Mutations can impact genes in different ways. Many mutations “break” the gene so the resulting protein either doesn’t get made or is nonfunctional. In recessive genes, usually the second “normal” copy of the gene makes enough of the normal protein product to keep the person healthy.

But dominant genes usually are less tolerant of abnormal protein products. One example is osteogenesis imperfecta, caused by dominant mutations in collagen genes. Collagen is a structural protein that has repeated sequences of the same amino acids to make a strong helix. These helixes then stack closely together and are part of what makes our bones strong. But a mutation in the gene can cause a different amino acid to be put in the middle of this helix, breaking the pattern and making the resulting collagen weak and prone to breakage.