Genetic Disorder & Chromosomal Abnormalities

Genetic Disorders: Core Definition & Scope

A genetic disorder is any pathology arising from abnormalities in an individual’s DNA.
- Abnormalities span a continuum:
- Single–base substitutions ("point mutations").
- Small or large deletions/insertions of genes.
- Gain or loss of whole chromosomes or entire chromosome sets (genomic imbalance).
Epidemiology
- Most single-gene or chromosomal conditions are rare, affecting ≈ $1$ person in several $10^3-10^6$ individuals.

Chromosome Fundamentals

Humans are diploid: $46$ chromosomes → $22$ homologous autosomal pairs + $1$ pair of sex chromosomes (allosomes).
Conventional display = karyotype (e.g., male karyotype $46,XY$ ).

Chromosomal Anomalies: Master Classification

Errors occur during meiosis (gametogenesis) or mitosis (somatic division).
Two umbrella categories
1. Numerical anomalies (aneuploidy) – abnormal chromosome number.
2. Structural anomalies – aberrant architecture of one or more chromosomes.

Numerical (Aneuploid) Anomalies

Monosomy – loss of one chromosome of a pair (e.g., Turner syndrome, $45,X$ ).
Trisomy – presence of three copies of a chromosome (e.g., Down syndrome, $47,+21$ ).
Etiology: nondisjunction—failure of homologues or chromatids to segregate.

Structural Anomalies – Spectrum & Prototypical Disorders

Deletion – loss of a segment.
- Wolf-Hirschhorn (4p–) & Cri-du-chat (5p–).
Duplication – repeat of a segment.
- Charcot–Marie–Tooth type 1A: duplication of PMP22 on chromosome $17$ .
Translocation – segment relocates to non-homologous chromosome.
- Reciprocal – mutual exchange.
- Robertsonian – entire acrocentric chromosome (13, 14, 15, 21, 22) fuses at centromere.
Inversion – segment breaks, rotates $180^{\circ}$ , re-inserts.
Insertion – segment excised from one chromosome and inserted into another.
Clinical Pearl: Structural events may be de novo or inherited; parental karyotyping clarifies recurrence risk.

Inheritance Patterns of Genetic Disease

Autosomal (non-sex chromosomes)
- Dominant – one mutant allele suffices → affected offspring has 50 % risk per pregnancy if one parent affected.
- Recessive – disease manifests only when both alleles are mutant → parents often carriers.
Allosomal (sex-linked)
- X-linked dominant†
- X-linked recessive†
- Y-linked (holandric) – transmitted father → son exclusively.
Mitochondrial
- Mutation in mtDNA or nuclear genes encoding mitochondrial proteins → ATP deficiency → organ failure.
- Strict maternal inheritance (sperm carry virtually no mitochondria).
- Prevalence ≈ $1$ in $4,000$ .

† Male hemizygosity for $X$ means recessive alleles manifest more readily in males.

Autosomal vs Allosomal Disorders – Key Contrasts

Mutation locus
- Autosomal → chromosomes $1–22$ .
- Allosomal → sex chromosomes (mainly $X$ ).
Sex predilection
- Autosomal → affects both sexes equally.
- Allosomal → males more often affected (hemizygosity).
Genotype of affected
- Autosomal → can be homozygous or heterozygous, dominant or recessive.
- Allosomal (recessive) → affected males are hemizygous.
Representative examples
- Autosomal: Down syndrome, sickle-cell anemia.
- Allosomal: Klinefelter syndrome, Turner syndrome.

Pre-disposing Cytogenetic Mechanisms for Autosomal Disease

Trisomy
Autosomal deletion
Microdeletion (sub-microscopic)
Chromosomal instability syndromes (e.g., Bloom, Fanconi anemia)

Trisomy (Polysomy) – Pathogenesis & Major Entities

Definition: Three copies of a chromosome (polysomic aneuploidy).
Origin: Nondisjunction during meiosis I, meiosis II, or (rarely) post-zygotic mitosis.
Survivable autosomal trisomies in humans
1. Trisomy 21 – Down syndrome
2. Trisomy 18 – Edwards syndrome
3. Trisomy 13 – Patau syndrome

Trisomy 13 (Patau Syndrome)

Cytogenetics: $47,+13$ or Robertsonian translocation involving chromosome $13$ .
Incidence: $1$ in $10{,}000–21{,}700$ live births.
Multisystem phenotype
- CNS: Severe intellectual disability, holoprosencephaly, absent corpus callosum, cerebellar hypoplasia.
- Craniofacial: Microphthalmia, bilateral cleft lip/palate, sloping forehead, scalp defects.
- Limbs: Post-axial polydactyly, rocker-bottom heels.
- Visceral: Congenital heart and renal malformations.
Prognosis: High neonatal mortality; median survival < $1$ year.

Trisomy 18 (Edwards Syndrome)

Cytogenetics: $47,+18$ .
Incidence: ≈ $1$ in $6{,}000$ live births; ≈ 80 % are female.
Hallmark features
- Growth: Intra-uterine growth restriction, low birth weight.
- Craniofacial: Prominent occiput, micrognathia, low-set ears, short sternum, webbed neck.
- Limbs: "Clenched fists" with overlapping fingers, hypoplastic nails, rocker-bottom feet.
- Cardiac: Ventricular septal defects.
- Renal anomalies.
Prognosis: Only <10 % survive beyond $1$ year; profound cognitive impairment.

Trisomy 21 (Down Syndrome)

Cytogenetics: $95 %$ due to free trisomy 21 via nondisjunction; $4 %$ Robertsonian translocation; $1 %$ mosaicism.
Discovery milestones
- Clinical delineation by J. L. Down (1866).
- Chromosomal basis identified by J. Lejeune (1959).
Epidemiology: Maternal age-dependent; overall ≈ $1$ in $700$ live births.
Clinical spectrum
- Intellectual disability: Mean IQ ≈ $50$ (vs. $100$ general population); cognitive decline with age.
- Craniofacial: Brachycephaly, flat occiput & face, epicanthal folds, upward-slanting palpebral fissures, flat nasal bridge, Brushfield spots.
- Oral: Protruding "scrotal" tongue, high-arched palate, dental anomalies.
- Limbs: Single transverse palmar crease (simian crease), clinodactyly of $5^{th}$ finger, short broad hands.
- Systemic: Hypotonia, congenital heart disease (AV canal, VSD), gastrointestinal atresia, increased leukemia & Alzheimer risk.

Autosomal Deletion Syndromes

Definition: Loss of chromosomal material on an autosome; may be microscopic (> $5$ Mb) or sub-microscopic (microdeletion).
Common entities
1. Wolf–Hirschhorn syndrome (4p–)
2. Cri-du-chat syndrome (5p–)
3. Langer–Giedion syndrome (8q23.1–q24.1)

Wolf–Hirschhorn Syndrome (WHS)

Genetics: Terminal deletion of short arm of chromosome $4$ (critical region = $4p16.3$ ).
Historical note: Described (1961) by Cooper & Hirschhorn; detailed by U. Wolf.
Key features
- Craniofacial "Greek warrior helmet" appearance – high forehead, wide nasal bridge, short philtrum.
- Microcephaly, seizures, severe growth & intellectual disability.
- Hypotonia, immunodeficiency, renal anomalies, hearing loss.

Cri-du-chat Syndrome (Cat-Cry, 5p–)

Genetics: Deletion on short arm of chromosome $5$ (critical region $5p15.2$ ).
Incidence: ≈ $1$ in $50{,}000$ ; female:male ≈ $4$ : $3$ .
Diagnostic cry: High-pitched, cat-like due to laryngeal hypoplasia.
Clinical picture
- Global developmental delay & intellectual disability.
- GI & cardiac malformations.
- Excessive drooling, behavioral disturbances.

Langer–Giedion Syndrome (Trichorhinophalangeal Syndrome II)

Genetics: Contiguous gene deletion at $8q24.11–q24.13$ .
Phenotype: Sparse hair, bulbous nose, multiple exostoses; included here for completeness (limited details in transcript).

Chromosomal Testing & Genetic Counseling

Parental karyotyping determines whether anomaly is de novo or inherited → influences recurrence risk.
Robertsonian carriers have $~10–15 %$ risk of Down syndrome in offspring if female, $1–3 %$ if male.
Ethical issues: Prenatal diagnosis, selective termination, psychosocial support.

Practical & Ethical Considerations

Early detection enables medical surveillance (e.g., cardiac echo in Down syndrome, renal ultrasound in Patau/Edwards).
Multidisciplinary care improves quality of life but raises resource-allocation questions.
Advances in CRISPR-based gene editing pose future therapeutic avenues yet raise germline-editing ethics.

These notes integrate foundational genetics, clinical correlates, epidemiology, and ethical reflections, providing a stand-alone study resource.