Ascert-derivation - Ascertainment derivation J.L Marsh Bio 137 B If we suspect that a disease is caused by a recessive mutation the simple Mendelian

Ascertainment derivation J.L. Marsh Bio 137 B © JLMarsh 1994 If we suspect that a disease is caused by a recessive mutation, the simple Mendelian expection is that 25% of the children from matings between 2 carriers should be affected. However, the method of finding families to analyze introduces biases that affect this expectation. The following derivation permits a precise prediction to be made that takes these biases into account. If we find families to study by finding affected children and then gathering data on the whole family, what proportion of offspring in these families will be expected to be affected if it is a simple Mendelian trait? To answer this, we define a term (q') as the percent affected offspring expected given the bias of complete truncate selection. Previous workers have said that that percent q' = q/[1-p n ] . How can we prove that the # affected/total # persons in our sample is described by q' = q/[1-p n ] where n = family size rather than by the simple mendelian prediction of 25% for a recessive?