Understanding inherited disease

Some conditions are written into a single gene and passed from parent to child in patterns first glimpsed in a monastery garden.

Long before anyone had seen a gene, a monk named Gregor Mendel worked out the rules by which traits pass from one generation to the next. Those same rules still govern many inherited diseases today.

One gene, one disease

A Mendelian, or monogenic, disease is caused by a mutation in a single gene.¹ There are five basic patterns by which such diseases are inherited: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.²

Dominant and recessive

The distinction matters enormously for families. In autosomal dominant conditions, a single faulty copy is enough to cause disease, it tends to appear in every generation, and each child of an affected parent has a 50% chance of inheriting it.² In autosomal recessive conditions, two faulty copies are needed; parents can be unaffected “carriers,” and each of their children has a 25% chance of being affected.²

The pattern of inheritance is often the first clue to the gene.

Individually rare, collectively common

Any single one of these disorders is rare. But taken together, single-gene diseases are a leading cause of illness and death in newborns and children.² It is part of why understanding inheritance sits at the very centre of rare-disease medicine.

Why it matters

Knowing how a disease is inherited shapes everything that follows, the search for the gene, the counselling of families, and, increasingly, the design of therapies aimed at the mutation itself.