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In this procedure one studies the coinheritance of a specific human phenotype (here a genetic disease) with an RFLP marker. If individuals who inherit the disease nearly always inherit the RFLP marker, then the gene causing the disease and the RFLP marker are likely to be close together on the chromosome, as shown here. To prove that an observed linkage is statistically significant, hundreds of individuals may need to be examined. Note that the linkage will not be absolute unless the RFLP marker is located in the gene itself. Thus, occasionally the RFLP marker will be separated from the disease gene by meiotic crossing-over during the formation of the egg or sperm: this has happened in the case of the chromosome pair on the far right.