Laboratory Genetics and Molecular Diagnostics
Core concepts
Understand the central dogma as applied to diagnostics: DNA to RNA to protein, and the techniques that interrogate each level (PCR, RT-PCR, sequencing, karyotyping, FISH, microarray, and next-generation sequencing). Know the nomenclature basics: karyotype description, deletion/duplication/translocation notation, and the difference between constitutional and somatic variants. Familiarise yourself with variant classification using the ACMG/AMP framework (pathogenic, likely pathogenic, variant of uncertain significance, likely benign, benign) and the evidence categories supporting each.
Know the common test indications: prenatal screening and diagnosis, carrier testing, newborn screening, predictive testing, and tumour profiling. Understand the limits of each method - karyotype detects balanced rearrangements but has low resolution; microarray detects copy-number changes but not balanced events; sequencing detects point variants and small indels but may miss large deletions and repeat expansions. Know trinucleotide repeat disorders, imprinting, and mitochondrial inheritance.
Traps in questions
A variant of uncertain significance must not be reported as pathogenic or used to change management. Somatic tumour variants require adequate tumour content and are reported against a reference transcript. Germline testing requires informed consent and consideration of incidental findings. Do not confuse analytical sensitivity with clinical validity. Remember that mosaicism may be missed in blood and require testing of a second tissue.
Rule of thumb
Match the test to the variant type: point variant = sequencing; copy-number change = microarray or MLPA; balanced rearrangement = karyotype or FISH; repeat expansion = specialised PCR/Southern blot.