11.3 Mendelian Inheritance, Chromosomal Abnormalities, and Molecular Genetics

11.3 Mendelian Inheritance, Chromosomal Abnormalities, and Molecular Genetics

Human clinical genetics bridges molecular biology and patient management, examining how genetic variation at the single-gene, chromosomal, and genomic levels produces physiological disease. Mastering the principles of Mendelian inheritance, non-traditional inheritance mechanisms, numerical and structural chromosomal aneuploidies, and high-fidelity DNA repair pathways is critical for board certification and integrative clinical diagnosis.

Classic & Non-Traditional Mendelian Inheritance Patterns

Single-gene (monogenic) disorders follow classic patterns of transmission originally identified by Gregor Mendel, modified by genomic and cellular phenomena:

1. Autosomal Dominant (AD)

• Mechanisms: Mutated allele on an autosome causes disease even in the presence of a normal allele (heterozygous state). Frequently affects structural proteins or receptor complexes (dominant-negative effects or gain-of-function).
• Pedigree Characteristics: Vertical transmission (disease appears in every generation); 50% probability of passing the mutant allele to offspring regardless of biological sex.
• Key Concepts:
  • Incomplete Penetrance: Not all individuals carrying the mutant genotype express the clinical phenotype (e.g., BRCA1 gene mutations). Expressed mathematically as the percentage of gene carriers who manifest clinical disease.
  • Variable Expressivity: Individuals bearing identical mutant genotypes exhibit varying degrees of clinical severity (e.g., Neurofibromatosis Type 1).
• Prototypic Disorders: Achondroplasia (FGFR3 gain-of-function mutation causing dwarfism), Marfan Syndrome (FBN1 gene defect encoding fibrillin-1; tall habitus, pectus excavatum, arachnodactyly, ascending aortic aneurysm/dissection, upward lens subluxation), Huntington Disease (trinucleotide CAG repeat expansion in HTT gene, demonstrating genetic anticipation), Neurofibromatosis Type 1 (NF1) (cafe-au-lait spots, neurofibromas, Lisch nodules of iris) and Type 2 (NF2) (bilateral vestibular schwannomas), and Familial Adenomatous Polyposis (FAP) (APC gene mutation).

2. Autosomal Recessive (AR)

• Mechanisms: Requires two mutant alleles (homozygous or compound heterozygous) for clinical manifestation. Most commonly involves enzymatic deficiencies where 50% enzyme activity in heterozygotes is sufficient for normal physiology.
• Pedigree Characteristics: Horizontal transmission (affected siblings within a single generation, unaffected carrier parents). 25% risk of affected offspring from carrier parents; 50% risk of carrier offspring. Strong association with consanguinity.
• Prototypic Disorders: Cystic Fibrosis (autosomal recessive CFTR delta-F508 mutation), Sickle Cell Anemia ($\beta$-globin point mutation), Thalassemias, Phenylketonuria (PKU), and Hereditary Hemochromatosis (HFE gene mutation causing systemic iron overload).

3. X-Linked Recessive (XLR)

• Mechanisms: Gene defect located on the X chromosome. Hemizygous males ($XY$) express the full disease phenotype because they lack a second protective X allele.
• Pedigree Characteristics: Marked male predominance. No male-to-male transmission (an affected father passes his Y chromosome to all sons and his mutated X chromosome to 100% of daughters, making all daughters obligate carriers). Heterozygous carrier females ($XX$) are usually asymptomatic due to random X-inactivation (lyonization), though skewed lyonization can produce mild symptoms.
• Prototypic Disorders: Duchenne and Becker Muscular Dystrophies (DMD gene frameshift or in-frame mutations), Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency), Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency, and Lesch-Nyhan Syndrome.

4. Mitochondrial Inheritance

• Mechanisms: Mitochondrial DNA (mtDNA) is inherited exclusively through the maternal oocyte cytoplasm; sperm mitochondria are selectively degraded post-fertilization.
• Pedigree Characteristics: Maternal transmission to ALL offspring. An affected mother passes the condition to 100% of her sons and daughters. An affected father passes the condition to 0% of his children.
• Clinical Variability: Driven by heteroplasmy, the random distribution and proportion of normal vs. mutant mtDNA molecules within individual daughter cells during mitosis and meiosis. Tissues with high metabolic oxygen demand (brain, skeletal muscle, cardiac tissue) are most severely affected.
• Prototypic Disorders: Leber Hereditary Optic Neuropathy (LHON) (sudden painless bilateral loss of central vision in young adults) and MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes).

Chromosomal Aneuploidies & Sex Chromosome Abnormalities

Numerical chromosomal abnormalities result primarily from meiotic nondisjunction (failure of homologous chromosomes or sister chromatids to separate during meiosis I or II).

Autosomal Aneuploidies

1. Down Syndrome (Trisomy 21):

   • Etiology: 95% caused by maternal meiotic nondisjunction (strongly correlated with advanced maternal age > 35); 4% caused by Robertsonian translocation (typically between chromosomes 14 and 21, independent of maternal age); 1% mosaicism.
   • Second-Trimester Quad Screen: Low $\alpha$-fetoprotein (AFP), low unconjugated estriol ($uE_3$), elevated $\beta$-hCG, and elevated Inhibin A ("HI up").
   • Clinical Features: Single palmar crease (simian crease), epicanthic folds, flat facial profile, hypotonia, duodenal atresia ("double bubble" sign on abdominal radiograph), Atrioventricular Septal Defect (AVSD) / endocardial cushion defects, early-onset Alzheimer disease (due to extra copy of the APP gene on chromosome 21), and increased risk of Acute Lymphoblastic Leukemia (ALL) and Acute Megakaryoblastic Leukemia (AML).

2. Edwards Syndrome (Trisomy 18):

   • Clinical Features: Micrognathia (small jaw), low-set malformed ears, clenched fists with overlapping fingers (2nd and 5th digits overlapping 3rd and 4th), rocker-bottom feet, congenital heart defects (VSD, PDA), severe intellectual disability, and mortality usually within the first year of life.

3. Patau Syndrome (Trisomy 13):

   • Clinical Features: Holoprosencephaly (failure of forebrain division), cleft lip/palate, microphthalmia, polydactyly, rocker-bottom feet, severe congenital heart disease, severe intellectual disability, and death typically occurring within days to weeks of birth.

Sex Chromosome Abnormalities

1. Klinefelter Syndrome (47,XXY):

   • Genotype & Pathophysiology: Male phenotype with an extra X chromosome ($47,XXY$). Testicular dysgenesis leads to seminiferous tubule hyalinization and Leydig cell dysfunction.
   • Clinical Presentation: Primary hypogonadism: testicular atrophy, infertility (azoospermia), gynecomastia, tall stature with eunuchoid body proportions (long legs), female pattern of hair distribution, low serum testosterone, and elevated serum FSH and LH (hypergonadotropic hypogonadism). Buccal smear reveals a single Barr body (inactivated extra X chromosome).

2. Turner Syndrome (45,XO):

   • Genotype & Pathophysiology: Female phenotype caused by complete or partial monosomy X ($45,XO$). Loss of the SHOX gene leads to short stature.
   • Clinical Presentation: Short stature, webbed neck (cystic hygroma / lymphatic obstruction), streak ovaries (premature ovarian failure with fibrous replacement of stroma), primary amenorrhea, delayed puberty, shield chest with widely spaced nipples, coarctation of the aorta, and bicuspid aortic valve. Laboratory evaluation shows elevated FSH and LH; buccal smear demonstrates no Barr bodies.

DNA Replication & High-Fidelity DNA Repair Mechanisms

Genomic integrity is maintained by sophisticated DNA repair systems that correct specific structural errors and lesions:

1. Nucleotide Excision Repair (NER):

   • Mechanism: Corrects bulky, helix-distorting DNA lesions, most notably pyrimidine dimers (thymine dimers) caused by ultraviolet (UV) radiation. Specific endonucleases cleave the damaged oligonucleotide strand, DNA polymerase fills the gap, and DNA ligase seals the phosphodiester backbone. Occurs during the G1 phase of the cell cycle.
   • Clinical Pathology: Defective in Xeroderma Pigmentosum (autosomal recessive endonuclease deficiency). Patients exhibit extreme photosensitivity, severe poikiloderma, and a >1000-fold increased risk of skin cancers (basal cell carcinoma, squamous cell carcinoma, melanoma).

2. Base Excision Repair (BER):

   • Mechanism: Corrects non-bulky, single-base alterations (e.g., spontaneous cytosine deamination to uracil, oxidation, alkylation).
   • Steps: Specific DNA Glycosylase recognizes and removes the altered base, creating an AP (apurinic/apyrimidinic) site. AP Endonuclease cleaves the 5' end, AP Lyase cleaves the 3' end, DNA Polymerase-$\beta$ inserts the correct nucleotide, and DNA Ligase seals the nick ("GEL-PL"). Occurs throughout the cell cycle.

3. Mismatch Repair (MMR):

   • Mechanism: Corrects mismatched bases and small insertion/deletion loops in newly synthesized daughter strands that escaped proofreading during DNA replication (S phase). Specialized proteins (MSH2, MLH1) recognize the unmethylated new strand, excise the mismatched segment, and resynthesize DNA.
   • Clinical Pathology: Defective in Lynch Syndrome (Hereditary Nonpolyposis Colorectal Cancer, HNPCC; autosomal recessive/dominant mutations in MSH2 or MLH1). Leads to Microsatellite Instability (MSI), conferring a high risk of colorectal, endometrial, ovarian, and urinary tract carcinomas.

DNA Repair Pathway Comparison