A study from Oxford, England studied DNA samples from 170 families and assessed 28 genes for their impact on bone mineral density. Eight genes were discovered to be linked to bone density and the strongest connection was the receptor for parathyroid hormone. This hormone plays a crucial role in determining the level of calcium in the blood. There may be subtle differences in the blood, which determine bone mineral density. Some genes may determine overall bone mineral density and others may act specifically at different sites, such as the hip and the spine. Brothers and sisters of people with osteoporosis are six times more likely to develop low bone mineral density than the general population itself.