It is believed that the main advantage of affected sib-pair tests is that their application requires no information about the underlying genetic mechanism of the disease. However, here it is proved that the mean test, which can be considered the most prominent of the affected sib-pair tests, is equivalent to lod score analysis for an assumed recessive mode of inheritance, irrespective of the true mode of the disease. Further relationships of certain sib-pair tests and lod score analysis under specific assumed genetic modes are investigated.