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Why Does One Sibling Have a Genetic Disease and The Other Is Healthy?

Reposted with permission

Recently I read a book about genetics, Inheritance; How Our Genes Change Our Lives – and Our Lives Change Our Genes, written by Dr. Sharon Moalem and it got me thinking about siblings. Why do I have a genetic inflammatory disease and not my brother? I’ll start by saying that I love my younger brother and I am very grateful that he doesn’t live with chronic illness. While reading Dr. Moalem’s book I learned that siblings who share the same mother and father can have vastly different health outcomes, even in the case of identical twins. Monozygotic twins (those having developed from one embryo, in early development, and separated to develop into two) have identical genes and yet they don’t automatically share the same health and wellness.


As Dr. Moalem explains, it’s not just the genes, it’s how the genes are treated and how they are expressed. Genetic expression is unique to each individual and can in fact, change over a lifetime. For example, it will be understood by many that smoking increases the risk of lung cancer. This is particularly true in people with genes that are more inclined to ‘express’ themselves to developing a disease, (examples: cancer, diabetes, lupus, multiple sclerosis etc.) versus having a genetic predisposition to developing that specific disease. Genetic expression can be thought of as ‘the turning on’, ‘the activation’, or ‘the start of communicating’ the genes presence within the overall portrait of our genetic make-up.

It’s possible for siblings to have the same genetic predisposition and yet each individual has a different mosaic of how the genes fit within the total person. For example, I know a set of twins who look identical but for one parts his hair on the left and the other, the right. Put them next to each other, and they truly look like a mirror image. Every part of a human is determined by how our genes are expressed, even the way in which our hair forms a natural part.

So what affects gene expression? As you probably guessed, the typical things like diet, activity level, what chemicals we are exposed to, all have an effect on gene expression. But it’s the combination of many factors and contributors that can make the difference as to whether the gene is expressed and activates a genetic disease. Twins may have identical genes and yet they may have very different diet, exercise levels and one may have exposure to an environment with a chemical that changes the genes and how they behave. It explains why one sibling may have very good health and the other may develop a chronic genetic disease, like inflammatory bowel disease, lupus, diabetes or others.

There is no definitive answer as to why one sibling develops a genetic disease and the other does not. The conditions of environment and development for cell growth from the beginning of embryo development through adulthood is vast and unique to each individual, even if their genes are the same. For many diseases, it is impossible to identify the exact factors that cause the gene to ‘turn on’.

The complexities of genetics and disease that focus on “what to change” in order to create a positive change in our health are difficult to identify. Just as we don’t know what changes have a direct effect on why a gene is expressed in a particular individual, we would be hard pressed to identify what changes would reverse the disease expression.

Jennifer Davagian Ensign
Inventor of the Sephure applicator


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