Chromosomes, Genes & Genome

1️⃣ Human Chromosome Count

• Somatic cells → 46 chromosomes (23 pairs) → Diploid (2n)
• Germ cells → 23 chromosomes → Haploid (n)

👉 Exam trigger: “Diploid vs Haploid” — always link somatic = 46, germ = 23.

2️⃣ Sex Determination

• XX = Female
• XY = Male
• Sex chromosomes = 23rd pair.

👉 Exam trigger: “Which chromosome determines sex?” → Y chromosome (presence = male).

3️⃣ Chromosome Structure

• DNA + histone proteins = Chromatin → Chromosome
• Centromere divides chromosome into:
• p arm = petite (short)
• q arm = long

👉 Exam trigger: p = short, q = long.

4️⃣ Types of Chromosomes (centromere position)

• Metacentric → equal arms
• Submetacentric → uneven arms
• Acrocentric → tiny p arm (13, 14, 15, 21, 22)

👉 High yield: acrocentric chromosome list (often asked).

5️⃣ Autosomes vs Sex Chromosomes

• Autosomes: first 22 pairs (1–22)
• Sex chromosomes: 1 pair (XX/XY)

👉 Exam trigger: “How many autosomes?” → 44 chromosomes.

6️⃣ What is a Gene?

• A segment of DNA coding for a protein.
• Human genome ≈ 20,000 genes.

👉 High yield: 20,000 = exam’s favourite number.

7️⃣ How Chromosomes Are Identified

• Length
• Banding pattern (G-banding(giemsa),Q- banding)

👉 “Banding = barcode” → unique ID.

🌟 SUPER-HIGH-YIELD SUMMARY (10 sec revision)

• 46 chromosomes in somatic → diploid
• 23 chromosomes in gametes → haploid
• XX female, XY male
• Centromere → p & q arms
• * Meta / Submeta / Acrocentric**
• Autosomes = 1–22, Sex = 23
• Gene = DNA → protein
• Genome ≈ 20,000 genes

CHROMOSOMES

1️⃣ Chromosomes – Absolute Essentials

• Somatic cells → 46 chromosomes (23 pairs), diploid (2n)
• Germ cells → 23 chromosomes, haploid (n)
• Autosomes = 22 pairs, Sex chromosomes = 1 pair (XX/XY)
• Centromere divides chromosome into p (short) & q (long) arms
• Chromosomes identified by length + banding pattern
• Types by centromere:
• Metacentric (equal)
• Submetacentric (unequal)
• Acrocentric (very short p arm)

👉 If you know these 6 points, you can answer 80% of chromosome questions.

2️⃣ Genes – High-Yield Core

• Chromosomes = DNA wrapped around histones
• Gene = DNA sequence that codes a protein
• Human genome ≈ 20,000 genes

👉 Most questions revolve around structure + number + definition.

3️⃣ Mitosis – High-Yield Marks

• Occurs in somatic cells
• Purpose: growth + repair
• DNA duplicates → cell divides → 2 identical diploid cells (46 each)
• No recombination
• Genetically identical daughters

👉 If they ask: “Does mitosis maintain chromosome number?” → YES.

4️⃣ Meiosis – High-Yield Marks

• Occurs only in germ cells
• Purpose:
• Maintain species number (46 → 23)
• Generate genetic diversity (recombination)
• Two divisions:
• Meiosis I → reduction division (46 → 23 duplicated)
• Meiosis II → mitosis-like (chromatids separate)
• End-product = 4 haploid (23) genetically unique gametes
• Sex-specific output:
• Female: 1 ovum + 3 polar bodies
• Male: 4 spermatozoa

👉 If a question involves recombination → always in Meiosis I, between homologous non-sister chromatids.

🧬 Cell Cycle — clear, exam-ready overview

What the cell cycle is

The cell cycle is the ordered sequence by which a cell grows, duplicates its DNA, and divides to form two identical daughter cells.

Main phases

1) Interphase (≈90% of cycle)

The cell prepares for division.

• G₁ (Gap 1) 10 hrs
• Cell growth, protein synthesis
• Organelles increase
• Decision point: divide or exit to G₀
• S (Synthesis) 9 hrs
• DNA replication (chromosomes duplicate → sister chromatids)
• Histones synthesized
• G₂ (Gap 2) 4 hrs
• Further growth
• DNA repair
• Preparation for mitosis (spindle proteins)

2) M phase (Mitosis + Cytokinesis)

Actual division happens.

• Prophase – chromosomes condense, spindle forms
• Metaphase – chromosomes align at equatorial plate
• Anaphase – sister chromatids separate
• Telophase – nuclear membranes reform
• Cytokinesis – cytoplasm divides → two cells

Checkpoints (quality control)

• G₁/S checkpoint (Restriction point)
• Cell size, nutrients, growth signals, DNA damage checked P53
• G₂/M checkpoint
• DNA fully replicated? DNA damage repaired?
• Spindle (Metaphase–Anaphase) checkpoint
• All chromosomes properly attached to spindle?

➡️ Failure to halt here → mutations, aneuploidy, cancer

Molecular control (high-yield)

• Cyclins: regulatory proteins (levels rise & fall)
• CDKs (Cyclin-dependent kinases): enzymes that drive phase transitions
• Cyclin–CDK complexes phosphorylate target proteins to move the cycle forward

Key pairs to remember

• Cyclin D–CDK4/6 → G₁ progression
• Cyclin E–CDK2 → G₁ → S transition
• Cyclin A–CDK2 → S phase
• Cyclin B–CDK1 → G₂ → M transition

Tumor suppressors & brakes

• p53 → DNA damage → ↑ p21 → CDK inhibition → cell cycle arrest
• RB protein → blocks G₁ → S transition until phosphorylated

Loss of these controls = uncontrolled proliferation

G₀ phase (exit lane)

• Cells that temporarily or permanently stop dividing
• Examples: neurons, skeletal muscle (permanent); hepatocytes (can re-enter)

Exam reflex lines (use these)

• “Interphase is the longest phase of the cell cycle.”
• “DNA replication occurs in S phase.”
• “Cyclin–CDK complexes regulate progression.”
• “p53 is the guardian of the genome.”
• “Loss of checkpoints predisposes to cancer.”

5️⃣ Chromosome Counts – The Golden Rules

• After mitosis: 46 → 46 + 46
• After meiosis I: 46 → 23 (duplicated) + 23 (duplicated)
• After meiosis II: 23 → four 23s (single chromatids)

👉 If they ask: “At which stage is the chromosome number halved?” → Meiosis I.

CHROMOSOMAL ABNORMALITIES

🧬 1. Aneuploidy & Nondisjunction

❓What is aneuploidy?

Answer:

Having too many or too few chromosomes (≠ 46 in a somatic cell).

• Trisomy = +1 chromosome (3 copies instead of 2)
• Monosomy = −1 chromosome (only 1 copy)

🧠 Analogy: Book set with 1 extra or 1 missing volume.

❓What causes aneuploidy?

Answer:

Mainly nondisjunction:

• Meiosis I → homologous chromosomes fail to separate
• Meiosis II / mitosis → sister chromatids fail to separate

Result → gametes with too many or too few chromosomes → trisomy/monosomy in zygote.

🔺 Risk increases with advanced maternal age.

🌟 2. High-Yield Autosomal Trisomies

❓What are the 3 important autosomal trisomies that survive to birth?

• Trisomy 21 – Down syndrome
• Trisomy 18 – Edwards syndrome
• Trisomy 13 – Patau syndrome

Most other trisomies/monosomies → do not survive to term.

🧩 Down Syndrome (Trisomy 21)

Cause:

• ~95%: nondisjunction of chromosome 21
• Others: Robertsonian translocation involving 21

Key features:

• Moderate learning difficulty
• Facial appearance: upslanting palpebral fissures, epicanthic folds, flat midface, brachycephaly
• ↑ risk of AV septal defects

🧩 Edwards (Trisomy 18) & Patau (Trisomy 13)

Both:

• Usually due to meiotic nondisjunction
• Often detected on prenatal screening/anomaly scan
• Very poor prognosis → high fetal loss, most babies die early; survivors have severe impairments & anomalies

Patau (13):

• Often linked with Robertsonian translocations involving 13.

Clinically-

• Noonan → increased nuchal thickness ,cystic hygroma, webbed neck, Pulmonay stenosis, ptosis
• Patau (Trisomy 13) → severe CNS defects, cleft lip/palate, holoprosencephaly
• Edwards (Trisomy 18) → clenched hands, rocker-bottom feet, growth restriction
• Turner syndrome → cystic hygroma, coarctation, renal anomalies like horseshoe kidney,left valve problems,streaky ovaries,webbed neck,can get pregnant with IVF

⚤ 3. Sex Chromosome Abnormalities

❓What are the key sex chromosome aneuploidies?

• 47,XXY → Klinefelter syndrome (male)
• 45,X → Turner syndrome (female)
• 47,XXX → Triple X (female)
• 47,XYY → XYY syndrome (male)

🧍‍♂️ Klinefelter Syndrome (47,XXY)

Cause:

• Extra X in a male → 47,XXY

Detection:

• Often found incidentally during:
• Prenatal testing (CVS/amniocentesis), or
• Infertility workup in adulthood
• Usually no big anomalies on scan, no ↑ NT.

🧍‍♀️ Turner Syndrome (45,X)

Key facts:

• Most common monosomy at birth
• Many affected pregnancies miscarry (often due to fetal hydrops)
• On first-trimester scan: NT > 4 mm → think Turner.

Cognition:

• IQ ~10–15 points lower than siblings, but most attend normal school.

Counseling if diagnosed in 1st trimester:

• Explain high risk of spontaneous loss, but some survive with typical Turner phenotype.
• web neck, coarctation + left valve issues, kidney horse shoes, Streaky ovaries

❓What about 47,XXX and 47,XYY?

• 47,XXX (Triple X) – female, usually mild/no symptoms, often found by chance.
• 47,XYY – male, often unnoticed, usually normal intelligence,long limbs

🔄 4. Translocations (Reciprocal & Robertsonian)

❓What is a reciprocal translocation?

Answer:

Exchange of segments between two non-homologous chromosomes.

• Balanced → all genetic material present (just rearranged) → usually no phenotype
• Unbalanced → extra/missing segments → trisomy/monosomy of parts → anomalies + learning difficulty

🧠 Analogy: Two books swapping pages. If all pages are still there → balanced. If some pages lost/duplicated → unbalanced.

❓What are the possible pregnancy outcomes if a parent has a balanced reciprocal translocation?

1. Normal karyotype, normal baby
2. Balanced translocation, healthy baby
3. Miscarriage due to unbalanced translocation
4. Child with anomalies/learning difficulties due to unbalanced translocation

Risk depends on:

• Which chromosomes are involved
• How big the translocated segments are

❓What is a Robertsonian translocation?

Answer:

Fusion of the long arms of two acrocentric chromosomes (13, 14, 15, 21, 22) with loss of the short arms.

• Short arm genes are duplicated elsewhere → balanced carrier = usually normal.

Common ones:

• rob(13q;14q) → risk of Patau (13)
• rob(14q;21q) → risk of Down (21)

❓What are the pregnancy outcomes with a balanced Robertsonian translocation?

• Normal child + normal karyotype
• Normal child + balanced translocation
• Miscarriage (unbalanced trisomy/monosomy, or imprinting problems)
• Liveborn child with trisomy syndrome (e.g. Down, Patau)
• Rarely: liveborn imprinting disorder

🧪 Risk is higher if mother is the carrier (~10–15%) than if father (<1%).

🧬 5. Imprinting & Uniparental Disomy (UPD)

❓What is genomic imprinting?

Answer:

Some genes are switched off (silenced) depending on whether they come from mother or father. Only one parental copy is active.

• Important on chromosomes 14 & 15.

❓What is uniparental disomy (UPD)?

Answer:

Both copies of a chromosome pair come from one parent.

• Often due to trisomy rescue: embryo starts with 3 copies → one copy is lost → but both remaining come from same parent.

❓Which classic syndromes are due to imprinting on chromosome 15?

• Angelman syndrome → loss of maternal gene (paternal imprinting pattern)
• Ataxia, severe learning difficulty, “happy” demeanour, distinct face
• Prader–Willi syndrome → loss of paternal gene (maternal imprinting pattern)
• Neonatal hypotonia, poor feeding → later hyperphagia, obesity, short stature

(Also UPD14 maternal vs paternal → different growth/neurological problems.)

✂️ 6. Microdeletions & aCGH

❓What are chromosome microdeletions?

Answer:

Very small missing pieces of a chromosome that can still cause major syndromes.

🧠 Analogy: Tiny tear in a blueprint that removes a critical detail → building fails.

❓How are microdeletions/duplications detected now?

Answer:

By microarray aCGH (array comparative genomic hybridisation).

• Detects both microdeletions (loss) and microduplications (gain).

❓Are all copy number changes harmful?

Answer:

No. They fall into 3 classes:

1. Benign copy number variants – harmless
2. Susceptibility loci / variants of uncertain significance – may increase risk, not clearly causal
3. Pathogenic variants – definitely disease-causing

GENE ABNORMALITIES

🧬 1. Core Idea – How Can a Gene “Go Wrong”?

❓ Big picture: How is gene function disrupted?

✅ By changing the DNA code so that the protein is wrong, incomplete, missing, or misregulated.

Main high-yield mechanisms you must know:

1. Missense mutation
2. Nonsense mutation
3. Frameshift mutation
4. Splice site mutation
5. Exon / whole gene deletion
6. Triplet repeat expansion

If you know these 6 + 3 classic triplet diseases, you’re safe for exams.

🔠 2. Point Mutations – Single Base Problems

❓What is a missense mutation?

➡️ Single base change → different amino acid in protein.

• Protein may still work, work less, or be harmful.

🧠 Analogy: Recipe says “2 spoons salt” instead of “2 spoons sugar”. Dish is changed but not necessarily cancelled.

❓What is a nonsense mutation?

➡️ Single base change → premature STOP codon

• Protein is cut short → usually non-functional.

🧠 Analogy: Movie suddenly ends halfway.

❓What is a frameshift mutation?

➡️ Insertion or deletion of 1–2 bases (not in 3s) → reading frame shifts.

• All codons after that are wrong → often early STOP.

🧠 Analogy:

“THE CAT ATE”

delete first T → “HEC ATA TE” → everything after is nonsense.

❓What are truncating mutations?

➡️ Mutations that make short, incomplete proteins:

• Nonsense
• Frameshift

KEY EXAM LINE:

Frameshift + nonsense = truncating → usually severe loss of function.

✂️ 3. Splicing & Deletions

❓What is a splice site mutation?

➡️ Mutation at exon–intron boundary → abnormal splicing.

• Wrong pieces kept or removed → abnormal protein.

🧠 Analogy: Film editor cuts the wrong parts of the movie.

❓What is an exon deletion?

➡️ One or more exons removed from a gene.

• Protein partly missing → partial loss of function.

❓What is a whole gene deletion?

➡️ Entire gene missing.

• No protein produced → complete loss of function.

🧠 Analogy:

• Exon deletion = missing chapters in a book.
• Whole gene deletion = whole book gone.

🔁 4. Triplet Repeat Expansions – “Too Much of a Good Thing”

❓What is a triplet repeat expansion?

➡️ A 3-base sequence (e.g., CAG, CGG) is repeated too many times → disrupts gene or protein.

High-yield diseases:

Just memorise: Fragile X – CGG, Huntington – CAG, Myotonic – CTG.

❓What 3 key features are shared by triplet repeat disorders?

1. Phenotypic variability
• Same mutation → different severity in different people.
2. Anticipation
• Disease worse and earlier in each generation because repeat length increases.
3. Parent-of-origin effect
• Severity depends on whether mutation came from mother or father
• e.g. Fragile X – worse if passed by mother.

🧠 Analogy: Each generation “adds more repeats to the word” until it completely destroys the sentence.

📊 5. One-Line Comparison: Chromosome vs Gene Abnormalities

Exam trick: “Microdeletion syndrome” = chromosome level; “missense in CFTR” = gene level.

• Chromosome abnormalities = big structural or number changes (trisomies, monosomies, translocations, microdeletions).
• Gene abnormalities = smaller changes inside a gene (point mutations, frameshifts, deletions, repeat expansions).

🧬 GENE ABNORMALITIES — SINGLE MASTER COMPARISON TABLE (ZERO-OMISSION)

🧠 INTEGRATED CLINICAL SCENARIOS — GENE ABNORMALITIES IN REAL LIFE

🧩 SCENARIO 1: The Newborn With Failure to Thrive

🏥 Presentation

A 3-week-old male infant presents with:

• Poor weight gain
• Recurrent chest infections
• Greasy, bulky stools
• Salty taste noticed by parents when kissing the baby

📌 Antenatal history unremarkable. Parents are consanguineous.

🧬 Genetic Investigation

Genetic testing reveals different mutations in the same gene in different patients with the same disease.

🔬 Mutation A: Missense Mutation

• Single base substitution
• Changes one amino acid in the protein
• Protein is produced but functions poorly

🧠 Clinical effect:

• Milder disease
• Some residual protein activity
• Later onset complications

📌 Classic exam idea:

“Patient has disease but protein is detectable at low levels.”

🔬 Mutation B: Nonsense Mutation

• Single base change → premature STOP codon
• Protein is truncated
• Rapid degradation → no functional protein

🧠 Clinical effect:

• Severe neonatal disease
• Early respiratory failure

📌 KEY LINK:

Nonsense = truncating mutation → severe loss of function.

🔬 Mutation C: Frameshift Mutation

• Insertion/deletion not in multiples of 3
• Entire downstream reading frame altered
• Early STOP codon generated

🧠 Clinical effect:

• Even more severe than missense
• Often lethal or very early presentation

📌 EXAM LINE (must write):

Frameshift mutations usually cause severe loss of function because all downstream amino acids are altered.

🧠 CONNECTION MADE

• Missense → partially working protein
• Nonsense + Frameshift → truncating → severe disease
• Same disease, different severity, depending on mutation type

✂️ SCENARIO 2: The Child With Unusual Protein Structure

🏥 Presentation

A 6-year-old child with:

• Developmental delay
• Abnormal protein detected on Western blot
• Normal gene size but abnormal mRNA

🔬 Mutation: Splice Site Mutation

• Mutation at exon–intron junction
• Abnormal splicing:
• Exon skipped
• Intron retained

🧠 Result:

• Protein with wrong internal sequence
• Length may appear “normal” but function is defective

🧠 Analogy (exam-friendly):

Movie scenes are present, but edited in the wrong order.

📌 EXAM TRICK:

“Normal gene length + abnormal protein” → think splice site mutation.

📖 SCENARIO 3: Partial vs Complete Loss — Two Siblings

👨‍👩‍👧 Family History

Two siblings with similar disease but different severity.

🔬 Sibling 1: Exon Deletion

• One or more exons missing
• Protein shorter but partially functional

🧠 Clinical effect:

• Milder disease
• Some preserved function

📌 Analogy:

Missing chapters, but book still readable.

🔬 Sibling 2: Whole Gene Deletion

• Entire gene absent
• No protein at all

🧠 Clinical effect:

• Severe disease
• Early onset
• Often incompatible with life if gene is essential

📌 Analogy:

Entire book missing.

🧠 CONNECTION

• Exon deletion → partial loss
• Whole gene deletion → complete loss

🔁 SCENARIO 4: The Multigenerational Family With Worsening Disease

🧓➡️👨➡️👦 Family Story

• Grandfather: mild symptoms at 50
• Father: symptoms at 35
• Son: symptoms at 18

Each generation:

• Earlier onset
• Worse severity

🔬 Mutation: Triplet Repeat Expansion

• Repetition of 3-base sequences
• Repeat length increases across generations

🧠 Three Core Features (EXAM GOLD)

1️⃣ Anticipation

→ Worse + earlier in next generation

2️⃣ Phenotypic variability

→ Same mutation, different severity

3️⃣ Parent-of-origin effect

→ Severity depends on transmitting parent

• Fragile X worse via mother
• Huntington often worse via father

📌 Analogy:

Each generation adds extra letters to a word until the sentence collapses.

🧠 SCENARIO 5: Exam Trap — Chromosome vs Gene

🧪 Question Stem

A child has:

• Developmental delay
• Dysmorphic features
• FISH shows a small missing segment on chromosome

➡️ Diagnosis: Microdeletion syndrome

📌 Key distinction:

🧠 EXAM RULE (WRITE THIS):

Microdeletion = chromosome-level

Missense in CFTR = gene-level

🧠 FINAL MASTER CONNECTION (ONE-LOOK MEMORY)

• Missense → wrong amino acid → partial function
• Nonsense → STOP → truncation → severe
• Frameshift → reading frame destroyed → severe
• Splice site → wrong mRNA editing → abnormal protein
• Exon deletion → partial loss
• Whole gene deletion → complete loss
• Triplet repeats → anticipation + variability + parent effect
• Chromosome vs gene → size of genetic damage

INHERITANCE

1️⃣ General Rules – The Foundation

❓How are genes inherited?

• Every gene comes in pairs → one copy from each parent.
• Genes encode proteins → when mutated, disease occurs.

❓What are the 4 classical inheritance patterns?

1. Autosomal Dominant
2. Autosomal Recessive
3. X-linked Recessive
4. X-linked Dominant

👉 If you can identify the pedigree pattern, you can solve 90% of questions.

2️⃣ Autosomal Dominant – “One Bad Gene Is Enough”

Key Rules:

• Only one mutated allele needed → 50% risk to each child.
• Both sexes equally affected.
• Seen in every generation.

Classic disorders:

• Marfan syndrome
• NF1
• ADPKD
• Huntington disease
• Tuberous sclerosis
• BRCA-related cancers

👉 Exam clue: Affected parent + vertical pattern.

3️⃣ Autosomal Recessive – “Silent Carriers”

Key Rules:

• Disease happens only if both alleles mutated.
• 25% affected,25%unaffected child, always50% carriers.
• Often skips generations.
• More common in consanguinity.

Classic disorders:

• Cystic fibrosis
• Spinal muscular atrophy
• CAH (21-hydroxylase deficiency)
• Tay–Sachs
• Haemochromatosis
• α1-antitrypsin deficiency
• alpha/Beta thalassemia

👉 Exam clue: A single affected child in a healthy family.

4️⃣ X-Linked Recessive – “Affected Sons, Carrier Mothers”

Key Rules:

• Males affected (only one X).
• Females = carriers (one healthy X compensates).
• NO male-to-male transmission (dad gives Y to sons).

Classic disorders:

• Duchenne muscular dystrophy
• Haemophilia A (8)
• X-linked adrenoleukodystrophy
• Ocular albinism

👉 Exam clue: Affected boys + healthy mother.

5️⃣ X-Linked Dominant – “Too Strong in Males”

Key Rules:

• One mutated X → disease in females.
• Often lethal in males → fewer/no affected boys.
• No male-to-male transmission.

Classic disorders:

• Rett syndrome
• Incontinentia pigmenti

👉 Exam clue: Affected females > males.

6️⃣ Mitochondrial Inheritance – “Mother Only”

Key Rules:

• Only mothers transmit (mitochondria from egg).
• Affects both sexes.

Classic disorders:

• ME-LA-S-Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes
• ME-RRF-myoclonic epilepsy with ragged red fibers
• Leigh syndrome

👉 Exam clue: All affected children have an affected mother.

7️⃣ Imprinting – “Which Parent Sent the Gene?”

Key Rules:

• One allele is silenced depending on mother or father.
• Disease depends on which parent’s copy is missing.

Key chromosome: 15

Classic disorders:

• Prader–Willi → loss of paternal
• Angelman → loss of maternal

👉 Exam clue: Parent-of-origin effect.

8️⃣ Triplet Repeat Disorders – “Anticipation”

Key features:

• Repeat expansions (e.g., CAG, CGG, CTG)
• Worsen + earlier onset in each generation

Classic examples:

• Huntington (CAG)
• Fragile X (CGG)
• Myotonic dystrophy (CTG)

👉 Exam clue: Disease more severe in next generation.

9️⃣ Prenatal Testing – The Core

Screening (NOT diagnostic):

• Nuchal translucency (NT)
• Maternal serum screening

Screens for:

• T21, T18, T13

Diagnostic tests:

• CVS (late 1st trimester)
• Amniocentesis (2nd trimester)

👉 Exam clue: High-risk screening → offer CVS/amnio.

🧬 MASTER CLINICAL SCENARIO — “The Genetics Clinic Day”

🧑‍⚕️ SCENARIO 1: The Referral (General Rules in Action)

A 26-year-old primigravida attends the antenatal genetics clinic at 11+5 weeks gestation after her booking ultrasound.

She is referred because:

• Her husband’s brother died young from a neurological illness.
• Her cousin has developmental delay.
• Her mother has cardiomyopathy.
• She herself is completely asymptomatic.

👉 First principle applied

Every disease here must be explained by genes inherited in pairs, encoding proteins, with disease occurring when mutations disrupt function.

Your job:

👉 Identify the inheritance pattern from the pedigree before touching any lab test.

🧬 SCENARIO 2: Vertical Transmission Appears (Autosomal Dominant)

On drawing the pedigree, you notice:

• The patient’s mother, maternal uncle, and maternal grandfather all had:
• Tall stature
• Long limbs
• Cardiac issues
• The disease is present in every generation
• Both males and females affected

🔍 Interpretation

This fits AUTOSOMAL DOMINANT inheritance:

• Only one mutated allele required
• 50% risk to each child
• Vertical transmission
• Sexes equally affected

🧠 Likely diagnoses (classic AD list activated):

• Marfan syndrome
• NF1
• ADPKD
• Huntington disease
• Tuberous sclerosis
• BRCA-related cancers

👉 Exam trigger phrase:

“Affected parent + disease in every generation”

🧬 SCENARIO 3: A Healthy Couple, One Sick Child (Autosomal Recessive)

The patient then mentions:

“Doctor, my sister has a child with salt-wasting crisis at birth. Parents are normal.”

Details:

• No prior family history
• One affected child
• Parents are healthy
• Parents are first cousins

🔍 Interpretation

This is AUTOSOMAL RECESSIVE:

• Disease only if both alleles mutated
• 25% affected, 50% carriers
• Skips generations
• Consanguinity increases risk

🧠 Classic AR diagnoses:

• Cystic fibrosis
• Spinal muscular atrophy
• Congenital adrenal hyperplasia (21-hydroxylase deficiency)
• Tay–Sachs
• Haemochromatosis
• α1-antitrypsin deficiency

👉 Exam trigger phrase:

“Single affected child born to healthy parents”

🧬 SCENARIO 4: Only Boys Are Affected (X-Linked Recessive)

Next, the husband’s family history:

• His maternal uncle died in childhood from muscle weakness
• His mother is healthy
• Only boys affected
• No father-to-son transmission

🔍 Interpretation

This is X-LINKED RECESSIVE:

• Males affected (one X)
• Females are carriers
• No male-to-male transmission
• Passed via carrier mothers

🧠 Classic XLR disorders:

• Duchenne muscular dystrophy
• Haemophilia A
• X-linked adrenoleukodystrophy
• Ocular albinism

👉 Exam trigger phrase:

“Affected boys + healthy mother”

🧬 SCENARIO 5: A Disorder Lethal in Males (X-Linked Dominant)

The patient recalls a distant relative:

• Multiple affected females
• Very few surviving males
• Some male infants died in early life
• Disease never passed from father to son

🔍 Interpretation

This is X-LINKED DOMINANT:

• One mutant X causes disease
• Females affected
• Often lethal in males
• No male-to-male transmission

🧠 Classic XLD disorders:

• Rett syndrome
• Incontinentia pigmenti

👉 Exam trigger phrase:

“Affected females >> males”

🧬 SCENARIO 6: Mother-Only Transmission (Mitochondrial)

The patient adds:

“Doctor, in my mother’s family, all children of affected women are affected, but affected men have normal children.”

Symptoms include:

• Seizures
• Myopathy
• Lactic acidosis
• Early neurological deterioration

🔍 Interpretation

This is MITOCHONDRIAL inheritance:

• Only mothers transmit
• Both sexes affected
• Mitochondria come from the egg

🧠 Classic mitochondrial disorders:

• MELAS
• MERRF
• Leigh syndrome

👉 Exam trigger phrase:

“All affected children have an affected mother”

🧬 SCENARIO 7: Same Chromosome, Different Disease (Imprinting)

A genetic test shows a deletion on chromosome 15.

Two cousins have:

• One with hyperphagia, obesity, hypotonia
• Another with severe intellectual disability and laughter

🔍 Interpretation

This is GENOMIC IMPRINTING:

• One allele is silenced depending on parent
• Disease depends on parent of origin

🧠 Classic imprinting disorders:

• Prader–Willi → loss of paternal allele
• Angelman → loss of maternal allele

👉 Exam trigger phrase:

“Same deletion, different disease → parent matters”

🧬 SCENARIO 8: Disease Gets Worse Every Generation (Triplet Repeats)

Pedigree shows:

• Grandfather: late-onset disease
• Father: earlier onset
• Child: severe early disease

🔍 Interpretation

This is TRIPLET REPEAT DISORDER with anticipation:

• Repeat expansion (CAG / CGG / CTG)
• Earlier onset
• Increased severity each generation

🧠 Classic examples:

• Huntington disease — CAG
• Fragile X — CGG
• Myotonic dystrophy — CTG

👉 Exam trigger phrase:

“Earlier + worse in next generation”

🧬 SCENARIO 9: Prenatal Testing Decision (Screening vs Diagnostic)

At 12 weeks, NT scan is increased.

🧪 Screening (NOT diagnostic):

• Nuchal translucency
• Maternal serum screening
• Screens for:
• T21
• T18
• T13

🧬 Diagnostic confirmation:

• CVS → late 1st trimester
• Amniocentesis → 2nd trimester

👉 Exam trigger phrase:

“High-risk screening → offer CVS or amniocentesis”

🧠 FINAL EXAM LOCK

If you:

1. Identify the pedigree pattern
2. Match it to inheritance rules
3. Select the correct disorder
4. Choose screening vs diagnostic correctly

👉 You will solve >90% of genetics questions.

If you want next:

• One single SBA exam question built from this
• A pedigree-only rapid recognition drill
• Or a table to memorize in 5 minutes

Just say the word.

SCREENING TESTS

1️⃣ Nuchal Translucency (NT) – Core Idea

❓What is NT?

• Fluid space behind fetal neck seen on US at 11–14 weeks.

❓Why do we care if NT is increased?

↑ NT = ↑ risk of:

• Chromosomal abnormalities – esp. T21, T18, T13
• Cardiac defects
• Single-gene disorders – e.g. Noonan (PTPN11)
• Skeletal dysplasias

Performance (for T21):

• NT alone → detects ~70–75%
• False positive ~5%

👉 Key exam line:

NT is a screening marker for trisomies + structural/genetic problems, best used 11–14 weeks.

2️⃣ Combined First-Trimester Test – The Workhorse

❓What is in the “combined test”?

• NT thickness
• Maternal serum PAPP-A ↓ in T21
• Maternal serum β-hCG ↑ in T21

Detection:

• About 90% for Trisomy 21

👉 Think: NT + (PAPP-A↓ + β-hCG↑) → 90% detection

3️⃣ Extra First-Trimester Markers (High-Yield Names Only)

Used as add-ons, technically demanding:

• Nasal bone: absent / hypoplastic → raises suspicion of T21
• Ductus venosus A-wave: absent / reversed → raises suspicion of T21

Each alone gives ~80–85% detection when used properly, but not routine everywhere due to difficulty.

4️⃣ Second Trimester Screening – Triple vs Quad

Triple test:

• α-FP ↓, β-hCG ↑, uE3 ↓
• Detection for T21 ≈ 70%

Quadruple test:

• Triple + Inhibin A ↑
• Detection ≈ 81%

👉 Easy memory:

• More markers → slightly better detection (Triple 70%, Quad 81%).

5️⃣ Integrated & Sequential Strategies

• Integrated test
• Uses 1st (NT + PAPP-A) + 2nd trimester (quad) together
• Results only given after 2nd trimester
• Detection ≈ 86%
• Sequential test
• Give 1st trimester result (NT + PAPP-A)
• If high risk → proceed with 2nd trimester test
• Detection ≈ 95% ✅ (best overall)

👉 High-yield line:

Sequential screening gives the highest detection (~95%) for T21.

6️⃣ Cell-Free Fetal DNA (cffDNA) – The Modern Game-Changer

❓What is cffDNA?

• Fetal DNA fragments in maternal blood, released from placenta.

Currently can detect:

• Fetal sex
• Rhesus D status
• Trisomies 13, 18, 21

How?

• Uses NGS to count DNA fragments; extra chr21 reads → T21 risk.

Future direction:

• Subchromosomal deletions, microduplications, gene mutations – all non-invasive from maternal blood.

👉 Exam phrase:

cffDNA is a highly sensitive screening test, not diagnostic.

7️⃣ One-Glance Detection Table (T21)

• NT only → 70–75%
• Combined (NT + PAPP-A↓ + β-hCG↑) → 90%
• Triple → 70%
• Quad → 81%
• Integrated → 86%
• Sequential → 95%

Antenatal Screening Tests for Trisomy 21 (Down Syndrome) — Complete Comparison Table

Ultra-High-Yield Exam Lock 🔒

• Best overall detection (non-diagnostic): Sequential screening (~95%)
• Best modern screening test: cffDNA (NIPT)
• NT timing: 11–14 weeks ONLY
• All are screening tests → diagnosis requires CVS / amniocentesis

CLINICAL SCINARIO

A 30-year-old woman (G2P1) comes to antenatal clinic at 10+6 weeks. Her first baby was normal. This pregnancy was unplanned, and she’s anxious because a cousin has a child with Down syndrome. She asks, “Doc, can we check the baby early without doing invasive tests?”

Visit 1: 11–14 weeks scan booking (the “first big fork in the road”)

At 12+4 weeks, she comes for the dating scan.

The sonographer explains Nuchal Translucency (NT)

On the screen, the sonographer shows a thin black fluid space behind the fetal neck.

• “This is NT—we measure it only in the 11–14 week window.”
• She’s told: increased NT doesn’t mean “Down syndrome only.” It flags higher risk of:
• Chromosomal abnormalities: T21, T18, T13
• Cardiac defects
• Single-gene disorders (example the doctor mentions: Noonan syndrome, classically linked with PTPN11)
• Skeletal dysplasias

The doctor adds an exam-style line in plain language:

• “NT is a screening marker for trisomies and other structural/genetic problems—best used at 11–14 weeks.”

“If we do NT alone, how good is it?”

She’s told:

• NT alone picks up about 70–75% of T21, with about 5% false positives

So it’s useful, but not the strongest option by itself.

The doctor offers the Combined First-Trimester Test (the workhorse)

Because she is already in the 11–14 week window, the doctor offers the standard combined approach:

What’s included

1. NT thickness (ultrasound)
2. Maternal blood markers:

• PAPP-A (tends to be low in T21)
• β-hCG (tends to be high in T21)

The doctor sums it up the way examiners like:

• “NT + (PAPP-A↓ + β-hCG↑) → ~90% detection for T21.”

She agrees and gives blood the same day.

Optional “extra first-trimester markers” (high-yield add-ons)

While scanning, the fetal medicine unit mentions two add-ons (not always routine everywhere because they’re more operator-dependent):

• Nasal bone: if absent/hypoplastic, suspicion for T21 rises
• Ductus venosus A-wave: absent or reversed raises suspicion for T21

They explain: these are “extra markers” used as add-ons; each can give roughly 80–85% detection when used properly, but they’re not routine everywhere due to technical difficulty.

Results come back: “Screen positive” (but the doctor keeps it clear: screening ≠ diagnosis)

Her report says the combined test shows high risk for T21.

The doctor is careful:

• “This is screening—it estimates risk. It does not prove the baby has T21.”

Now the patient asks: “What’s the best next step if we want high accuracy but still avoid invasive testing?”

The modern next step: Cell-Free Fetal DNA (cffDNA / NIPT)

At 13 weeks, the doctor offers cffDNA (maternal blood test):

What it is

• Placenta releases fetal DNA fragments into maternal blood (cffDNA).

What it can detect (currently)

• Fetal sex
• Rhesus D status
• Trisomies 13, 18, 21

How it works (simple explanation)

• Uses NGS counting: if there are extra chromosome 21 fragments, the risk for T21 becomes high.

Key exam line the doctor says clearly

• “cffDNA is highly sensitive screening, but it is not diagnostic.”

She chooses cffDNA.

Alternative pathway (if she had presented later): Second trimester serum screening

The doctor also explains what would happen if she missed the first-trimester window and came at 15–20 weeks.

Triple test (second trimester)

Markers pattern for T21:

• AFP ↓
• β-hCG ↑
• uE3 ↓(unconjugated estriol)

Detection for T21: ~70%

Quadruple test

• Triple plus Inhibin A ↑

Detection: ~81%

The doctor’s “easy memory” line:

• “More markers → slightly better detection (Triple ~70%, Quad ~81%).”

Bigger strategies: Integrated vs Sequential (how clinics design the whole screening program)

Because she’s a “planner,” she asks: “Doc, what’s the best overall strategy if the clinic is using multi-step screening?”

The doctor explains:

Integrated screening

• Combines:
• 1st trimester: NT + PAPP-A
• 2nd trimester: quad
• One final result only after the 2nd trimester
• Detection about ~86%

Sequential screening

• You get a 1st trimester result (NT + PAPP-A) early
• If high risk, you proceed to 2nd trimester testing / next steps
• Detection about ~95% (best overall)

High-yield line:

• “Sequential screening gives the highest detection (~95%) for T21.”

One-glance recap the doctor writes for her in the clinic book (T21 detection)

• NT only: 70–75%
• Combined (NT + PAPP-A↓ + β-hCG↑): ~90%
• Triple: ~70%
• Quad: ~81%
• Integrated: ~86%
• Sequential: ~95% ✅

Ending the case (bringing it back to real clinic flow)

Her cffDNA returns as low risk for T21/T18/T13. The doctor reassures her again:

• “This is very reassuring as a screening test. We still continue routine anomaly scanning.”

Because increased NT can also link to cardiac defects, the doctor also plans:

• a careful fetal anatomy scan, and if NT was truly high, consideration of fetal echocardiography (because NT elevation isn’t only about trisomies).

She leaves understanding the key idea:

• NT and serum tests are risk screens
• cffDNA is a newer, very sensitive screen
• and none of these are “diagnosis” by themselves—diagnosis needs invasive testing if ever indicated.

DIAGNOSTIC TESTS

1️⃣ When do we offer diagnostic tests? (core triggers)

• High-risk screening result
• Abnormal scan findings
• Advanced maternal age
• Family history of chromosomal/genetic disease

👉 If screening → abnormal OR scan → abnormal → offer CVS/Amnio.

2️⃣ Chorionic Villus Sampling (CVS)

When: ≥ 11 weeks (NEVER before → limb defects)

What sampled: Placental tissue (cytotrophoblast + mesenchyme)

Results:

• Cytotrophoblast → 2–3 days (rapid)
• Mesenchymal core → 1–3 weeks (confirmatory)

Key exam line:

CVS gives fastest results but has risk of confined placental mosaicism → abnormal placenta, normal fetus.

Miscarriage risk: ~ 1%

3️⃣ Amniocentesis

When: ≥ 15 weeks

What sampled: Amniotic fluid + fetal cells

Results: 1–3 weeks (needs culture; no rapid test)

Key exam line:

Amniocentesis reflects true fetal karyotype better than CVS.

Miscarriage risk: ~ 1% (same as CVS in experienced hands)

4️⃣ Cordocentesis (PUBS)

When: ≥ 18 weeks

What sampled: Fetal blood from umbilical vein

Used for:

• Fetal anemia
• Infections
• Rapid karyotype

Key downside:

Highest miscarriage risk (1–2%) + technically difficult.

5️⃣ High-Yield 3-Line Comparison

🌟 If you remember only this logic:

• CVS = Early + Fast + Placenta + Mosaicism
• Amnio = Later + Accurate + Fetal cells
• Cordocentesis = Late + Risky + Fetal blood (anemia/infection)

…you will answer 80% of all prenatal diagnostic exam questions

CLINICAL SCINARIO

A 36-year-old primigravida comes to clinic at 10+5 weeks.

1) Why she gets offered diagnostic testing (core triggers)

She has two triggers:

1. High-risk screening result

• Her combined first-trimester screen comes back high-risk for trisomy 21.

1. Abnormal scan finding

• The dating/NT scan also shows an increased nuchal translucency (abnormal scan finding).

Because screening is abnormal OR scan is abnormal, you counsel her that she should be offered an invasive diagnostic test (rather than only repeating screening).

You also mention other standard reasons diagnostic tests are offered in general: advanced maternal age (she’s 36) and family history of chromosomal/genetic disease (you ask and document it—even if negative, it’s one of the classic triggers).

2) The early option: CVS (and the key “NEVER before” rule)

She is anxious and says, “I want the fastest definite answer.”

You explain:

• CVS timing: can be done at ≥ 11 weeks
• NEVER before 11 weeks because earlier CVS is associated with limb defects (exam-critical rule).
• What CVS samples: placental tissue (specifically cytotrophoblast + mesenchyme).

What happens after CVS (results timeline)

You describe the two “layers” of results:

• Cytotrophoblast gives a rapid result in 2–3 days.
• Mesenchymal core is the confirmatory culture, taking 1–3 weeks.

The key counselling line

You warn her about the signature CVS limitation:

• CVS is fastest, but because it samples placenta, there is a risk of confined placental mosaicism → abnormal placenta, normal fetus.

Risk

You quote the expected miscarriage risk:

• Miscarriage risk ~1%.

Clinical decision point:

At 11+2 weeks, she chooses CVS because she wants early, fast certainty.

3) The “what if CVS is confusing?” turn (mosaicism → amnio to confirm)

At 11+5 weeks, the rapid cytotrophoblast result returns as “mosaic pattern / possible mosaicism.”

You explain calmly:

• This could be confined placental mosaicism (placenta abnormal, fetus normal).
• So, an amniocentesis may be recommended later to reflect the true fetal karyotype more reliably.

4) The later, more fetal-representative option: Amniocentesis

You schedule follow-up counselling and she returns at 15+2 weeks.

You explain:

• Amniocentesis timing: ≥ 15 weeks.
• What it samples: amniotic fluid + fetal cells.

Result speed (and the “no rapid test” line)

• Results typically take 1–3 weeks because fetal cells need culture.
• No rapid test is the exam line you emphasize here (in your note: “requires culture → 1–3 weeks”).

Key exam line

You tell her:

• Amniocentesis reflects true fetal karyotype better than CVS.

Risk

• Miscarriage risk ~1% (and you add: similar to CVS in experienced hands).

Clinical decision point:

She chooses amniocentesis to clarify the CVS mosaic result.

5) The late/high-stakes option: Cordocentesis (PUBS) when you need fetal blood

At 19 weeks, she is referred urgently because the anomaly scan plus Dopplers suggest possible fetal anemia, and there is also concern about infection (you explain both are classic reasons you’d want direct fetal blood information).

You counsel:

• Cordocentesis (PUBS) timing: ≥ 18 weeks.
• What it samples: fetal blood from the umbilical vein.

What it’s used for (must-say indications)

• Fetal anemia
• Infections
• Rapid karyotype (because you can test directly from fetal blood when needed)

Key downside (must-say)

You document the counselling line:

• Highest miscarriage risk (1–2%) and it is technically difficult.

Clinical decision point:

Because the clinical question is now specifically anemia/infection, cordocentesis becomes the most targeted tool despite higher risk—so the team offers it when it changes management.

6) How you summarize it to her in one “exam safe” wrap-up (your 3-line logic)

You finish the counselling session exactly like this:

• CVS = Early (≥11 wks) + Fast (2–3 days rapid) + Placenta + Mosaicism risk + ~1% miscarriage; never before 11 weeks (limb defects).
• Amnio = Later (≥15 wks) + Amniotic fluid/fetal cells + 1–3 weeks culture + best reflection of fetal karyotype + ~1% miscarriage.
• Cordocentesis = Late (≥18 wks) + Fetal blood (umbilical vein) + used for anemia/infection/rapid karyotype + highest risk (1–2%) + technically difficult.

🔥 Molecular & Cytogenetic Prenatal Diagnosis

1️⃣ Karyotyping (Conventional Cytogenetics) – “Full Chromosome Picture”

What it does:

• Grows fetal cells → stops them in mitosis → stains chromosomes → views number + structure.

What it detects:

• Aneuploidy
• Large deletions/duplications
• Translocations (balanced + unbalanced)

Limitations:

• Needs cell culture → slow
• Cannot detect small microdeletions

👉 Exam clue: Best for whole chromosome abnormalities.

2️⃣ FISH – “Fluorescent Targeted Check”

Key idea:

• Fluorescent DNA probe binds to specific chromosome region.

Detects:

• Aneuploidy (extra/missing signals)
• Microdeletions
• Translocations
• Marker chromosomes

Strength:

• Fast, targeted, works on interphase cells

👉 Exam clue: Use FISH when you know what region you’re looking for.

3️⃣ QF-PCR – “Rapid Aneuploidy Test”

Tests for:

• Trisomy 13, 18, 21, and sex chromosomes only

Strength:

• Very fast (1–2 days)
• Cheap

Limitation:

• Cannot detect structural rearrangements
• Only covers 4 chromosome groups

👉 Exam clue: First-line rapid test for common trisomies.

4️⃣ MLPA – “Copy Number Detector”

What it does:

• Uses probes + PCR amplification to measure dosage.

Detects:

• Deletions (0.5)
• Duplications (1.5)
• Some unbalanced translocations

Best for:

• Gene-level CNVs (e.g., Duchenne deletions)

👉 Exam clue: When asked about gene deletions/duplications, answer MLPA.

5️⃣ Array-CGH – “Genome-Wide Dosage Map”

How it works:

• Test DNA vs control DNA hybridized on chip → color shift shows gain/loss.

Detects:

• Microdeletions
• Microduplications
• Aneuploidy
• Genome-wide copy number changes

Limitations:

• Cannot detect balanced translocations or inversions

👉 Exam clue: Best test for unexplained anomalies, microdeletion syndromes.

6️⃣ DNA Sequencing – “Reading the Code”

Two types:

• Sanger sequencing → small targeted regions
• NGS → multi-gene panels / exome / whole genome

Detects:

• Single-gene mutations
• Sequence variants

Limitation:

• Variant interpretation can be difficult.

👉 Exam clue: For monogenic disorders, choose sequencing (Sanger or NGS).

7️⃣ PGD (Preimplantation Genetic Diagnosis) – “Test Before Pregnancy Starts”

When used:

• Couples at high risk of inherited disease using IVF.

Method:

• IVF → ICSI → biopsy 1–2 cells from 8-cell embryo → genetic test → transfer only unaffected embryos.

👉 Exam clue: Only available via IVF + embryo biopsy, regulated by HFEA (UK).

Elaborative integrated clinical scenario: “One pregnancy, many tests, each for a reason”

A 31-year-old primigravida comes at 12+4 weeks for her booking scan.

Step 0 — Why she’s “high-yield complicated”

• Combined screening comes back high risk for trisomy 21.
• On scan, the sonographer also notes:
• Increased nuchal translucency
• A possible conotruncal cardiac defect (needs fetal echo)
• History reveals:
• Her brother had Duchenne muscular dystrophy.
• Her husband’s sister has a child with 22q11.2 deletion syndrome.
• They also say: “We may consider IVF if we can avoid an affected pregnancy.”

Now you must choose the right test at the right time—and explain what each test contributes.

1️⃣ QF-PCR(Quantitative Fluorescent PCR): the rapid “common trisomies” answer (fast triage)

Because the screening suggests a common aneuploidy risk, the couple opts for CVS at 12+6 weeks.

The lab offers a rapid result first:

✅ QF-PCR is done immediately

• What it tests for: Trisomy 13, 18, 21 + sex chromosomes only
• Why it’s chosen here: It’s very fast (1–2 days) and cheap, ideal when you want a quick answer for the common trisomies.
• What it cannot do: It cannot detect structural rearrangements and it only covers those 4 chromosome groups.

Result (day 2): QF-PCR shows trisomy 21 pattern.

👉 This gives fast direction, but the team still completes the full chromosomal workup because you also saw anomalies and there may be structural issues.

2️⃣ Karyotyping: the “full chromosome picture” confirmation + structure check

Even though QF-PCR already strongly suggests T21, the lab proceeds to conventional karyotyping.

✅ Karyotyping is ordered

• How it works (exam wording): fetal cells are cultured, arrested in mitosis, stained, and chromosomes are examined for number + structure.
• What it detects best:
• Aneuploidy
• Large deletions/duplications
• Translocations (balanced + unbalanced)
• Key limitation: Needs cell culture → slow, and it cannot detect small microdeletions.

Result (about 10–14 days): Karyotype confirms 47,XX,+21.

👉 Now counseling changes: this is confirmed Down syndrome, but the scan also raised the question of other submicroscopic problems (especially with the heart finding), so the team discusses microdeletion-level testing too—because a normal karyotype would not exclude those, and even in an aneuploidy case, families sometimes request a fuller explanation if ultrasound findings are atypical.

3️⃣ FISH: “fluorescent targeted check” when you know what you’re looking for

Because the fetal echo suggests a conotruncal defect and there’s family concern about 22q11.2 deletion, the geneticist says:

“If we suspect a specific region like 22q11.2, use a targeted test.”

✅ FISH is performed (targeted probe)

• Core idea: a fluorescent probe binds to a specific chromosome region.
• What it can detect:
• Aneuploidy (extra/missing signals)
• Microdeletions
• Translocations
• Marker chromosomes
• Strength: Fast, targeted, and can work on interphase cells (no need to wait for metaphase culture).
• Limitation: only detects what you probe for—you must suspect the region.

Example outcome for learning: FISH for 22q11.2 returns normal signals → no deletion detected by that probe.

👉 This shows the classic role of FISH: quick confirmation/exclusion of a specific suspected region.

4️⃣ Array-CGH: “genome-wide dosage map” for unexplained anomalies

Now imagine an alternative branch (very exam-realistic):

Alternate scan pathway

Instead of a clear trisomy result, suppose QF-PCR is normal and karyotype is normal, but the fetus still has:

• multiple congenital anomalies on the anomaly scan (e.g., heart + renal + facial markers), and
• no single syndrome is obvious.

The genetics team then chooses:

✅ Array-CGH

• How it works: patient DNA vs control DNA hybridised to a chip → a signal change shows gain/loss.
• Detects:
• Microdeletions
• Microduplications
• Aneuploidy
• Genome-wide copy number changes
• Key limitation: cannot detect balanced translocations or inversions (because copy number is unchanged).

👉 Exam clue use-case: “unexplained anomalies” or “microdeletion syndrome suspected but not sure which one” → Array-CGH.

5️⃣ MLPA: the “copy number detector” for gene-level deletions/duplications

Back to this couple’s family history: her brother had Duchenne muscular dystrophy.

The genetic counselor says:

“Duchenne is often due to exon deletions/duplications in the DMD gene—use a dosage method.”

✅ MLPA is selected for the DMD gene

• What it does: probe-based amplification measures dosage.
• Detects best:
• Deletions (0.5 dosage)
• Duplications (1.5 dosage)
• Some unbalanced translocations
• Best for: gene-level CNVs (classic example: Duchenne deletions)
• Limitation: not genome-wide; you’re checking a defined set of targets.

Outcome (example): Mother is found to be a carrier of a DMD exon deletion → future pregnancies need targeted testing early.

👉 This locks the MLPA role perfectly: when the question is “gene deletions/duplications” → MLPA.

6️⃣ DNA sequencing: “reading the code” for single-gene disorders

The couple also mentions a cousin with a suspected monogenic disorder (e.g., a metabolic disease) where the mutation is a single base change—not a deletion/duplication.

The genetics team explains:

✅ Sequencing is used when the problem is at “letter level”

• Types:
• Sanger sequencing: small targeted region
• NGS: multi-gene panel / exome / whole genome
• Detects: single-gene mutations / sequence variants
• Limitation: variants can be difficult to interpret (variant of uncertain significance).

👉 Exam clue: “monogenic disorder” → sequencing (Sanger if one gene/known mutation; NGS if many genes/unknown).

7️⃣ PGD: “test before pregnancy starts” (IVF + embryo biopsy)

After counseling, the couple says:

“We want to avoid an affected pregnancy from the start—especially for Duchenne.”

They choose IVF.

✅ PGD (Preimplantation Genetic Diagnosis) pathway

• When used: couples at high risk of inherited disease, using IVF.
• Method (exam steps):
• IVF → often ICSI
• biopsy 1–2 cells from an 8-cell embryo
• perform the selected genetic test (e.g., MLPA/targeted sequencing/PGT methods depending on the mutation)
• transfer only unaffected embryos
• Exam clue: requires IVF + embryo biopsy, and in the UK it is regulated by HFEA.

👉 This is the prevention end of the story: instead of diagnosing an established pregnancy, you select embryos before implantation.

The entire case in one tight “decision logic” (no omissions)

• QF-PCR: first rapid answer for T13/18/21 + sex chromosomes only; fast/cheap; not structural.
• Karyotype: confirm and fully visualize whole chromosomes; detects aneuploidy + large rearrangements + translocations (balanced/unbalanced); slow and misses microdeletions.
• FISH: targeted fluorescent probe for a known region; detects aneuploidy/microdeletions/translocations/marker chromosomes; fast, can use interphase; only finds what you look for.
• MLPA: dosage for gene-level deletions/duplications (classic: DMD); can show 0.5/1.5 patterns; not genome-wide.
• Array-CGH: genome-wide copy number gains/losses; great for unexplained anomalies + microdeletion syndromes; cannot detect balanced rearrangements.
• Sequencing (Sanger/NGS): detects sequence variants for monogenic disorders; interpretation can be hard.
• PGD: IVF + embryo biopsy testing before pregnancy truly begins, transfer only unaffected embryos; regulated (UK: HFEA).

✔️ If you remember ONLY this:

• Karyotype = whole chromosomes
• FISH = targeted glowing probe
• QF-PCR = rapid trisomy test
• MLPA = gene dosage
• Array-CGH = genome-wide copy number
• NGS = single-gene sequencing
• PGD = IVF embryo testing

…you will answer most molecular/cytogenetic prenatal exam questions correctly.

🔥 Gynecological Cancer Genetics

1️⃣ BRCA1 & BRCA2 — MOST IMPORTANT

Memorize this:

• BRCA1 → higher ovarian risk
• BRCA2 → lower ovarian risk

Lifetime risks:

Key exam points:

• <2% of ALL breast cancers are due to BRCA1/2
• BRCA mutations cause a small proportion of ovarian cancers
• BRCA carriers benefit from prophylactic BSO after childbearing →

✔️ Almost eliminates ovarian cancer (residual 1% peritoneal risk)

✔️ Reduces breast cancer by 50%

❌ Causes abrupt menopause

2️⃣ Lynch Syndrome (HNPCC) — SECOND MOST IMPORTANT

Caused by mutations in:

• MLH1, MSH2, MSH6, PMS2

Key cancer risks:

• Endometrial cancer risk = up to 50% (VERY high)
• Ovarian cancer risk ≈ 4% (2–3× population)

Hallmark:

• Increased risk of colon, endometrial, gastric, ovarian cancers.

Management:

• No perfect endometrial screening → rely on:
• PMB evaluation
• Period pattern changes
• Consider risk-reducing hysterectomy after childbearing

3️⃣ Ovarian Cancer — Essential Stats

• General population lifetime risk = 1 in 70
• Only a small inherited fraction, mostly from:

✔️ BRCA1/2

✔️ Mismatch repair genes (Lynch)

Screening limitations:

• CA-125 + pelvic ultrasound = poor sensitivity/specificity
• Many false positives & negatives

4️⃣ Other Syndromes — Minimal Facts Needed

Peutz–Jegher Syndrome (LKB1 mutation)

• Features:

✔️ Pigmented mucosa

✔️ GI polyps

• Risks:
• GI cancers ~30%
• Endometrial cancer ~40%

Cowden Syndrome (PTEN mutation)

• Features:

✔️ Hamartomas

✔️ Breast, thyroid, endometrial cancer risk ↑

• Endometrial risk: Not well defined

Clinical Scenario: “One clinic day — four families, four syndromes (and the ovarian screening trap)”

Setting

You’re in a gynae oncology / genetics clinic. The referral letter says:

“Family history of breast/ovarian/colon cancers. Wants advice on risk + prevention.”

Patient 1 — BRCA story (and the ovarian-risk contrast)

Presentation

Ms A, 36 years, 2 kids, finished family.

Her mother had breast cancer at 41. Maternal aunt died of ovarian cancer at 49. A cousin has a known BRCA mutation.

You take the key step: offer genetic testing (after counselling).

Result options (this is where the exam tests you)

If she is BRCA1 positive

You explain the core memory line:

• BRCA1 → higher ovarian risk
• Lifetime risks
• Breast: 60–90%
• Ovary: 40–60%

If she is BRCA2 positive

You explain the contrast:

• BRCA2 → lower ovarian risk
• Lifetime risks
• Breast: 45–85%
• Ovary: 10–30%

The “big population” exam trap (you say it clearly)

Even though BRCA is famous, you counsel her that:

• <2% of ALL breast cancers are due to BRCA1/2
• BRCA mutations cause only a small proportion of ovarian cancers (but in carriers, the risk is very high and prevention matters).

Management decision (she’s completed childbearing)

You discuss risk-reducing bilateral salpingo-oophorectomy (prophylactic BSO).

What you MUST connect (exam checklist):

• Benefit 1: “Almost eliminates” ovarian cancer risk
• But you warn: residual ~1% risk of primary peritoneal cancer (because peritoneum can still behave like ovarian epithelium).
• Benefit 2: Reduces breast cancer risk by ~50%
• Downside: Abrupt menopause (sudden symptoms + long-term consequences), so she needs counselling and menopausal planning.

She agrees to proceed after counselling.

Patient 2 — Lynch syndrome (HNPCC) and the “no perfect endometrial screening” line

Presentation

Ms B, 42 years, heavy irregular bleeding + new “period pattern changes.”

Her dad had colon cancer at 45. Her sister had endometrial cancer at 39.

This pattern screams Lynch syndrome (HNPCC).

Genetics you state explicitly

You explain Lynch is due to mismatch repair gene mutations:

• MLH1, MSH2,6, PMS2

Risk counselling (must say the numbers)

• Endometrial cancer risk: up to 50% (very high)
• Ovarian cancer risk: about 4% (around 2–3× population)

Hallmark cancer cluster (say it as one line)

“Increased risk of colon + endometrial + gastric + ovarian cancers.”

Screening reality (the key management logic)

You tell her:

• There is no perfect endometrial screening test that reliably prevents cancer.

So your plan relies on:

• Prompt evaluation of PMB
• Attention to period pattern changes (like what she has now)

Because she’s symptomatic, you treat it as needs urgent assessment (not reassurance).

Risk-reducing surgery counselling

She’s finished childbearing → you discuss:

• Consider risk-reducing hysterectomy after childbearing (this is the exam management line you must connect).

Patient 3 — “I want ovarian screening” (and you shut down the false reassurance)

Presentation

Ms C, 33 years, anxious because a neighbour died of ovarian cancer. No strong family history. She asks for CA-125 and ultrasound every year.

You anchor the population statistic

You explain baseline risk first:

• General population lifetime risk of ovarian cancer = 1 in 70

Then you add the inherited fraction logic:

• Only a small inherited fraction, mostly due to:
• BRCA1/2
• Mismatch repair genes (Lynch)

Screening limitation (must be explicit)

You say clearly:

• CA-125 + pelvic ultrasound have poor sensitivity and specificity
• Meaning: many false positives AND false negatives

So routine screening can:

• Miss cancers (false reassurance), and
• Trigger unnecessary anxiety/surgery (false positives).

You redirect her to risk assessment: if no strong family history/red flags, reassure and educate; if red flags appear, refer for genetics pathway.

Patient 4 — “Spots on lips + polyps” (Peutz–Jeghers, LKB1)

Presentation

Ms D, 19 years, comes with her mother. She has:

• Dark pigmented spots on lips and buccal mucosa
• Past history of GI polyps

You link the syndrome immediately:

• Peutz–Jeghers syndrome due to LKB1 mutation

The risk facts you must connect

• GI cancers ~30%
• Endometrial cancer ~40%

You explain she needs long-term surveillance planning (and family counselling), because the pattern is inherited and cancer risk is meaningful.

Patient 5 — “Hamartomas + thyroid + breast” (Cowden, PTEN) with the “endometrium not well defined” line

Presentation

Ms E, 28 years, known benign lumps and multiple hamartomas, plus family history of thyroid cancer and early breast cancers.

You connect:

• Cowden syndrome due to PTEN mutation
• Features: hamartomas
• Risks increased: breast, thyroid, endometrial cancers
• AUTOSOMAL DOMINANT

And you say the key exam qualifier:

• Endometrial risk is not well defined (so you counsel risk awareness, symptom vigilance, and individualized follow-up rather than quoting a fixed number).

One closing summary you’d say to students (ties the whole clinic together)

• BRCA1: breast 60–90%, ovary 40–60% (higher ovarian)
• BRCA2: breast 45–85%, ovary 10–30% (lower ovarian)
• BRCA <2% of all breast cancers, and inherited ovarian cancers are a small fraction overall, mainly BRCA + Lynch
• Prophylactic BSO (after childbearing): almost eliminates ovarian cancer, residual ~1% peritoneal risk, breast cancer ↓ ~50%, but abrupt menopause
• Lynch (MLH1/MSH2/MSH6/PMS2): endometrial risk up to 50%, ovarian ~4%, cancers include colon/endometrial/gastric/ovarian, and no perfect endometrial screening → act on PMB + cycle changes, consider risk-reducing hysterectomy after childbearing
• Ovarian screening: CA-125 + USS = poor sensitivity/specificity → false positives + false negatives
• Peutz–Jeghers (LKB1): pigmented mucosa + GI polyps, GI cancers ~30%, endometrial ~40%
• Cowden (PTEN): hamartomas, breast/thyroid/endometrial risk ↑, but endometrial risk not well defined

❌ Li-Fraumeni syndrome ≠ MEN 2

Li-Fraumeni syndrome

• Gene: TP53 (tumor suppressor)
• Inheritance: Autosomal dominant
• Core problem: Loss of p53-mediated DNA damage control
• Cancer spectrum (early age):
• Soft-tissue sarcoma
• Osteosarcoma
• Breast cancer (young women)
• Brain tumors
• Leukemia
• Adrenocortical carcinoma

👉 Mnemonic: SBLA

Sarcoma, Breast, Leukemia, Adrenal

❗ NOT an endocrine neoplasia syndrome

✅ MEN 2 (Multiple Endocrine Neoplasia type 2)

MEN 2 = Sipple syndrome

• Gene: RET proto-oncogene (gain-of-function)
• Inheritance: Autosomal dominant
• Key tumors:
• Medullary thyroid carcinoma (100%)
• Pheochromocytoma
• Parathyroid hyperplasia (MEN 2A)
• Mucosal neuromas + marfanoid habitus (MEN 2B)

👉 Mnemonic: M + P + P

• Medullary thyroid carcinoma
• Pheochromocytoma
• Parathyroid hyperplasia (2A)

🧠 Exam lock (very high yield)

• Li-Fraumeni → TP53 → multiple early non-endocrine cancers
• MEN 2 → RET → medullary thyroid carcinoma ± pheochromocytoma
• Li-Fraumeni is NOT MEN
• MEN 2 always involves endocrine tumors