Gene cloning permits direct detection of gene mutations.
When the normal gene has not been cloned, a closely linked RFLP marker may indicate the presence of an abnormal allele if the RFLP marker is frequently co-inherited with the disease. Blood samples from relatives can be used to determine which RFLP marker is linked to the abnormal allele and which is linked to the normal allele. The RFLP markers must be different for the normal and abnormal alleles. Under these conditions, the RFLP marker variant found in the person being tested can reveal whether the normal or abnormal allele is likely to be present. Alleles for cystic fibrosis and Huntington's disease can be detected in this manner.