MRCOG Part 1: Genetics
Genetics is one of the four highest-yield subjects in MRCOG Part 1, with approximately 135 questions. Questions are highly specific and formula-driven — particularly around inheritance patterns, recurrence risks and chromosomal disorders. This is a subject where learning the rules pays dividends.
Topics Covered
Mendelian Inheritance
- ›Autosomal dominant — 50% recurrence, variable expressivity, incomplete penetrance
- ›Autosomal recessive — 25% affected offspring from carrier × carrier cross
- ›X-linked recessive — affected males, carrier females (no male-to-male transmission)
- ›X-linked dominant — rarer, examples include hypophosphataemia
- ›Codominance vs incomplete dominance — blood groups (ABO) as classic example
- ›Common AD conditions: Huntington's, achondroplasia, Marfan syndrome, BRCA1/2
- ›Common AR conditions: cystic fibrosis, PKU, sickle cell, haemochromatosis
Chromosomal Disorders
- ›Trisomy 21 (Down syndrome) — features, risk with maternal age, recurrence risk
- ›Trisomy 18 (Edwards) and Trisomy 13 (Patau) — features and prognosis
- ›Monosomy X (Turner syndrome) — 45,X, features, fertility implications
- ›Klinefelter syndrome (47,XXY) — features, infertility
- ›Translocation Down syndrome — 14;21 Robertsonian translocation, recurrence risk calculation
- ›Inversions, deletions, microdeletions (22q11 DiGeorge, 5p Cri du Chat)
- ›Mosaicism — mitotic error after fertilisation, milder phenotype
Molecular Genetics
- ›DNA structure, replication and repair mechanisms
- ›Transcription and translation — mRNA, tRNA, codon table
- ›Gene mutations — missense, nonsense, frameshift, splice-site
- ›DNA repair pathways — BRCA1/2 and homologous recombination
- ›Polymerase chain reaction (PCR) — principles and diagnostic applications
- ›Southern blot, FISH, array CGH — when each is used clinically
Population Genetics & Screening
- ›Hardy-Weinberg equilibrium — allele frequency calculations
- ›Carrier frequency calculation from disease incidence
- ›Down syndrome screening — first trimester combined (NT + PAPP-A + hCG)
- ›NIPT (cell-free fetal DNA) — sensitivity/specificity, what it screens for
- ›Imprinting — Prader-Willi (paternal deletion) vs Angelman (maternal deletion)
- ›Anticipation — trinucleotide repeats expanding with generations (myotonic dystrophy)
Exam Tips for Genetics
Practice risk calculations until they are automatic: recurrence risks for AR (25%), AD (50%), X-linked recessive (50% of sons).
Robertsonian translocations: the translocation Down risk depends on which parent carries it — know the carrier-sex difference.
Hardy-Weinberg: q² = disease frequency, q = carrier frequency × 2pq. Practice using simple numbers.
Imprinting is a guaranteed question type every few sittings — know Prader-Willi vs Angelman and why the same deletion gives different disease.
Anticipation and trinucleotide repeats: myotonic dystrophy, Fragile X and Huntington's are the key examples.
Recommended Book
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