Genetic Foundations to Human Behavior: Part I — Behavioral Genetics & Molecular Genetics (Week 2)
Behavioral Genetics: Key Concepts
Examines how genetics and environment influence the development of behaviors/traits (phenotypes).
Phenotype: An observable characteristic that can be measured (e.g., hair color, height, IQ).
Goal: Break down the variation in a trait/behavior into environmental and genetic components.
Variance: A statistic that captures heterogeneity (differences) among people who share a phenotype.
Example: Height – everyone in the class has height as a phenotype, but individuals vary in height; variance measures how different heights are across the class.
Course Plan for the Week (Context for the Reading/Quiz)
Wednesday: Cover Behavioral Genetics section of Chapter 2.
Friday: Cover molecular genetics section from Chapter 2.
Reading Quiz – due Friday at 11:59pm; covers Chapter 1 and Chapter 2 (pp. 35-69).
Quiz format: 10 questions, 30-minute timer, on Canvas; open-note/book; must be taken individually.
Correct answers released after all submissions; late quizzes not accepted for credit.
Two Types of Environments
Shared Environment (c^2): settings shared by siblings that tend to make them more similar.
Examples: parents, socioeconomic status (SES), neighborhood, shared school.
Nonshared Environment (e^2): different settings across siblings that tend to make them different.
Examples: different peer groups, prenatal environments, unique experiences; can also arise when siblings interpret the same event differently (e.g., divorce, death of a loved one).
Heritability and Variance Decomposition
Heritability (h^2): the extent to which individual differences in a behavior/trait are due to differences in DNA sequences.
BG studies partition phenotypic variance into components: genetic (h^2), shared environment (c^2), and nonshared environment (e^2).
Fundamental equation (sum-to-one):
h2 + c2 + e_2 = 1.0Interpretations:
If h_2 = 0, then genetic variance does not contribute to differences in the phenotype.
If h_2 = 0.50, then 50% of phenotypic variance is accounted for by genetic factors.
If c_2 = 1, then 100% of the phenotypic variance is due to shared environment (note: this is the slide’s example; real-world values typically vary).
Heritability of Traits
Most behaviors and personality traits show moderate to large heritability estimates (roughly between 0.50 and 0.90).
Implications:
Individual differences on observed traits can be explained by genetic factors to a substantial degree.
Nonshared environment is typically the second most influential source of variability for behavioral outcomes.
The shared environment tends to have the lowest influence on explaining differences in behaviors (as studied by traditional criminologists).
Disclaimer: You do not need to perform calculations of heritability or environmental values; you should understand what the terms mean and why they matter.
Polderman et al. (2015) – Meta-analysis Overview
Large-scale meta-analysis of twin studies from the past 50 years.
Data scope: over 17,000 traits (phenotypes) across more than 2,700 studies and 14.5 million twin pairs.
Key finding: about 49% of variability in roughly 70% of traits analyzed is explained by genetic differences.
Interpretation: approximately 50% of differences in traits are due to genes and the remainder to environmental factors (in line with the h^2 + c^2 + e^2 framework).
How Are Behavioral Genetics (BG) Studies Conducted?
Classic Twin Study
Monozygotic Twins Reared Apart (MZAs)
Adoption Studies
Family Studies
Classic Twin Method
Comparison groups:
Monozygotic (MZ) twins: share ~100% of their DNA.
Dizygotic (DZ) twins: share ~50% of their DNA.
Interpretive rule:
If MZ twins are more similar to each other than DZ twins are to each other, genes have a greater influence on the phenotype.
If DZ twins are as similar as MZ twins, environment has a greater influence on the phenotype because DZ twins share the environment but share less DNA.
Rationale: Twins (both MZ and DZ) share an environment; differences in similarity between MZ and DZ twins point to genetic influence.
Sibling and Twin Illustrations (Illustrative Diagram in Slides)
Regular siblings share about 50% of their genetic material; fraternal twins are conceptually similar to regular siblings in this respect.
Fraternal twins are conceived in the same pregnancy but from two different sperm and eggs; identical twins are conceived in the same pregnancy from a single sperm and egg that split.
Quick relationships:
Regular Sibling: 50% genetic similarity
Fraternal (Dizygotic) Twins: 50% genetic similarity
Identical (Monozygotic) Twins: 100% genetic similarity
MZ Twins Separated at Birth (MZAs)
Very rare studies where MZ twins are raised apart.
Logic: Similarities between MZAs would be due to shared genes, since they do not share environments.
If only environment mattered, separated MZ twins would be dissimilar.
Research has found uncanny similarities in some MZAs.
Example reference: a well-known case described in the Jim Twins (see video linked in slides).
Fictional illustration: “The Parent Trap.”
Adoption Studies
Study families that raise children to whom they are not genetically related.
If the adoptee more closely resembles biological parents, genetics have a stronger influence on the phenotype.
If the adoptee resembles the adoptive (environmental) parents, environment has a stronger influence on the phenotype.
Family Studies
Look at family dynamics beyond twins/adoptees (full-siblings, half-siblings, step-siblings, etc.).
Require measuring at least two people within the same household with known degrees of biological relatedness.
How do these findings compare with SSSMs (special sibling similarity models) from previous lectures?
Family Member Relatedness (Illustrative Table)
Shared genetic material:
MZ Twins: 100%
DZ Twins: 50%
Full Siblings (same mom and dad): 50%
Half Siblings (different mom or dad): 25%
Cousins: 12.5%
Unrelated Siblings (e.g., step-siblings): 0%
Disclaimer on BG Methods
No method is perfect; there are always improvements to be made in both genetic and environmental spaces.
Overall, methods that measure both environmental and genetic influences tend to explain a greater portion of phenotypic variation in traits related to criminal behavior.
Molecular Genetics – Preview for Friday
Friday’s session will cover molecular genetics: DNA, genes, and types of genetic differences that can lead to phenotypic differences.
Expect terms you don’t know yet; take notes and use office hours if needed.
Reading quiz on Friday will cover molecular genetics in addition to Chapter 2 content.
Molecular Genetics
Key concept: DNA is the genetic code; most individuals (except MZ twins) have unique DNA.
Four nucleotide bases:
Adenine, Thymine, Guanine, Cytosine
Base-pairing rules: A
ightleftharpoons T, \, T
ightleftharpoons A, \, G
ightleftharpoons C, \, C
ightleftharpoons G
DNA is like an instruction manual; it cannot do things by itself without cellular machinery.
What Do Genes Do?
Gene: a set of base pairs within DNA that codes for a protein.
There are about 25{,}000 genes in the human genome.
Each gene is about ext{~}1{,}000 base pairs long (approximate).
Genes code for all body functions – even brain structures, gut functioning, fingernail shape, etc.
Protein: composed of amino acids; proteins are the workers that execute the instructions encoded by DNA.
From Gene to Protein (Overview)
Two-step process:
Transcription: RNA reads a codon in DNA and carries instructions to a ribosome.
Codon: a section of a gene that codes for a specific amino acid.
Translation: ribosome reads RNA instructions and assembles amino acids into a protein.
Once the protein is complete, it leaves the ribosome to perform its functions (e.g., determine hair color, eye color, fingernail growth).
The process is complex, but for this course, focus on the conceptual flow: DNA -> RNA (transcription) -> protein (translation).
Gene Terminology: Variants and Genotypes
Genotypic variance: differences in DNA sequences lead to different phenotypes.
Allele: alternative versions of a gene that can produce different phenotypes.
Genotype: the combination of alleles that determines the phenotype.
Types of Genotypes (Illustrative Hair Color Example)
Gene with two alleles (polymorphism): the two alleles can be B (brown) and b (blonde).
Genotype possibilities: BB, Bb, bb.
Homozygous: BB or bb (same allele from both parents).
Heterozygous: Bb (different alleles from each parent).
Genetic Variations: Base-Pair Differences
The amount of DNA is vast; even a single base-pair difference can alter the RNA instruction and the resulting protein.
The human genome has many such differences that contribute to phenotype variation and disease risk.
Consequences range from hair color to serious conditions like sickle cell anemia, rheumatoid arthritis, and some cancers.
Single Nucleotide Polymorphisms (SNPs)
SNP: a difference at a single base pair in the DNA sequence.
Impact: a different base leads to a different codon, potentially altering the amino acid and the resulting protein.
Prevalence: ~90% of DNA differences are SNPs.
Disease relevance: ~85% of genetic disorders are due to SNPs.
Microsatellites and Minisatellites
Repeats: these polymorphisms involve repeated base-pair sequences that are longer than usual in the DNA strand.
Minisatellites have more repeat units than microsatellites.
For this course, the key point is that they differ from SNPs and can contribute to genotype differences that influence phenotypes.
Quick Visual Reference (DNA Coding Overview)
Letters in a sequence (A, T, G, C) form a code that determines proteins.
The exact arrangement and repetition of bases determine the final phenotype via proteins.
Summary and Next Steps
This material covers the foundations of molecular genetics and how genetic variation can influence phenotype.
On Monday, we will finish Chapter 2 by discussing specific polymorphism differences, neurons, and neurotransmitters.
Reading Quiz 2 will cover these topics.
Attendance will be tracked in Canvas (lowest two attendances dropped at semester end).
Attendance and Quiz Reminders
Attendance check will be on Canvas and is not graded directly (lowest two drops).
Reading Quiz is due tonight at 11:59pm.
If you have questions, you can request one-on-one office hours Thursday or before/after Friday class.