Genetic counseling for multi factorial disorders follows in general the principles outlined briefly below:
1. The recurrence risk is much higher for first-degree relatives of affected family members than for more distant relatives.
2. The best estimate of the recurrence risk is the empiric risk, which is simply the recurrence risk observed in similar families, for the same degree of relationship. It is often useful to state it as a multiple of the population risk of the defect.
The empiric risk is based entirely on past experience and does not imply that the genetic and environmental factors in the pathogenesis of the malformation are understood. An empiric risk is an average for the population and is not necessarily for a specific family.
3. The recurrence risk is increased by:
* The presence of more than one affected relative;
* a severe form of the disorder;
* an affected person of the sex less likely to be affected;
* consanguineous parentage.
4. Two common errors in risk calculation should be avoided:
* If the parent of a child with a multi factorial birth defect has another child by a different partner, the children are second-degree, not first-degree, relatives, and the empiric risk for the second child is much lower than if the children had both parents in common (usually, the risk is approximately 1 percent instead of approximately 5 percent).
* When an unaffected uncle or aunt of a child with a multi factorial defect inquiries about the risk of the same defect in his or her own offspring, the relevant risk is not the risk to the consultand (a second-degree relative of the index patient) by the risk to the offspring of the consultand (third-degree relatives).