Complex diseases

Complex Disorders

  • Adult Onset: Diabetes Mellitus, Epilepsy, Glaucoma, Hypertension, Ischemic heart disease, Manic depression, Schizophrenia.

  • Congenital: Cleft lip/palate, Congenital dislocation of the hip, Congenital heart defects, Pyloric stenosis, Talipes.

Genetic and Environmental Contributions

  • Genetic Factors: Simple, unifactorial, high recurrence rate (e.g., Duchenne muscular dystrophy, Hemophilia).

  • Environmental Factors: Influences multifactorial diseases, which are common, complex, and have low recurrence rates (e.g., Peptic ulcer, Diabetes, Tuberculosis).

  • Most diseases with a genetic component display multifactorial inheritance rather than Mendelian inheritance.

Polygenic Inheritance

  • Definition: Involvement of many genes where each contributes a small amount to the final phenotype. ALso environment is a factor

  • Examples: Skin color.

  • Model: Multiple alleles at multiple loci contribute to a single trait.

Some diseases are defined clinically in terms of quantitative traits meaning it can be measured, example- hypertension

Other multifactorial disorders differ qualitatively from normal states e.g. cleft palate which means that normally there would not be a gap between the nose and lip, however, in cleft palate there is.

Multifactorial Inheritance

  • Definition: Controlled by many genes with small additive effects (polygenic) along with environmental factors.

  • Phenotype Determination: Dependent on genetic, environmental, and developmental factors, including protective and susceptibility factors.

  • Genetic Predisposition: Inherited from both parents, but the exact genes and environmental factors vary among individuals.

  • Model Characteristics:

    • Several loci are involved without dominance or recessively.

    • Additive effects of loci.

    • Interaction of genotype and environment to produce phenotype.

  • Normal Variation: Most phenotypic variations in normal individuals (e.g., skin colour, height, intelligence) are due to multifactorial traits.

If an individual liability exceeds a certain threshold value, he or she will have the disease.

Tools in the Study of Complex Diseases

  • Twin Studies: Assess genetic effects on variation. Monozygotic (identical) twins share 100% genes, and dizygotic (fraternal) twins share 50% genes.

    • Concordance Rates: High concordance in monozygotic twins for genetic traits indicates a stronger genetic influence.

    • Utility: Distinguish between genetic, non-genetic, and combined effects. Limitations include small sample sizes and biases.

  • Adoption Studies: Compare adopted children with their biological and adoptive parents.

    • Advantages: Isolate genetic factors from environmental influences.

    • Limitations: Selective placement and non-representative samples.

  • Migrant Studies: Compare disease patterns in immigrants with citizens and kin in the homeland.

    • Purpose: Assess the roles of genetic, environmental, and lifestyle factors in disease occurrence.

  • Linkage Studies: Used for identifying genes or genetic variants responsible for diseases by studying large families with affected individuals across generations.

    • Advantages: Localize disease risk areas across the genome and study multiple genetic markers.

    • Limitations: Require large numbers of affected families, and less helpful for complex traits.

  • Association Studies: Compare the DNA of people with a disease (cases) and without (controls) to find associated genetic variants.

    • Genome-Wide Association Studies (GWAS): Investigate the entire genome for disease-associated SNPs.

Factors Influencing Recurrence Risk

  • Higher Risk: More affected relatives, severe cases, and relatives of the less commonly affected sex.

  • Determination: Through family studies and empiric risk observation.

Multifactorial Threshold Model

  • Concept: Disease manifests only when a certain threshold of genetic and environmental risk factors is exceeded.

  • Examples: Spina bifida, and pyloric stenosis.

Family Studies

  • Proband: The affected individual from whom the study starts.

  • Recurrence Risk Calculation: Based on the proportion of affected siblings.