Genetic Counseling Knowledge and Skills
Core knowledge to command
This domain rewards fluency across inheritance patterns (autosomal dominant and recessive, X-linked, mitochondrial, imprinting, trinucleotide repeat and anticipation, mosaicism), risk calculation, variant interpretation and classification, and test methodology (karyotype, FISH, microarray, sequencing panels, exome/genome, deletion/duplication analysis, carrier and prenatal screening, noninvasive prenatal testing). You should be able to read a pedigree, apply Bayesian reasoning to carrier risk, and explain positive and negative predictive value, sensitivity, and penetrance in plain language.
Traps to watch
Common errors include treating a variant of uncertain significance as pathogenic, confusing analytic validity with clinical utility, and misapplying Bayes by ignoring prior probability. Watch for questions that hinge on residual risk after a negative test (a negative result rarely reaches zero) and on the difference between carrier and affected. Another trap is assuming a condition is inherited when it is sporadic or de novo, or forgetting that reduced penetrance and variable expressivity change counseling for relatives. Prenatal items often test whether screening versus diagnostic testing is being described.
Worked rule of thumb
For an autosomal recessive condition, if both parents are known carriers, each child has a 1 in 4 chance of being affected, independent of prior children. If a healthy sibling of an affected child is tested and found to carry one pathogenic variant, the sibling's carrier status is established (assuming both parental variants are known); without testing, the sibling's prior carrier risk is 2 in 3. Always state the assumptions behind a calculation - unknown parental status or unidentified variants change the number.
Skill focus
Practice translating technical results into plain language and checking understanding, since many items ask for the most appropriate communication rather than the most precise statistic.