Heritability and Genetics in Behavioral Science

Heritability

Estimates the proportion of variance in a trait within a population that can be attributed to genetic differences. It is population-specific, not individual-specific.

Heritability Range

Range from 0 (no genetic contribution) to 1 (entirely genetic).

0.5 suggests ~50% variability due to genetics.

Heritability Calculation

Calculated using twin studies: Heritability ≈ 2(rMZ - rDZ),

Where MZ twins share 100% of genes and DZ twins share ~50%.

Vp Formula

Vp = A² + C² + E²,

A² = Additive genetic influence,

C² = Shared environment,

E² = Non-shared environment.

High Heritability Misconception

High heritability does not mean a trait is immutable or unaffected by the environment.

Heritability other misconceptions

Estimates vary across time, populations, and methods.

Heritability says nothing about causation in individuals.

Environmental Variability and Heritability

Heritability increases if environmental variability decreases (e.g., schooling is equal).

Flynn's Multiplier Effect

Small genetic differences may influence environments that reinforce these differences

Mendelian Inheritance - basic principle

Inheritance is based on discrete units (genes), where traits are determined by alleles.

Traits can be dominant or recessive.

Genotype

Genetic makeup (e.g., GG, Gg, gg).

Phenotype

Observed traits (e.g., green or yellow peas).

Punnett Squares

Predict outcomes of genetic crosses.

Heterozygous Cross Phenotypic Ratio

A heterozygous cross (Gg × Gg) yields a 3:1 phenotypic ratio (dominant:recessive).

Serotonin Transporter Gene

Has two alleles: long (l) and short (s).

Effects are additive, not strictly dominant/recessive.

Gene Transcription Function

Replication - copying for new cells or offspring; Protein Synthesis - via transcription and translation.

Transcription Process

DNA → mRNA in the nucleus; RNA polymerase reads DNA and synthesizes complementary RNA.

mRNA Function

mRNA leaves the nucleus and is translated into proteins in the ribosome.

Tryptophan Hydroxylase

Genes transcribed into enzymes like tryptophan hydroxylase (involved in serotonin production).

Caspi et al. (2003) Study

Key Finding: Short allele (especially ss) associated with increased risk of depression only when combined with life stress.

No direct effect of gene or stress alone — but a significant gene-environment interaction (G×E).

Zhang et al. (2005) Study - Tryptophan Hydroxylase Gene

Finding: Mutant form associated with lower serotonin production in vitro.

Higher prevalence in depressed individuals, but not all depressed people have it → other factors involved.

DAT1: Dopamine transporter gene

linked to ADHD, cocaine addiction, schizophrenia

GABRA2: GABA receptor gene

linked to conduct disorder, epilepsy;

CHRNA5-A3-B4 cluster: Nicotinic receptor gene

linked to smoking and nicotine addiction.

Polygenic Influences

Single genes rarely cause psychiatric conditions.

Environmental Context

Environmental context matters: E.g., stress, parenting, peer groups.

Replication Problems

Not all studies consistently replicate associations.