Human genetic disorders encompass a broad spectrum of conditions caused by alterations in the genome, ranging from single-nucleotide changes to large chromosomal rearrangements. Understanding their molecular basis, inheritance patterns, and clinical manifestations is essential for diagnosis, genetic counseling, and the development of targeted therapies.

Single-Gene (Mendelian) Disorders

Single-gene disorders follow predictable inheritance patterns defined by Mendel and are caused by mutations in individual genes. Autosomal dominant disorders, such as Huntington disease, neurofibromatosis type 1, and Marfan syndrome, manifest when a single mutant allele is sufficient to produce the phenotype, often because the mutant protein exerts a dominant-negative or gain-of-function effect. Affected individuals have a 50% chance of passing the mutation to each offspring. Autosomal recessive disorders, including cystic fibrosis, sickle cell disease, and Tay-Sachs disease, require mutations in both alleles of a gene and are more common in consanguineous populations. Carriers (heterozygotes) are typically asymptomatic. X-linked recessive disorders, such as Duchenne muscular dystrophy, hemophilia A, and glucose-6-phosphate dehydrogenase deficiency, predominantly affect males who inherit a single mutant X chromosome, while heterozygous females are usually carriers with variable expressivity due to X-chromosome inactivation.

Chromosomal Disorders

Chromosomal disorders result from numerical or structural abnormalities of chromosomes. Aneuploidies, gains or losses of individual chromosomes, arise from nondisjunction during meiosis. Down syndrome (trisomy 21) is the most common viable human aneuploidy, with an incidence of approximately 1 in 700 births, increasing markedly with maternal age. Features include intellectual disability, characteristic facial appearance, congenital heart defects, and increased risk of Alzheimer disease and leukemia. Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13) are more severe with limited survival. Sex chromosome aneuploidies such as Turner syndrome (45,X) and Klinefelter syndrome (47,XXY) have milder phenotypes due to X-inactivation buffering the gene dosage effect. Structural chromosomal abnormalities include deletions (such as the 22q11.2 deletion causing DiGeorge syndrome), duplications, inversions, and translocations, which may cause disease through gene disruption, altered gene dosage, or position effects.

Trinucleotide Repeat Disorders

Trinucleotide repeat disorders arise when short tandem repeat sequences expand beyond a critical threshold, causing disease through gene disruption or toxic RNA or protein products. Huntington disease caused by CAG repeat expansion in the HTT gene follows an autosomal dominant pattern with anticipation: the repeat length tends to increase in successive generations through paternal transmission, leading to earlier onset and more severe disease in descendants. Unaffected individuals have 6–35 CAG repeats, while 36–39 repeats show reduced penetrance and 40 or more repeats cause fully penetrant disease. Fragile X syndrome, the most common inherited cause of intellectual disability, is caused by CGG repeat expansion in the FMR1 gene promoter, leading to hypermethylation and transcriptional silencing. Myotonic dystrophy type 1 involves CTG expansion in the DMPK gene, producing a toxic RNA that sequesters splicing factors and disrupts the regulation of multiple downstream genes.

Mitochondrial Disorders

Mitochondrial disorders are caused by mutations in either mitochondrial DNA (mtDNA) or nuclear genes encoding mitochondrial proteins, and they characteristically affect tissues with high energy demands such as muscle, brain, and heart. mtDNA is maternally inherited, and cells contain hundreds to thousands of mtDNA copies, so mutations can exist in a state of heteroplasmy (mixture of mutant and wild-type molecules). Phenotypic expression depends on the threshold proportion of mutant mtDNA in affected tissues. Leber hereditary optic neuropathy (LHON), caused by mtDNA mutations in complex I genes, leads to acute vision loss in young adults. MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes) syndrome is most commonly caused by the m.3243A>G mutation in the tRNA leucine gene. Because mtDNA has limited repair capacity and high mutation rates, mitochondrial disorders are among the most common inherited metabolic diseases.

Complex (Multifactorial) Disorders

Complex disorders result from the combined effects of multiple genetic variants, each contributing modestly to risk, together with environmental factors. This category includes common conditions such as type 2 diabetes, coronary artery disease, hypertension, schizophrenia, and autoimmune diseases like rheumatoid arthritis and type 1 diabetes. Genome-wide association studies have identified thousands of common variants associated with complex diseases, but each typically has a small effect size (odds ratios of 1.1–1.5). The missing heritability problem refers to the gap between heritability estimated from family studies and the variance explained by identified variants. Polygenic risk scores aggregate the effects of many variants to predict disease risk, but their clinical utility remains limited by modest predictive power and poor portability across ancestral populations.

Genetic Testing and Counseling

Genetic testing encompasses molecular, cytogenetic, and biochemical approaches to detect disease-causing genetic variants. Sanger sequencing remains the gold standard for single-gene testing, while next-generation sequencing enables panel testing, exome sequencing, and genome sequencing for broader diagnostic applications. Prenatal testing options include chorionic villus sampling and amniocentesis for karyotyping and molecular analysis, as well as non-invasive prenatal testing (NIPT) using cell-free fetal DNA from maternal blood for aneuploidy screening. Newborn screening programs test for treatable genetic conditions such as phenylketonuria and congenital hypothyroidism. Genetic counseling involves risk assessment, education about the genetic condition, discussion of testing options and their implications, and psychosocial support for affected individuals and families.