Chromosomal Disorders - USMLE Step 1

SECTION 1

Historical Context & Motivation

The study of chromosomal disorders began in earnest only after scientists could reliably visualize and count human chromosomes. For decades, the incorrect assumption that humans possessed 48 chromosomes persisted until improved cytogenetic techniques revealed the true diploid number of 46. Once this foundational fact was established, clinicians rapidly linked specific chromosomal aberrations to recognizable clinical syndromes, transforming both genetics and clinical medicine. The ability to correlate karyotypic abnormalities with phenotypic presentations has since become a cornerstone of medical genetics, reproductive counseling, and prenatal diagnosis.

1956 • Human Chromosome Number Established

Joe Hin Tjio and Albert Levan used improved hypotonic techniques and colchicine arrest to demonstrate that humans possess 46 chromosomes (2n = 46), correcting the longstanding error of 48.

1959 • Trisomy 21 Identified

Jérôme Lejeune, Marthe Gautier, and Raymond Turpin identified an extra copy of chromosome 21 in patients with Down syndrome, establishing the first link between a chromosomal abnormality and a clinical disorder.

1959 • Sex Chromosome Aneuploidies Described

Patricia Jacobs and John Strong reported 47,XXY (Klinefelter syndrome), while Charles Ford described 45,X (Turner syndrome), revealing that sex chromosome number is critical for normal sexual development.

1968 • Chromosomal Banding Techniques

Torbjörn Caspersson developed Q-banding using quinacrine fluorescence, enabling identification of individual chromosomes and subtle structural rearrangements. G-banding soon followed and became the clinical standard.

1990s–present • Molecular Cytogenetics

Fluorescence in situ hybridization (FISH) and chromosomal microarray analysis (CMA) enabled detection of submicroscopic deletions and duplications, refining the diagnosis of microdeletion syndromes such as DiGeorge and Williams.

The central question these discoveries addressed was deceptively simple: how does the gain, loss, or rearrangement of chromosomal material translate into the complex, multi-system phenotypes observed in clinical practice? Understanding the mechanisms of nondisjunction, structural rearrangement, and gene dosage imbalance is essential for every medical student preparing for the USMLE Step 1, as these concepts underpin a substantial proportion of genetics questions.

SECTION 2

Core Principles & Definitions

Chromosomal disorders arise when the normal complement of 46 chromosomes (22 pairs of autosomes plus two sex chromosomes) is altered in either number or structure. These alterations disrupt gene dosage — the precise quantity of genetic material required for normal development. Even modest imbalances involving as little as a single chromosomal segment can produce recognizable clinical syndromes because hundreds of genes may reside within that region. The principles below form the conceptual scaffold upon which all chromosomal disorders are classified.

Aneuploidy

An abnormal chromosome number that is not an exact multiple of the haploid set (n = 23). Common forms include trisomy (2n + 1 = 47) and monosomy (2n − 1 = 45). Most arise from nondisjunction during meiosis I or II.

Polyploidy

The presence of a complete extra set of chromosomes (e.g., 69 = triploidy; 92 = tetraploidy). Triploidy is usually lethal, resulting from dispermy or failure of a meiotic division. It accounts for approximately 20% of spontaneous first-trimester abortions.

Nondisjunction

Failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly, resulting in gametes with an abnormal chromosome number. Maternal age is the strongest risk factor for meiotic nondisjunction, especially for trisomy 21.

Structural Rearrangements

Alterations in chromosome architecture including deletions, duplications, inversions, translocations, ring chromosomes, and isochromosomes. May be balanced (no net gain/loss of genetic material) or unbalanced (with dosage imbalance and clinical consequences).

Mosaicism

The coexistence of two or more cell lines with different karyotypes in a single individual, typically from mitotic nondisjunction after fertilization. The phenotype depends on the proportion and distribution of abnormal cells. Classic example: mosaic Turner syndrome (45,X/46,XX).

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KEY TAKEAWAY

Think of chromosomes as volumes in an encyclopedia. Aneuploidy is like having a duplicate volume (trisomy) or a missing one (monosomy) — the total information is unbalanced, leading to errors when the 'factory' of the cell tries to use the instructions. Structural rearrangements are like torn pages, swapped chapters, or flipped sections within a volume — sometimes the text is still intact (balanced), but if pages are lost (unbalanced), the story no longer makes sense. This gene dosage imbalance is the unifying principle behind all chromosomal disorders.

SECTION 3

Visual Explanation — Nondisjunction & Aneuploidy

This diagram contrasts nondisjunction occurring at meiosis I (left) versus meiosis II (right). In meiosis I nondisjunction, homologous chromosomes fail to separate, resulting in all four gametes being abnormal (two with an extra chromosome and two missing one). In meiosis II nondisjunction, sister chromatids fail to separate in one secondary cell, producing two normal and two abnormal gametes. The distinction is clinically important: meiosis I errors are more common and affect all resulting gametes.

When studying this diagram, focus on the key distinguishing point tested on USMLE: if nondisjunction occurs at meiosis I, all four resulting gametes carry an abnormal chromosome complement, and the trisomic offspring will possess both copies of the parental homolog (heterodisomy). In contrast, meiosis II nondisjunction produces only two abnormal gametes out of four, and the trisomic offspring will carry two identical copies of one homolog (isodisomy). This distinction has implications for conditions involving genomic imprinting, where the parental origin of the extra chromosome matters clinically, as seen in some cases of Prader-Willi or Angelman syndrome that arise through uniparental disomy.

SECTION 4

Mechanisms of Chromosomal Abnormalities

Numerical Abnormalities

Numerical chromosomal abnormalities fall into two broad categories. Aneuploidy refers to the gain or loss of individual chromosomes, most commonly resulting from nondisjunction during gametogenesis. The vast majority of autosomal trisomies are incompatible with life; only trisomies 13, 18, and 21 survive to term with any regularity, reflecting the relatively low gene density on these chromosomes. Sex chromosome aneuploidies (47,XXY; 47,XXX; 47,XYY; 45,X) are generally better tolerated due to X-inactivation and the low gene content of the Y chromosome. Polyploidy (e.g., triploidy, 69 chromosomes) typically results from fertilization of an egg by two sperm (dispermy) or from failure of a meiotic division, and is almost always lethal.

Structural Abnormalities

Structural chromosomal rearrangements occur when chromosome breakage is followed by abnormal repair. Translocations involve exchange of segments between non-homologous chromosomes and are classified as reciprocal (exchange of terminal segments) or Robertsonian (fusion of two acrocentric chromosomes at their centromeres, losing the short arms). Robertsonian translocations are the most clinically significant structural rearrangement for USMLE, particularly the t(14;21) translocation that accounts for approximately 4% of Down syndrome cases and can be familial. Deletions remove chromosomal material and are always unbalanced. Inversions (pericentric if they include the centromere, paracentric if they do not) are usually balanced in the carrier but can produce unbalanced gametes during meiosis. Isochromosomes form when the centromere divides transversely rather than longitudinally, producing a chromosome with two identical arms — the isochromosome of the long arm of X, i(Xq), is a recognized cause of Turner syndrome.

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HIGH-YIELD FOR USMLE

Robertsonian translocation carriers (e.g., balanced t(14;21)) have only 45 chromosomes but are phenotypically normal because no genetic material is lost — only the short arms of the acrocentric chromosomes, which contain redundant rRNA genes. However, their offspring are at risk for unbalanced karyotypes (trisomy 21 or monosomy 21). A young mother with a child with Down syndrome should raise suspicion for a translocation carrier parent.

Maternal Age & Nondisjunction Risk

The risk of nondisjunction increases markedly with advancing maternal age, particularly for trisomy 21. Human oocytes arrest in prophase I of meiosis during fetal development and do not complete meiosis I until ovulation — potentially decades later. The prolonged arrest weakens cohesins that hold homologous chromosomes together, increasing the probability of missegregation. At maternal age 20, the risk of trisomy 21 is approximately 1 in 1,500 live births; by age 35, it rises to approximately 1 in 350; and by age 45, it reaches approximately 1 in 30. Paternal age has a far weaker association with aneuploidy but is linked to increased point mutations in sperm.

RECURRENCE RISK — ROBERTSONIAN TRANSLOCATION

Theoretical risk (carrier mother, t(14;21)) = 1/3 trisomy 21, 1/3 carrier, 1/3 normal

The theoretical risk for trisomy 21 offspring from a female Robertsonian translocation carrier is 1/3 (monosomy 21 gametes are non-viable). In practice, the empiric risk is ~10–15% if the mother is the carrier and ~2–5% if the father is the carrier, reflecting selection against unbalanced gametes.

SECTION 5

Major Chromosomal Disorders — Classification & Features

The following table summarizes the most commonly tested chromosomal disorders on USMLE Step 1, organized by their underlying mechanism — autosomal trisomies, sex chromosome aneuploidies, and structural abnormality syndromes. Mastering the associations between karyotype, clinical features, and underlying mechanism is essential for board success.

Disorder	Karyotype	Key Clinical Features	High-Yield Associations
Down Syndrome	Trisomy 21 (95%); Robertsonian translocation (4%); Mosaic (1%)	Intellectual disability, flat facies, epicanthal folds, single palmar crease, hypotonia, gap between 1st and 2nd toes	Duodenal atresia, Hirschsprung disease, AV septal defects, ALL, early-onset Alzheimer (by age 40), atlantoaxial instability, increased nuchal translucency
Edwards Syndrome	Trisomy 18	IUGR, prominent occiput, micrognathia, clenched fists with overlapping fingers, rocker-bottom feet	Congenital heart defects (VSD), horseshoe kidney, omphalocele; death by age 1 in most cases
Patau Syndrome	Trisomy 13	Holoprosencephaly, cleft lip/palate, polydactyly, microphthalmia, cutis aplasia	Severe intellectual disability, congenital heart defects; median survival ~10 days
Turner Syndrome	45,X (monosomy X)	Short stature, webbed neck, shield chest, lymphedema at birth, streak gonads, primary amenorrhea	Bicuspid aortic valve, coarctation of aorta, horseshoe kidney, cystic hygroma on prenatal ultrasound; NO Barr body
Klinefelter Syndrome	47,XXY	Tall stature, long extremities, gynecomastia, small firm testes, infertility, mild intellectual disability	Dysgenesis of seminiferous tubules, ↑FSH/LH, ↓testosterone; 1 Barr body; increased risk of breast cancer and SLE
Cri-du-chat	Deletion of 5p (5p−)	High-pitched mewing cry, microcephaly, intellectual disability, epicanthal folds	Congenital heart defects; the cry results from laryngeal abnormalities and diminishes with age
DiGeorge / 22q11.2 Deletion	Microdeletion 22q11.2	CATCH-22: Cardiac defects, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia	Truncus arteriosus, tetralogy of Fallot; T-cell deficiency; detected by FISH; due to abnormal 3rd/4th pharyngeal pouch development
Williams Syndrome	Microdeletion 7q11.23	'Elfin' facies, intellectual disability, hypercalcemia, extreme friendliness ('cocktail party' personality)	Supravalvular aortic stenosis; deletion of elastin gene (ELN); good verbal skills despite low IQ

High-yield chromosomal disorders for USMLE Step 1

This classification tree organizes chromosomal disorders into numerical (left branch) and structural (right branch) categories, with specific syndromes listed under each subtype. The mnemonic box at the bottom provides high-yield memory aids commonly used for board preparation.

SECTION 6

Worked Example — Diagnosing a Chromosomal Disorder

Clinical Vignette: A Newborn with Dysmorphic Features

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Step 1 — Read the Clinical Stem

A 38-year-old G3P2 woman delivers a male infant at 37 weeks. Physical examination reveals hypotonia, upslanting palpebral fissures, a flat nasal bridge, a single transverse palmar crease, and a gap between the first and second toes. Auscultation reveals a systolic murmur. An abdominal radiograph shows a 'double-bubble' sign.
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Step 2 — Identify Key Findings

The constellation of hypotonia, upslanting palpebral fissures, single palmar crease, sandal-gap deformity, and the double-bubble sign (indicating duodenal atresia) in a child born to a mother of advanced maternal age strongly points to a single unifying diagnosis.

Findings cluster: Down syndrome (Trisomy 21)
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Step 3 — Determine the Mechanism

Given the mother's age of 38, the most likely etiology is meiotic nondisjunction (accounting for ~95% of Down syndrome cases). However, you should consider Robertsonian translocation (~4%) if the question provides a family history of recurrent trisomy 21 or if the mother is young. The cardiac murmur likely reflects an atrioventricular septal defect (endocardial cushion defect), the most common congenital heart defect in Down syndrome.

Karyotype: 47,XY,+21 (full trisomy 21)
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Step 4 — Confirm with Diagnostic Studies

The definitive diagnosis requires a karyotype analysis demonstrating three copies of chromosome 21. Prenatal screening may have already shown elevated β-hCG, decreased AFP, decreased estriol, and increased inhibin A (the 'quad screen' pattern for trisomy 21). Increased nuchal translucency on first-trimester ultrasound is another screening marker. Cell-free fetal DNA (cfDNA) testing from maternal blood now offers high sensitivity and specificity as a non-invasive prenatal screening tool.

Quad screen pattern for T21: ↑β-hCG, ↓AFP, ↓estriol, ↑inhibin A
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Step 5 — Consider Long-Term Complications

Patients with Down syndrome are at increased risk for acute lymphoblastic leukemia (ALL) and acute megakaryoblastic leukemia (AMKL), hypothyroidism, atlantoaxial instability, and early-onset Alzheimer disease (virtually universal by age 40 due to three copies of the APP gene on chromosome 21, leading to amyloid β accumulation). This child will need an echocardiogram, thyroid function monitoring, hearing screening, and ophthalmologic evaluation.

APP gene (chr 21) → 3 copies → ↑amyloid β → early Alzheimer disease

SECTION 7

Comparing Autosomal Trisomies — Features & Prognosis

Feature	Trisomy 21 (Down)	Trisomy 18 (Edwards)	Trisomy 13 (Patau)
Incidence	1 in 700 live births (most common viable autosomal trisomy)	1 in 5,000–8,000 live births	1 in 10,000–15,000 live births
Characteristic Hands	Single transverse palmar (simian) crease	Clenched fists with overlapping fingers (index over 3rd, 5th over 4th)	Polydactyly (postaxial)
Head/Face	Flat facies, epicanthal folds, Brushfield spots, protruding tongue	Prominent occiput, micrognathia	Holoprosencephaly, cleft lip/palate, microphthalmia
Cardiac Defect	AV septal defect (endocardial cushion defect)	VSD, PDA	VSD, ASD, PDA, dextrocardia
Feet	Sandal-gap (wide space between 1st and 2nd toes)	Rocker-bottom feet	Rocker-bottom feet
GI Finding	Duodenal atresia, Hirschsprung disease	Omphalocele	Omphalocele (less common)
AFP on Maternal Serum Screen	↓ AFP	↓ AFP	Not routinely screened on quad screen
Median Survival	~60 years (most common cause of inherited intellectual disability)	~1 year (>90% die in first year)	~10 days (median); very few survive past first year

Side-by-side comparison of the three viable autosomal trisomies

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KEY TAKEAWAY

On the USMLE, distinguishing the three autosomal trisomies often comes down to the hand findings: a single palmar crease (trisomy 21), clenched fists with overlapping fingers (trisomy 18), and polydactyly (trisomy 13). Think of it as a progression: chromosome 21 is the mildest (longest survival), 18 is intermediate, and 13 is the most severe (shortest survival). This inversely correlates with chromosome size — smaller chromosomes have fewer genes, so their trisomy is better tolerated.

SECTION 8

Connection to Advanced Concepts — Genomic Imprinting & Uniparental Disomy

Chromosomal disorders intersect with several advanced genetic concepts that are increasingly tested on USMLE Step 1. Uniparental disomy (UPD) occurs when both copies of a chromosome (or chromosome segment) are inherited from a single parent. This can arise from trisomy rescue — a trisomic cell line loses one extra chromosome during mitosis, but if the remaining two copies originate from the same parent, UPD results. When the affected region harbors imprinted genes (genes expressed only from one parental allele), UPD can cause disease even with normal chromosome number: maternal UPD 15 causes Prader-Willi syndrome (loss of paternal expression), while paternal UPD 15 causes Angelman syndrome (loss of maternal expression).

Concept	Basic Chromosomal Disorders	Advanced Extension
Gene Dosage	Extra or missing chromosome → altered protein levels → phenotype	Copy number variants (CNVs) detected by microarray; sub-microscopic dosage changes cause ~15% of unexplained intellectual disability
Nondisjunction	Meiotic errors → aneuploidy (trisomy, monosomy)	Mitotic nondisjunction → mosaicism → variable phenotype; may explain why some Down syndrome patients have milder features
Translocations	Robertsonian → familial Down syndrome; Reciprocal → balanced carrier	Philadelphia chromosome t(9;22) → BCR-ABL fusion → CML; t(15;17) → PML-RARA → APL; somatic translocations as oncogenic drivers
X-inactivation	Explains tolerance of sex chromosome aneuploidies (XXY, XXX)	Skewed X-inactivation can unmask X-linked recessive disorders in heterozygous females; relevant for manifesting carriers
Imprinting & UPD	Trisomy rescue can lead to UPD	Prader-Willi (maternal UPD 15), Angelman (paternal UPD 15), Beckwith-Wiedemann (paternal UPD 11p15.5)

Basic chromosomal disorder concepts and their advanced extensions

As you advance through medical genetics, you will encounter these concepts in clinical contexts ranging from preimplantation genetic testing (PGT) in reproductive medicine to tumor cytogenetics in oncology. The foundational understanding of chromosomal structure, nondisjunction, and gene dosage effects developed in this lesson provides the scaffolding upon which more nuanced topics — including epigenetics, chromothripsis, and precision medicine approaches — are built.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A carrier of a balanced Robertsonian translocation t(14;21) has 45 chromosomes but is phenotypically normal. Explain why a person with only 45 chromosomes can appear completely healthy, while a person with 47 chromosomes (trisomy 21) has significant clinical abnormalities.

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In a balanced Robertsonian translocation carrier, the long arms of chromosomes 14 and 21 are fused into a single derivative chromosome. Although the chromosome count is 45, there is no net loss of essential genetic material — the lost short arms of the acrocentric chromosomes contain only redundant ribosomal RNA genes and satellite DNA, which are present on multiple other acrocentric chromosomes and are therefore functionally dispensable. Gene dosage for all protein-coding genes remains normal. In contrast, a patient with trisomy 21 has three complete copies of chromosome 21, resulting in a ~50% increase in expression of the approximately 200–300 genes on that chromosome. This dosage imbalance disrupts developmental pathways and protein stoichiometry, producing the Down syndrome phenotype. The critical distinction is gene dosage balance, not chromosome number per se.

Question 2: BASIC CALCULATION

A woman who is a balanced Robertsonian translocation carrier t(14;21) and her chromosomally normal husband plan to have children. List the six possible gametic combinations at fertilization, identify which are viable, and state the theoretical risk of having a child with Down syndrome.

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The carrier mother can produce six types of gametes based on segregation of the translocation chromosome: (1) normal 14 + normal 21 → normal child; (2) t(14;21) derivative → balanced carrier (phenotypically normal); (3) t(14;21) + free 21 → trisomy 21 (Down syndrome); (4) t(14;21) + free 14 → trisomy 14 (lethal); (5) free 14 only (no 21) → monosomy 21 (lethal); (6) free 21 only (no 14) → monosomy 14 (lethal). Of the three viable outcomes, the theoretical risk of Down syndrome is 1/3 (33%). However, the empiric risk is lower (~10–15% for a female carrier) because selection against unbalanced gametes and embryos reduces the observed frequency. If the father is the carrier, the empiric risk drops further to ~2–5%.

Question 3: INTERMEDIATE

A 25-year-old woman with no family history of chromosomal abnormalities gives birth to a child with Down syndrome. Karyotype of the child shows 46,XY,der(14;21)(q10;q10),+21. The mother's karyotype is 45,XX,der(14;21)(q10;q10). What is the recurrence risk for Down syndrome in her future pregnancies, and how does this differ from the recurrence risk if the child's trisomy 21 were due to de novo nondisjunction?

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The child's karyotype reveals a Robertsonian translocation form of Down syndrome (the derivative chromosome plus a free chromosome 21 gives three functional copies of chromosome 21 material). The mother is a balanced carrier with t(14;21), explaining why she is phenotypically normal despite having 45 chromosomes. Her empiric recurrence risk is approximately 10–15% for each future pregnancy, significantly higher than the ~1% recurrence risk associated with de novo nondisjunction trisomy 21 in a 25-year-old. This case illustrates why parental karyotyping is essential when Down syndrome occurs in offspring of young parents. If the trisomy were due to simple nondisjunction, the recurrence risk would be approximately 1% or the age-related risk, whichever is greater. Genetic counseling should address the option of prenatal diagnosis (amniocentesis or CVS) in future pregnancies.

Question 4: APPLIED

A 2-week-old female infant presents with lymphedema of the hands and feet, a webbed neck, and a systolic ejection murmur heard best at the right upper sternal border. Echocardiography reveals a bicuspid aortic valve. The karyotype returns as 45,X/46,XX. (a) What is the diagnosis? (b) Why might this patient's clinical presentation be milder than a patient with a pure 45,X karyotype? (c) What additional evaluations should be performed in infancy and adolescence?

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(a) The diagnosis is mosaic Turner syndrome (45,X/46,XX). The clinical features of lymphedema, webbed neck, and bicuspid aortic valve are classic for Turner syndrome. (b) The presence of a normal 46,XX cell line alongside the 45,X line means that some tissues have normal chromosomal complement. The proportion and distribution of each cell line determine the severity; mosaic patients often have milder features, may have some spontaneous pubertal development, and occasionally achieve natural fertility. (c) In infancy: renal ultrasound (horseshoe kidney in ~30%), thyroid function tests, hearing evaluation. In adolescence: monitor for growth failure (consider growth hormone), assess for primary amenorrhea and streak gonads, evaluate for gonadal dysgenesis requiring hormone replacement therapy, and screen for aortic root dilation with serial echocardiography.

Question 5: CRITICAL THINKING

A couple has a child with Prader-Willi syndrome. Genetic testing reveals no deletion on chromosome 15q11-q13 and no methylation abnormality on standard testing, but further analysis demonstrates that the child has two copies of the maternal chromosome 15 and no paternal chromosome 15 (maternal uniparental disomy). Explain the mechanism by which this most likely arose, discuss how it relates to the chromosomal disorder concepts covered in this lesson, and contrast the recurrence risk with that of a de novo deletion causing Prader-Willi syndrome.

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This case illustrates trisomy rescue as a mechanism linking aneuploidy to imprinting disorders. The most likely sequence is: (1) maternal nondisjunction during meiosis produced an egg with two copies of chromosome 15; (2) fertilization by a normal sperm created a trisomy 15 zygote (which is normally lethal); (3) during early mitotic divisions, one chromosome 15 was lost ('rescued'), restoring disomy; (4) by chance, the lost chromosome was the paternal copy, leaving maternal uniparental disomy (UPD) for chromosome 15. Since the Prader-Willi critical region on 15q11-q13 is paternally imprinted (the maternal copy is silenced), both copies are functionally silent, producing Prader-Willi syndrome despite having two intact chromosome 15s. This directly connects to nondisjunction — the same process that causes trisomy 21 can, through trisomy rescue, cause imprinting disorders. The recurrence risk for UPD-mediated Prader-Willi is very low (<1%), similar to de novo deletion cases (~1%), because both result from sporadic errors. However, if the mother has a Robertsonian translocation involving chromosome 15, the recurrence risk increases substantially, necessitating parental karyotyping.

SUMMARY

Lesson Summary

Summary

Chromosomal disorders are caused by numerical abnormalities (aneuploidy and polyploidy) or structural rearrangements (translocations, deletions, inversions, isochromosomes, and ring chromosomes). The three viable autosomal trisomies — trisomy 21 (Down), trisomy 18 (Edwards), and trisomy 13 (Patau) — primarily result from nondisjunction, with risk increasing sharply with advanced maternal age. Sex chromosome aneuploidies (45,X Turner; 47,XXY Klinefelter; 47,XXX; 47,XYY) are generally better tolerated because of X-inactivation and low Y-chromosome gene content. Robertsonian translocations (especially t(14;21)) are high-yield because balanced carriers are phenotypically normal but have significantly elevated recurrence risk for Down syndrome offspring.

Key microdeletion syndromes include DiGeorge (22q11.2) with the CATCH-22 mnemonic, Williams (7q11.23) with supravalvular aortic stenosis and hypercalcemia, and Cri-du-chat (5p−) with the characteristic high-pitched cry. Advanced concepts linking to chromosomal disorders include uniparental disomy (arising from trisomy rescue), genomic imprinting (Prader-Willi and Angelman syndromes), and mosaicism (explaining phenotypic variability in Turner syndrome and other conditions). For USMLE success, focus on recognizing the specific clinical features, karyotype, screening markers, and complications unique to each disorder.