Collagen 1 makes up to 90% of the bone matrix. The presence of the risk allele T (its frequency in Europeans is 18-20%) leads to osteoporosis — decreased bone mass and more frequent bone fractures.
The gene encodes the components of type I collagen fibers, which predominate in fibrous ligaments and tendons. In the T/T genotype carriers, the gene expression is increased resulting in a weakened and more flexible form of type I collagen fibers, which leads to a tenfold decrease in the risk of ligament and tendon ruptures or sprains, while simultaneously increasing the risk of bone fractures twice.