A 52-year-old man of Indian origin presents with type 2 diabetes mellitus diagnosed 2 years ago. His mother, maternal uncle, and eldest son all have type 2 diabetes. His BMI is 28 kg/m², and he has central obesity. Genetic testing for single-locus mutations (MODY, neonatal diabetes genes) is negative. The pedigree pattern shown in the diagram at **A** represents the inheritance pattern most consistent with his family history. Which of the following BEST characterizes the genetic architecture and inheritance mechanism illustrated at **A**?

Explanation

## Why option 1 is correct The pedigree pattern at **A** — familial clustering of late-onset type 2 diabetes without clear Mendelian (dominant or recessive) segregation, variable age of onset, and strong environmental associations (obesity, lifestyle) — is the hallmark of **polygenic (multifactorial) inheritance**. Type 2 diabetes mellitus is the prototypic polygenic disease of adulthood. Genome-wide association studies (GWAS) have identified >150 loci conferring small individual effects (odds ratios typically 1.05–1.4 per risk allele), with TCF7L2 being the strongest common genetic risk factor. The combined effect of many genetic variants interacting with environmental determinants (obesity, sedentary lifestyle, diet, central adiposity) determines disease susceptibility and manifestation. Even with hundreds of risk alleles, polygenic risk scores explain only ~10–20% of heritability — the remainder is environmental. This pattern is characteristic of common complex diseases and is the most common mode of inheritance for type 2 diabetes, coronary artery disease, essential hypertension, and common cancers (Harrison's Internal Medicine 21e Ch 404; Thompson and Thompson Genetics 8e Ch 8). ## Why each distractor is wrong - **Option 2 (Autosomal dominant MODY)**: MODY (Maturity-Onset Diabetes of the Young) is a monogenic form of diabetes caused by a single autosomal dominant mutation (e.g., HNF1A, GCK). It presents with onset <25 years, multi-generational dominant pedigree, and is non-obese. The proband's age of onset (52 years), obesity, negative single-locus genetic testing, and lack of strict Mendelian segregation rule out MODY. This corresponds to **B** in the diagram, not **A**. - **Option 3 (Mitochondrial diabetes MIDD)**: Mitochondrial diabetes (MIDD) is caused by maternal inheritance of mitochondrial DNA mutations (most commonly m.3243A>G) and is typically associated with sensorineural hearing loss (maternally inherited deafness). The proband's family history shows affected individuals through both maternal and paternal lines (maternal uncle, father's side implied), and there is no mention of hearing loss, making mitochondrial inheritance unlikely. This corresponds to **C** in the diagram, not **A**. - **Option 4 (Autosomal recessive Wolfram syndrome)**: Wolfram syndrome (DIDMOAD) is a rare autosomal recessive disorder caused by WFS1 mutations, presenting with diabetes, optic atrophy, deafness, and diabetes insipidus. The proband shows none of these extra-pancreatic features, and the pedigree does not show the consanguinity or recessive pattern expected. This corresponds to **D** in the diagram, not **A**. **High-Yield:** Type 2 diabetes in a family without Mendelian segregation, negative single-locus testing, and environmental risk factors = **polygenic inheritance**; counsel first-degree relatives on lifestyle intervention and screening, not genetic testing of single loci. [cite: Harrison's Internal Medicine 21e Ch 404 (Diabetes); Thompson and Thompson Genetics 8e Ch 8 (Multifactorial Inheritance)]