Fragile-X syndrome, like Huntington disease, is due to an allele that has an exc

Question

Fragile-X syndrome, like Huntington disease, is due to an allele that has an excessive number of trinucleotide repeats (in this case, CAG) relative to the wild type. Which of the following methods would provide the most practical genetic test to learn if an infant's impaired mental development is due to fragile-X syndrome?

A. Extract DNA, digest it with restriction enzymes, prepare a Southern blot, hybridize it with a probe complementary to CAG sequences, and examine the size of the hybridizing bands.   
B. Extract DNA, run a PCR using primers that hybridize to regions flanking (on both sides) the CAG repeats, and analyze the size of the amplified fragments on a gel.   
C. Extract DNA, digest it with restriction enzymes, clone the fragments, isolate the fragile-X gene, and sequence the DNA.   
D. Extract DNA, sequence the genome, and search for sequences corresponding to the CAG expansion.   
E. Extract DNA, run a PCR using primers that hybridize to the CAG repeats, and analyze the size of the amplified fragments on a gel.   

Explanation / Answer

B. Extract DNA, run a PCR using primers that hybridize to regions flanking the CAG repeats, and analyze the size of the amplified fragments on a gel. This gives the most precise data on the number of CAG repeats and size of the amplified fragment.

PCR test is relatively inexpensive and quick to perform. REsults are obtained faster. It is accurate to determine premutation and normal gene repeat numbers.

Option A is also a choice of test for diagnosis of the syndrome. Southern blot analysis determines if the gene has a full mutation, the size of the mutated region, and if there is any mosaic pattern of the gene. However, in recent years, PCR took over Southern hybridization.

The answer is both A and B can be used to diagnose the syndrome

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