Bayes theorem can be used to adjust the risk of being a carrier for a genetic condition if additional modifying information is available.

Carrier 
Noncarrier 
prior probability 
(risk) 
1  (risk) 
conditional probability 
(conditional risk carrier) 
(conditional risk noncarrier) 
joint risk 
(risk) * (conditional risk carrier) 
(1  (risk)) * (conditional risk noncarrier) 
where:
• The prior probability for being a carrier in many genetic conditions if one parent is a known carrier is 0.5.
• The conditional probability for being a noncarrier is 1 if offspring are normal.
posterior probability of being a carrier =
= ((risk) * (conditional risk carrier)) / (((risk) * (conditional risk carrier)) + ((1  (risk)) * (conditional risk noncarrier)))
posterior probability of being a noncarrier =
= ((1  (risk))* (conditional risk noncarrier)) / (((risk) * (conditional risk carrier)) + ((1  (risk)) * (conditional risk noncarrier)))
For the situation where previous offspring are all normal (conditional risk for noncarrier = 1):
posterior probability of being a carrier =
= ((risk) * (conditional risk carrier)) / (((risk) * (conditional risk carrier)) + (1  (risk))) =
= ((risk) * (conditional risk carrier)) / (((risk) * ((conditional risk carrier)  1)) + 1)
posterior probability of being a noncarrier =
= (1  (risk)) / (((risk) * (conditional risk carrier)) + (1  (risk))) =
= 1  (posterior risk of being a carrier)
Specialty: Genetics