8.7 Genetic screening

Question 1

What are DNA probes?

Answer 1

• Short single-stranded pieces of DNA
• With a base sequence complementary to bases on a target allele
• Usually labelled with a fluorescent / radioactive tag

Question 2

Why are DNA probes longer than just a few bases?

Answer 2

• A sequence of a few bases would occur at many places throughout the genome
• Longer sequences only likely to occur in target allele

Question 3

How can genetic screening be used to locate specific alleles?

Answer 3

• Extract DNA and amplify by PCR
• Cut DNA at specific base sequences using restriction enzymes
• Separate DNA fragments using gel electrophoresis
• Transfer to nylon membrane and treat to form single strands with exposed bases
• Add labelled DNA probes with hybridise with target alleles
• Expose to UV light if fluorescent label used or use autoradiography if radioactive label used

Question 4

How does gel electrophoresis separate DNA fragments?

Answer 4

• DNA samples added to wells in gel and electrical current passed through
• DNA is negatively charged so moves towards positive electrode
• Shorter DNA fragments move faster so travel further

Question 5

How can gel electrophoresis results be interpreted?

Answer 5

• Run a standard with DNA fragments of known lengths
• Compare to position of unknown DNA fragments to estimate their size

Question 6

What are some uses of genetic screening?

Answer 6

• Screening for heritable conditions
• Screening for drug responses
• Screening for health risks

Question 7

What do genetic counsellors do?

Answer 7

• Explain results of genetic screening
• Discuss available treatments
• Discuss lifestyle choices / precautions to reduce risk of genetic disease developing
• Explain probability of condition being passed on to children, allowing couples to make informed decisions