When relatives who resemble each other in terms of a disease or other characteristics (i.e., phenotype1) share a certain genetic marker at a rate greater than expected based on chance alone, this strongly suggests that a gene located near the marker influences the disease. For example, two siblings share, on average, 50 percent of their genes and are therefore also expected to share a given marker 50 percent of the time. If both siblings have the same disorder, however, they should share those regions of the genome that contribute to the development of the disorder more than 50 percent of the time.
