The difference between dominant and recessive traits as causes of genetic disease used to be just an abstraction based on a great deal of observation.

If a genetic defect expressed itself only in patients who inherited the trait from both parents, it was called recessive; both copies of the gene coding for the trait were presumably defective, resulting in disease. If the trait was dominant, on the other hand, it meant that one defective copy of the gene was sufficient to spell disaster.

But why should some disorders require two mistakes, while others resulted from only one? Molecular biology has given a concrete and remarkably simple explanation.

"It now appears that these two categories [recessive and dominant] correspond pretty closely to the two fundamental categories of proteins: enzymatic and structural," Victor McKusick, of The Johns Hopkins University School of Medicine, said in a review of genetics research.

Recessive disorders tend to result from failures in genes that code for enzymes, the biological catalysts that do much of the body's chemical work. A person who has inherited the defective gene from only one parent often goes disease-free because the normal gene inherited from the other parent produces enough of the enzyme to serve the body's needs. The disorder appears only when the person inherits the same defect from both parents and therefore lacks any working copy of the normal gene.

If the genetic defect affects structural proteins, however—for example, collagen, a key component of connective tissues and bones—only one copy of the faulty gene is usually enough to cause disease. It is easy to see why. A four-engine airplane can still fly even if one of its engines fails, as long as the other engines provide enough power, but a single faulty strut that makes a wing fall off will cause the plane to crash.

— Harold Schmeck, Jr.

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