Biological Bases of Behavior: Genetics and Neurodevelopment

Genetics: The Human Genome, Chromosomes, and Molecular Structure

  • The Human Genome

    • The human genome contains approximately 19,00019,000 protein-coding genes.

    • Humans possess 2323 pairs of chromosomes, totaling 4646 chromosomes in each cell.

    • One set of chromosomes is inherited from each biological parent.

    • Deoxyribonucleic acid (DNA) is the molecule that carries all genetic instructions.

  • Genotype vs. Phenotype

    • Genotype: This refers to the specific genetic makeup of an individual.

    • Phenotype: This refers to the observable traits of an individual.

    • Interaction: The phenotype is the result of the interaction between genes and the environment (Genes+Environment\text{Genes} + \text{Environment}).

    • Examples:

      • Eye Color: Primarily determined by genetics.

      • Anxiety Risk: Determined by specific genes interacting with environmental factors.

    • Standard Male Karyotype: Written as 46 XY46 \text{ XY}.

  • DNA and Chromosomal Structure

    • DNA (Deoxyribonucleic acid): The molecule responsible for genetic instructions, tightly organized within the nucleus.

    • Organization Process: DNA wraps around proteins called histones to form nucleosomes. Nucleosomes then coil together to form the structure of chromosomes.

    • Key Structural Components:

      • Chromosome: Tightly coiled DNA and proteins.

      • Chromatids: The two identical halves of a single replicated chromosome.

      • Centromere: The specific region where two chromatids are joined together.

      • Telomere: Protective caps located at the ends of chromosomes.

      • Chromatin: The less condensed form of DNA, representing its active state.

Genes, Bases, and Gene Expression

  • Defining Genes

    • Genes are specific segments or sections of DNA that carry instructions for building proteins.

    • The function of a gene is determined by its DNA sequence.

    • Genes influence physiological traits such as eye color and the functionality of neurotransmitter systems.

  • Chemical Bases of DNA

    • DNA is built from four nitrogenous bases:

      • Adenine (A)

      • Thymine (T)

      • Cytosine (C)

      • Guanine (G)

    • Bonding Rules: Adenine always pairs with Thymine (A=TA=T) and Cytosine always pairs with Guanine (C=GC=G).

  • The Process of Gene Expression

    • The movement of genetic information follows the central dogma: DNA→RNA→Protein\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}.

    • Proteins are the workhorses that carry out most cellular functions.

    • The Two Main Steps:

      1. Transcription: The conversion of DNA into RNA.

      2. Translation: The process where RNA (read as codons) is converted into a Protein.

    • Regulation: Gene activity is strictly regulated by developmental stages and environmental factors. At any given time, only a small portion of genes (approximately 10–20%10\text{--}20\%) are active.

Alleles, Genetic Variation, and Inheritance Patterns

  • Alleles and Individual Variability

    • Alleles: Different versions or variations of the same gene.

    • Most individuals have two alleles for every gene, which contributes to individual variability.

    • Zygosity:

      • Homozygous: Possessing the same alleles (e.g., BBBB or bbbb).

      • Heterozygous: Possessing different alleles (e.g., BbBb).

  • Inheritance Patterns

    • Dominant: A trait that is expressed whenever at least one dominant allele is present (e.g., BBBB or BbBb).

    • Recessive: A trait that is expressed only if both alleles match the recessive type (e.g., bbbb).

    • Incomplete Dominance: A mixed or intermediate expression of traits.

    • Genomic Imprinting: A phenomenon where only one parent's copy of a gene is expressed, while the other is silenced. The expression depends entirely on the parent of origin.

    • Clinical Examples:

      • Huntington's Disease: Follows a dominant inheritance pattern.

      • Cystic Fibrosis: Follows a recessive inheritance pattern.

      • Serotonin Transporter Gene: Exhibits partial dominance.

      • Prader-Willi and Angelman Syndromes: Disorders resulting from atypical genomic imprinting.

  • Sex-Linked Traits

    • These involve genes located on the X chromosome.

    • Because males have only one X chromosome, these traits are expressed more commonly in males.

    • Females: Classified as a "mosaic" for X-linked genes due to random X-inactivation.

    • Example: Color blindness and Blood types (specifically A,B,OA, B, O variants).

Sources of Genetic Diversity and Mutation

  • Meiosis and Genetic Diversity

    • Meiosis: The process of creating gametes (egg and sperm).

    • It reduces the chromosome number from 4646 to 2323 per cell.

    • Meiosis creates unique individuals through more than 8 million8 \text{ million} possible combinations through independent assortment alone.

    • Mechanism of Variation:

      • Independent Assortment: Random distribution of maternal and paternal chromosomes.

      • Crossing Over: The physical exchange of DNA segments between chromosomes during division.

  • Genetic Mutations

    • Mutations are changes in the DNA sequence. While most have little to no effect, some are clinically relevant.

    • SNPs (Single Nucleotide Polymorphisms): A change in a single base pair within the DNA (e.g., the APOE4APOE4 variant).

    • CNVs (Copy Number Variations): Duplications or deletions of large segments of DNA. These are associated with:

      • Autism Spectrum Disorder (ASD)

      • Schizophrenia (SCZ)

Population Concepts and Behavioral Genetics

  • Heritability

    • Definition: A population-level concept describing the extent to which genetic influence accounts for variation in a trait within a specific population.

    • Range: Expressed on a scale from 00 to 11.

    • Key Distinctions:

      • Applies to groups, not individuals.

      • High heritability does not mean a trait is fixed or unchangeable.

      • Measurement depends heavily on environmental variability.

      • Example: Height is highly heritable, but also heavily influenced by nutrition.

  • Behavioral Genetics Principles (Plomin et al., 2016)

    1. All psychological traits show significant genetic influence.

    2. No psychological traits are 100%100\% heritable.

    3. Polygenic Nature: Most traits involve many genes, each exerting a small effect.

    4. Environment remains crucial, specifically non-shared environments.

    5. Heritability of many traits (e.g., intelligence) increases as an individual ages.

    6. Gene-Environment Correlation: Genes can influence the types of environments individuals select or experience.

    7. Dimensional View: "Abnormal" behavior is viewed as the extreme end of normal variation.

Epigenetics: Gene-Environment Interaction

  • Mechanism of Action

    • Epigenetics involves changes in gene expression that do not alter the underlying DNA sequence. These changes can be long-lasting and potentially transgenerational.

    • Primary Mechanisms:

      • DNA Methylation: Typically marks a gene to be turned "off."

      • Histone Modification: Changes the accessibility of a gene for transcription.

      • RNA Interference: Blocks the translation of messenger RNA (mRNA) into protein.

  • Environmental Influences

    • Factors that trigger epigenetic changes include toxins (such as BPA), smoking, chronic stress, and trauma.

Neurodevelopment: From Conception to Brain Structure

  • Early CNS Development

    • Development begins approximately 3 weeks3 \text{ weeks} post-conception.

    • The neural tube forms the precursor to the brain and spinal cord. Failure of this tube to close results in neural tube defects.

    • Embryogenesis Timeline:

      • ~3 weeks: Formation of the neural plate.

      • ~4 weeks: Development of the neural tube. The inner portion (lumen) becomes the ventricles and spinal cord; the outer portion develops into brain structures.

  • The Three Germ Layers

    1. Ectoderm: Origin of the nervous system, sense organs, and skin.

    2. Mesoderm: Origin of muscles, skeleton, and the circulatory system.

    3. Endoderm: Origin of internal organs, such as lungs and the digestive system.

  • Neurogenesis and Neuroblasts

    • Neurogenesis: The rapid production of neurons via mitosis, occurring primarily in the ventricular zone during the prenatal period.

    • The prenatal brain produces 2–3× more neurons2\text{--}3 \times \text{ more neurons} than the final adult brain.

    • Neuroblasts: Immature neurons that will eventually migrate and differentiate into specific cell types.

Cell Migration, Differentiation, and Apoptosis

  • Cell Migration

    • Neurons move to their final locations in the brain, guided by chemical signals and radial glial cells.

    • The majority of migration is completed by roughly 4 months4 \text{ months} post-conception.

  • Apoptosis (Programmed Cell Death)

    • Development involves a massive elimination of cells; approximately 50%50\% of neurons are eventually eliminated.

    • Apoptosis occurs when neurons fail to make functional connections or maintain sufficient activity.

Refinement: Myelination and Synaptic Pruning

  • Myelination (Increasing Efficiency)

    • This is the formation of a fatty myelin sheath around axons to speed up neural transmission.

    • Oligodendrocytes: Provide myelin in the Central Nervous System (CNS).

    • Schwann cells: Provide myelin in the Peripheral Nervous System (PNS).

    • Nodes of Ranvier: Gaps in the myelin sheath that increase conduction speed.

    • Timeline: Myelination continues through adolescence, with the prefrontal cortex (PFC) continuing to myelinate into the mid-20s20\text{s}.

  • Synaptic Pruning

    • The elimination of unused or inefficient synapses to refine neural networks.

    • Early childhood is characterized by high synaptic density and high flexibility.

    • Pruning begins between ages  2–7 years~2\text{--}7 \text{ years} and continues through early adulthood, shifting the brain toward lower density but higher efficiency.

    • Principle: "Use it or lose it."

    • Clinical Relevance:

      • Autism Spectrum Disorder (ASD): Linked to reduced pruning (excessive connections).

      • Schizophrenia (SCZ): Linked to over-pruning (excessive loss of connections).

      • ADHD: Linked to delayed or reduced pruning specifically in the PFC.

Evolutionary Windows: Critical vs. Sensitive Periods

  • Critical Periods

    • Specific time windows where environmental input is strictly required for normal development. There is a very limited opportunity for recovery if the window is missed.

    • Examples: Vision and language.

  • Sensitive Periods

    • Times when the brain is particularly responsive to experience, but development remains more flexible than in critical periods.

    • Examples: Emotion regulation and general learning.

Key Takeaways

  • Dynamic Development: Development is not deterministic; genes provide a blueprint, not a guaranteed outcome.

  • Polygenic and Dimensional: Behavior is influenced by many small genetic and environmental factors; no single gene is responsible for complex traits.

  • Growth through Elimination: Brain development relies on an initial overproduction followed by refinement through apoptosis and pruning for efficiency.

  • Time Sensitivity: The timing of experiences (critical and sensitive periods) is essential for proper neurological maturation.