Genetic Counseling Practice
What this domain tests
The CGC exam's Genetic Counseling Practice domain tests whether you can move from a clinical question to a defensible risk estimate and a counseling plan. Items combine pedigree analysis, inheritance logic, test selection, result interpretation, and psychosocial/ethical judgment. Expect vignettes with incomplete information; the task is to identify what is known, what must be asked, and which test or referral actually changes management.
Core rules and concepts
- Build a three-generation pedigree with standardized symbols; record ages, ancestry, consanguinity, affected status, carrier testing, and variants with classification.
- Match test to question: karyotype/CMA for copy-number, sequencing for point variants, deletion/duplication for exonic loss, cfDNA as screening not diagnosis.
- Use mode of inheritance plus penetrance, expressivity, anticipation, imprinting, mosaicism, and sex-limited expression.
- Treat VUS as uncertain; do not act on it as pathogenic or benign.
- Obtain informed consent before testing: purpose, limitations, possible results, incidental findings, cost, privacy.
- Address psychosocial needs, family communication, and confidentiality; know exceptions such as serious preventable harm.
Worked example
A woman's brother has cystic fibrosis; her parents are unaffected. Her partner has no family history and is of the same ancestry. What is the couple's risk of an affected child? Reasoning: CF is autosomal recessive. Unaffected parents of an affected child are obligate carriers, so each parent's carrier risk is 1. The woman is unaffected, so her possible genotypes are AA or Aa in a 1:2 ratio; P(carrier) = 2/3. Without testing, use population carrier frequency; if 1/25, partner risk = 1/25. If both carriers, each pregnancy has a 1/4 risk. Multiply: 2/3 x 1/25 x 1/4 = 1/150. Offer carrier testing to refine; if she tests negative, the residual risk depends on the test's detection rate, not zero.
Traps that cost marks
- Stopping at a dominant 50% risk without applying penetrance or the affected parent's tested status.
- Calling a VUS causative or using it to justify predictive testing.
- Forgetting that a negative screening test is not diagnostic.
- Missing consanguinity, ancestry-specific carrier frequencies, or a second affected relative.
- Ignoring the psychosocial plan, informed consent, or a duty-to-warn exception.