An understanding of the effect of selecting families through a high-valued proband on the major gene index (MGI), the offspring between parents function (OBP), and the pairwise midparental correlation coefficient (MPCC) is developed. It is shown that the interpretations of these statistics must be carefully modified to adjust for the biases created by the selection criteria. Computer simulations are used to examine sporadic, multifactorial, and major gene models, under moderate (85%) and extreme (95%) proband selection. Additional insights may be acquired into the nature of mode of inheritance by comparing and contrasting selected and unselected of populations.