Genes, Heritability, and Molecular Genetics in Psychopathology

Recap of Developmental Psychopathology

Psychopathology is viewed as a developmental outcome that emerges through various complex processes. Key concepts include:

• Approaches: The use of both categorical (disorder-based) and dimensional (trait-based) approaches to identify symptoms.

• Risk Factors: These can be transdiagnostic, meaning they contribute to multiple different types of disorders.

• Developmental Cascades: The process by which early challenges in development spread across different systems and time points, shaping long-term outcomes.

• Developmental Pathways:

  • Continuity: The persistence of traits or disorders over time.

  • Equifinality: Different early experiences or risk factors leading to the same outcome.

  • Multifinality: A single risk factor leading to many different possible outcomes.

• Lifespan Prediction: Early development is a critical predictor for psychopathology throughout the entire lifespan.

Defining Offspring in Psychopathology

Within the context of psychopathology research, the term "offspring" is used broadly. It does not solely refer to children; it includes grown-up children or adult descendants. This term is consistently applied when discussing:

• Genes and heritability.

• Environmental risk factors.

Behavioural Genetics: Genotype and Phenotype

Behavioural genetics explores the relationship between inherited material and observable traits.

• Genotype: The actual genes passed down from parents to offspring.

• Phenotype: The observable characteristics or traits of an individual.

• The Core Question: To what extent is the phenotype determined by genes versus environmental influences?

Understanding Heritability

Heritability refers to the genetic contribution to the variation of a trait or phenotype within a specific population.

• Definition: It is the proportion of variation in a trait that is associated with genetic variation.

• Scale: Measured on a scale of $0$ to $1$ (or $0\%$ to $100\%$).

• Example: A heritability estimate of $0.7$ means that $70\%$ of the differences in a trait among a population are associated with genetic differences.

Typical Research Methods in Behavioural Genetics

1. Adoption and Family Studies: These examine the similarities between parents and offspring, as well as siblings and half-siblings, to tease apart genetic and environmental influences.

2. Twin Studies: These are the gold standard for heritability research:

   • Monozygotic (MZ) Twins: Identical twins who share $100\%$ of their genes.

   • Dizygotic (DZ) Twins: Non-identical (fraternal) twins who share approximately $50\%$ of their genes.

Concordance and Shared Environments

Concordance refers to the rate at which pairs (such as twins or siblings) share a specific trait, such as eye color, height, or a psychopathology diagnosis.

• Relative Differences: Comparison is usually made between MZ and DZ twins, or full and half-siblings. Concordance is typically higher in MZ twins and full siblings due to greater genetic overlap.

• Assumptions: These studies often assume that the proportion of shared and unique environmental factors is the same for the groups being compared (e.g., assuming MZ and DZ twins both grew up together in the same household environment).

Disadvantages of the Concordance Approach

According to Whitney et al. (2025), there are several limitations to using concordance rates:

• Environmental Similarity: The assumption of equal environment is challenged; MZ twins may experience greater environmental similarity than DZ twins.

• Causation: High concordance does not technically prove causation.

• Sample Differences: Findings can differ significantly between clinical samples and broader population samples.

• Measurement Issues: Diagnosis is often treated as an "end-point" or a binary (present/absent), whereas many traits are dimensional.

• Censoring: Some individuals might develop a disorder later in life, meaning they are "censored" in studies with shorter follow-up periods.

Liability Threshold Models

As an alternative to simple present/absent diagnosis models, liability models suggest that risk exists on a continuum (Whitney et al., 2025).

• Liability: Everyone in the population has a certain degree of liability (risk) for a disorder. A higher score indicates higher liability.

• Threshold: A diagnosis represents the "extreme end" of this continuum. Once an individual's liability crosses a specific threshold, they meet the criteria for a disorder.

• Key Insight: While a diagnosis is binary (present/absent), the underlying risk is not.

Evidence for Heritability

Extensive research (Allegrini et al., 2020; Hilker et al., 2018; Pettersson et al., 2018; Tick et al., 2015) has established several findings:

• Higher MZ Concordance: Consistently higher rates in identical twins vs. fraternal twins provide strong evidence for genetic liability.

• Broad Phenotypes: Heritability is evident in both diagnostic thresholds and "broad phenotypes" (milder versions of traits).

• Dimensionality: Genetic factors are important across both categorical and dimensional approaches to psychopathology.

• Environmental Interaction: Heritability estimates vary because environment also matters and varies. Environments are particularly important for the expression of broad phenotypes.

• Long-term Data: Studies with longer follow-ups help reduce the problem of censoring.

Molecular Genetics and Psychopathology

While behavioural genetics estimates "how much" is inherited, molecular genetics looks at "what" is inherited.

Core Assumptions

• Specific disorders are linked to specific genes (either monogenic—one gene, or polygenic—many genes).

• Different diagnoses have different genetic signatures.

• There is a biological basis that exists independently of the environment.

Known Genetic Causes

Some conditions have confirmed genetic causes via testing for inherited or de novo (new) mutations, such as divisions or deletions:

• Phenylketonuria: Caused by a specific genetic mutation.

• Chromosomal Abnormalities: Examples include Down’s Syndrome, Fragile X, and Williams Syndrome.

• Note: Not all aspects of a phenotype are directly linked to genes. Genetic abnormalities often affect basic processes which then lead to developmental cascades (Whitney et al., 2025).

Types of Genetic Variants

According to Smoller et al. (2019) and Whitney et al. (2025), two main types of variants are studied:

1. Single Nucleotide Polymorphisms (SNPs):

   • Definition: A variant in a single DNA location.

   • Frequency: Common.

   • Effect: Small effects on the phenotype.

2. Copy Number Variants (CNVs):

   • Definition: Different number of copies of certain DNA segments.

   • Frequency: Rare.

   • Effect: Larger effects on the phenotype.

Genome-Wide Association Studies (GWAS)

GWAS involves scanning the entire genome to identify if common genetic variants (SNPs) are associated with a specific trait or disorder across many individuals.

• Method: Compares "cases" (individuals with a disorder) against "controls" (those without).

• Requirements: Needs massive sample sizes to reach statistical significance.

• Findings:

  • SNPs meet genome-wide significance for multiple psychopathologies.

  • Psychopathology is polygenic (influenced by many genes).

  • Individual effects are small—lower than heritability estimates from twin studies.

  • High cross-disorder association: Shared genetic variants exist between different disorders, overlapping with dimensional psychopathology.

Polygenic Risk Scores (PRS)

A PRS is calculated by adding together the weighted effects of various genetic variants identified in GWAS.

• Function: It measures genetic predisposition.

• Nature: Continuous across the population; a higher score indicates higher risk.

• Predictive Value:

  • PRS for adult diagnoses are related to childhood dimensional psychopathology (Allegrini et al., 2020; Neumann et al., 2021).

  • Specificity increases over time as individuals age.

  • PRS for Bipolar Disorder and Depression relate to symptom severity and functional outcomes across a range of psychopathologies (Mistry et al., 2018).

  • Effects remain relatively small.

Current State of Molecular Genetics Research

Contemporary research (Andreason et al., 2023; Smoller et al., 2019; Whitney et al., 2025) suggests:

• Complexity: Psychopathology is highly polygenic.

• Pleiotropy: Genes do not "read the DSM"; the same genes often influence multiple different disorders.

• Biological Processes: Genes do not directly code for behavior; they code for biological processes which then have cascading effects.

The Missing Heritability Problem

There is a significant discrepancy between estimated heritability from twin studies and the heritability explained by molecular genetics (SNPs).

• Possible causes: Overestimation in twin studies, missed small molecular effects, or the influence of rarer variants (Andreason et al., 2023; Joseph, 2012).

Influence of Parent Psychopathology

The timing of exposure to a parent's psychopathology is critical (Aktar et al., 2019; Bailey et al., 2021):

• Pregnancy (Perinatal): Biological pathways involve physiological changes (e.g., stress hormones) that influence fetal development.

• Childhood: Behavioural pathways include modeling of behaviors and stressors linked to the parent’s mental health.

• Intergenerational Transmission: This is both genetic and environmental. Importantly, it is not "destined."

• Bidirectionality: Some evidence suggests the relationship between parent and child psychopathology is bidirectional (Xerxa et al., 2020).

• Stability: Low levels of psychopathology in parents generally lead to less psychopathology in offspring.

Gene x Environment Interactions (G x E)

The Diathesis-Stress Model is the primary framework for understanding G x E interactions.

• Focus: Adversity (e.g., parent psychopathology) is the most studied environmental factor.

• Findings: Research is still emerging, with mixed findings and inconsistent methods.

• Complexity: Because people are unique, these interactions involve multiple, complex factors.

Summary and Key Takeaways

• Evidence supports the role of genes and heritability, but heritability is never $100\%$

• Common genetic variants (SNPs) make small individual contributions.

• Parental psychopathology is a predictor of offspring outcomes via both biological and behavioral pathways.

• Cross-disorder associations are common; many disorders share genetic roots.

• Warning: Do not view "faulty genes" as inevitable. Genetic risk may be a necessary factor for some, but it is rarely sufficient on its own and requires interaction with the environment.