystic fibrosis, or Tay-Sachs disease.
Birth defects: Heart problems, cleft palate, limb abnormalities.
Sensory issues: Hearing loss, vision problems.
Mental health: Higher chances of intellectual disabilities, schizophrenia, or depression.
Other conditions: Diabetes, cancer, blood disorders, and even lower IQ in some cases.

In kids from sibling marriages, risks are even higher – up to 5% abnormality rate vs. 2-2.5% in unrelated couples. Plus, more miscarriages, stillbirths, and infant deaths.

For instance, in communities where cousin marriages are common (like some parts of Pakistan or the Middle East), studies show more kids with congenital heart defects or metabolic disorders. In Nigeria, where sickle cell is prevalent, inbreeding could make it worse if both parents carry the trait.

But here’s a key point: Not every child from inbred marriages will have issues; some will turn out fine. However, the odds are stacked against them, and the risk not worth it at all.

A Simple Example with Genes

For every child that is born, each parent donates one gene for a trait.

For Unrelated Parents: Mom has genes 1 and 2; Dad has 3 and 4. Their child will get unique combos like 1+3 or 2+4. This combo create a low chance of matching faulty genes.
For Related Parents: Mom has genes 1 and 2; Dad has 2 and 3. Their child might get 2+2 …. and if that 2 is a faulty gene, trouble will surely erupt.

Using a Punnett square (a genetics tool):
If both parents carry a recessive fault (Aa x Aa):

25% AA (normal)
50% Aa (carrier, normal)
25% aa (disorder)

In unrelated couples, the chance both carry the same fault is tiny. Biut in siblings, the risk is much higher.

Historical and Cultural Shifts

Back in the day, inbreeding kept power in families – like Cleopatra marrying her brother. But as science advanced, we saw the patterns; royals like the Habsburgs had “Habsburg jaw” from generations of cousin marriages.

Major religions had to step in: Christianity, Islam, Hinduism all banned close unions. In Nigeria, most ethnic groups discourage it too. Laws in many countries outlaw sibling marriage to protect health.

Today, genetic counseling can help avoid this type of marriage, especially if you’re from a community where cousin marriages happen, and you’re not sure if you are related to the person you’re about to get married to or of they carry certain genes that could be of problem in the future, genetic testing can check carrier status.

Modern Medical Views and Advice

Doctors worldwide condemn inbreeding because the risks far outweigh the benefits. The World Health Organization notes higher disease burdens in consanguineous communities.

In 2025, with gene editing like CRISPR, maybe we can fix some faults. But for now, prevention is key: Marry outside the family, get premarital screening (especially for sickle cell in Nigeria).

If you suspect family history, see a geneticist. Simple blood tests can help spot carriers.

Final Thoughts: Health Over Tradition

In short, marrying siblings or close relatives is medically condemned because it boosts chances of kids inheriting double faulty genes – leading to disorders, defects, and higher death rates. It’s not about new mutations per se, it’s about uncovering hidden ones. Society evolved to ban it for a good reason.

Choose love wisely – your future kids will thank you.

Stay healthy and informed!

Useful Links

References

Ahmed S, et al. (2024). Consanguineous Marriage and Its Association With Genetic Disorders. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC10924896/
BBC News. (2025). Cousin marriage: The new evidence about children’s ill health. https://www.bbc.com/news/articles/c241pn09qqjo
Samitivej Hospital. Intrafamilial marriage could place your child at risk. https://www.samitivejhospitals.com/article/detail/intrafamilial-marriage
Bittles AH, Black ML. (2010). Consanguinity, human evolution, and complex diseases. PNAS. https://www.pnas.org/doi/10.1073/pnas.0906079106
Centre for Genetics Education. When parents are related. https://www.genetics.edu.au/PDF/When_parents_are_related_Consaguinity_fact_sheet-CGE.pdf
McGill OSS. (2025). Inbreeding: Behind the Stigma. https://www.mcgill.ca/oss/article/inbreeding-behind-stigma
El-Gilany AH, et al. (2017). Global distribution of consanguinity and their impact on complex diseases. ScienceDirect. https://www.sciencedirect.com/science/article/pii/S1110863017300174
Bittles AH. (2022). The impact of consanguinity on human health and disease. Springer. https://link.springer.com/article/10.1007/s44162-022-00004-5
Bener A, et al. (2018). Consanguineous Marriage and the Psychopathology of Progeny. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC6145769/