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Dr. Antoniou, a medical geneticist, stands at the front of the genetics lab, where a genetic testing poster is displayed on a large board. He smiles warmly and gestures for you and your fellow students to enter the lab. 

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Dr. Antoniou: ‘Welcome, everyone. Today, we will be discussing types of genetic disorders and the tools we have to diagnose them. Firstly, we’ll explore a family where Down’s Syndrome is suspected. Then we will explore a family with a history of with cystic fibrosis.’ 

You: ‘These sound like completely different diseases. Will we use the same tools?’

Dr Antoniou: ‘Actually, we want to see how you’ll use both karyotyping and genetic probability rules to unravel the situation. Let’s begin our discussion by looking at the family that suspects Down’s Syndrome. How would you determine if a person has Down’s Syndrome?’

Michael: ‘I’d say we should definitely look at external physical traits first. If someone looks like they have Down’s Syndrome then maybe there’s no need for further genetic testing.’

Ana: ‘I’m not sure, because subtle chromosome issues might not be obvious physically. Maybe we should do a direct karyotype test first. ‘

Dr. Antoniou: ‘I hear differing opinions. Which method do you think is the most logical first step?’