Consanguineous marriages

This article is intended to provide information from a purely scientific and medical standpoint. We recognize that marriage practices are deeply rooted in cultural, social, and religious traditions. The goal here is not to pass judgment but to explain the genetic principles that are important for prospective parents to understand when making decisions about their family's future health.

What is a Consanguineous Marriage?

A consanguineous marriage is a union between two individuals who are related by blood, meaning they share at least one common ancestor. In medical genetics, this most commonly refers to unions between second cousins or closer. The degree of relationship is categorized by how closely the individuals are related:

• First-degree relatives: Parents and children, or siblings.
• Second-degree relatives: Uncles/aunts and nieces/nephews, grandparents and grandchildren, or half-siblings.
• Third-degree relatives: First cousins. This is the most common form of consanguineous marriage.
• Fourth-degree relatives: First cousins once removed or second cousins.

Why Are Consanguineous Marriages Common in Some Communities?

For centuries, and in many parts of the world today, marrying within the extended family has been a common practice. The reasons are often multifaceted and include:

• Cultural and Social Cohesion: Keeping the family unit tight-knit, preserving family traditions, and ensuring compatibility between families.
• Economic Reasons: Keeping land, property, and wealth within the family.
• Geographical Proximity: In smaller or more isolated communities, the pool of potential partners is naturally limited to relatives.

It is a practice that has a long history and is often seen as a way to strengthen family bonds and ensure a stable and supportive marital environment.

Debunking Common Myths

• Myth: "Our family has always married cousins, and our children have always been healthy."
  • Fact: While many children from consanguineous unions are born perfectly healthy, this does not eliminate the risk. It simply means that, by chance, previous generations may not have passed on problematic gene combinations. The risk is a matter of statistical probability, not a certainty. Each new pregnancy carries its own specific risk.
• Myth: "It's better to marry someone you know and trust from your own family."
  • Fact: While familiarity is a comfort, it doesn’t influence genetics. The biological risks are linked to shared ancestry, regardless of the personal relationship between the partners.

A Simple Explanation of the Genetic Risk

Think of your genetic information (your DNA) as a massive library of instruction manuals. Each person has two copies of almost every manual – one from each parent. These "manuals" are called genes.

Sometimes, a manual has a small typo (a gene mutation). If the other copy of the manual is fine, the typo usually doesn't cause a problem. The good copy acts as a backup. A person carrying one faulty copy and one good copy is called a carrier. They are typically healthy and often don't even know they carry the faulty gene.

The issue arises when two people who are carriers for the same faulty gene have a child. Because blood relatives share a significant portion of their genetic makeup from common ancestors, they are more likely to be carriers for the same rare, faulty genes.

If two carriers have a child, there are three possibilities for that specific gene:

1. The child inherits two good copies (25% chance) - Healthy.
2. The child inherits one good copy and one faulty copy (50% chance) - Healthy carrier, just like the parents.
3. The child inherits two faulty copies (25% chance) - The child will have the genetic disorder.

This is illustrated below, where 'A' is the normal gene and 'a' is the recessive, faulty gene. If both parents are carriers (Aa):

This 1 in 4 risk applies to each pregnancy.

Why is This a Concern? The Scientific Explanation

The primary medical concern with consanguineous marriage is the increased risk of the child inheriting autosomal recessive disorders. The general population risk for having a child with a major congenital anomaly is about 2-3%. For a first-cousin union, this risk approximately doubles to 4-6% (Hamamy, 2012).

This happens because individuals from the same family share a higher percentage of their DNA.

• Siblings share about 50% of their genes.
• First cousins share about 12.5% of their genes.
• Second cousins share about 3.125% of their genes.

While the increased risk might seem small, it is significant for severe, lifelong conditions.

Examples of Autosomal Recessive Disorders:

• Thalassemia: A blood disorder that affects the body's ability to produce hemoglobin, leading to severe anemia.
• Cystic Fibrosis: A condition that causes severe damage to the lungs, digestive system, and other organs.
• Spinal Muscular Atrophy (SMA): A disease that affects motor nerve cells in the spinal cord, leading to muscle weakness and atrophy.
• Phenylketonuria (PKU): A metabolic disorder where the body can't break down an amino acid called phenylalanine, leading to intellectual disability if not treated from birth.
• And many others

What Can You Do?

Knowledge is power. If you are considering a consanguineous marriage, or are already in one and plan to have children, medical science offers options (Warning: Some of the laboratory or instrumental investigation methods mentioned in this article may not be available in Uzbekistan at the time of writing):

• Genetic Counseling: A genetic counselor can assess your family history, explain your specific risks, and discuss available testing options.
• Carrier Screening: A simple blood or saliva test can determine if you and your partner are carriers for common or specific recessive disorders prevalent in your community.
• Prenatal Diagnosis: If both partners are found to be carriers, tests during pregnancy like Chorionic Villus Sampling (CVS) or amniocentesis can determine if the fetus has inherited the disorder.

Understanding the potential risks allows you to make informed decisions for the health and well-being of your future children.

Sources:

1. Hamamy, H. (2012). Consanguineous marriages: Preconception consultation in primary health care settings.Journal of community genetics, 3(3), 185–192. https://doi.org/10.1007/s12687-011-0072-y
2. World Health Organization (WHO). (2023). Genetic counselling. https://www.who.int/news-room/fact-sheets/detail/genetic-counselling
3. Bittles, A. H. (2001). Consanguinity and its relevance to clinical genetics. Clinical Genetics, 60(2), 89–98. https://doi.org/10.1034/j.1399-0004.2001.600201.x

National Human Genome Research Institute (NHGRI). (n.d.). Frequently Asked Questions About Genetic Disorders. https://www.genome.gov/FAQ/Genetic-Disorders