We thank Dr Jantz for the interest and attention with which he read our commentary and agree with his assessment concerning our statement about twin studies (lines 5-7 on page 317). The statement was incorrect and should have read, "Twin studies are based on the premise that monozygotic twins share all of their genes, whereas dizygotic twins share, on average, half of their genes. Thus, if a birth defect resulted exclusively from fully penetrant genetic factors, monozygotic twins would share the malformation in 100% of twins studied, whereas dizygotic twins would be expected to share the malformation in 50% of twins studied." It is unfortunate that this mistake was not identified prior to publication and we apologize for any confusion that this may have caused.
Hobbs CA, Cleves MA, Simmons CJ. Genetics Update. Arch Pediatr Adolesc Med. 2002;156(10):1058. doi: