SUMMARYParent‐child correlations of approximately zero and sib‐sib correlations of approximately one‐half have beefi obtained in respect of the grade of seventy in the nail‐patella syndrome. The nail defect and the patellar size have been used as indices of severity.These findings are interpreted as evidence for the operation of a series of modifying genes at the nail‐patella locus itself and a possible method of confirming this interpretation is suggested in which use is made of the known linkage between this locus and the ABO locus.