What was the most significant birth defect you have noticed before?
This is a great way to show the interviewer that you have seen something that is different in the genetic field. This will also let the interviewer know that you have the depth of knowledge need to excel.
"The most significant birth defect that I noticed was Marfan's syndrome. While I was interning with a genetic counselor post graduate school, a couple came in who were expecting a baby in five months. They had a family history of Marfan's and were worried that their baby might have it too. This syndrome is a common inherited disorder of the connective tissue and is located in the FBN1 gene. We ran a panel and confirmed the findings that their fetus did indeed have a high percentage of possibility for that symptom. The couple now had a difficult decision to make regarding their pregnancy."