Genetics, Brain Development, and Visual Perception

Genetics and Biological Foundations

The Human Genome: The complete set of instructions for constructing a human being.

• Genotype: A person’s 23 pairs of chromosomes.
• Chromosome: Thread-like structures made of protein and a single molecule of DNA.
• Gene: A segment of DNA that produces a single protein.
• Histones: Proteins that provide structural support for a chromosome.
• Nucleotide: A molecule consisting of a nitrogen-containing base.

Gene Expression and Epigenetics

Gene Expression: The translation of genotype into phenotype.

• Phenotype: Observable characteristics.
• Imprinted Genes: Only the copy from the mother or the father, but not both, will be expressed. Approximately 1% of human genes are imprinted.
• Epigenetics: How behaviors and environment cause changes that affect gene function. Gene expression can be switched on or off; some epigenetic changes can be passed to offspring (e.g., smoking leads to genome-wide methylation).
• DNA Methylation: Adding chemical groups to DNA to block transcription, effectively switching genes off. This can be reversed following smoking cessation.

Sources of Genetic Variability

Alleles: Different versions of a gene.

• Single-nucleotide polymorphisms (SNPs): Alleles that differ in only one location.
• Dominant vs. Recessive: A dominant allele overrules the recessive one. For example, Huntington’s disease is an autosomal dominant trait requiring only a single copy of the huntingtin (HTT) allele.

Sex Chromosomes and Inheritance

The 23rd pair of chromosomes determines sex: XX (female) and XY (male). The X chromosome is always present in the ovum, while the sperm carries either X or Y.

• Sex-linked characteristics: Phenotypes resulting from gene expression on the X chromosome (e.g., Hemophilia).
• Copy-number variations (CNVs): Variations in the number of gene copies, often associated with autism spectrum disorder and schizophrenia.

Heritability and Environment

Heritability refers to the contribution of genetics to trait variation within a population, not individuals. It must be assessed alongside environmental factors (nature vs. nurture). Twin studies, such as the Minnesota Study of Twins Reared Apart, help isolate these variables.

Alzheimer’s Disease Example: Nature (inherited APOE4 allele) increases baseline risk, while nurture (lifestyle) modifies it. Regular exercise and cognitive stimulation increase brain plasticity and cognitive reserve, protecting against the disease.

Neural Development and Plasticity

Cell Migration: Most cells are guided by radial glia to become excitatory neurons in the cerebral cortex. Other cells migrate tangentially to become inhibitory neurons.

• Differentiation: Precursor cells become specific motor or sensory neurons via extracellular signaling factors (e.g., NGN2).
• Neural Tube: The precursor of the CNS, organized along dorsal/ventral and rostral-caudal axes.
• Growth Cones: Mobile structures at the end of axons and dendrites that explore the environment. Guidepost cells release chemicals like netrins to attract or repel growth.
• Synaptic Specificity: The identification of the correct target neuron.

Synapse Formation and Pruning

1. Growth cone approaches dendrite.
2. Synaptic vesicles are transported to the connection site.
3. Vesicles dock and are released.
4. Dendritic spine forms and receptors are inserted.

Cell Death (Apoptosis): 40-75% of new neurons die during development due to a lack of neurotrophins. Synaptic pruning continues throughout the lifespan to refine functional connections.

Brain Plasticity

Plasticity depends on age, exercise (which increases BDNF), and experience. Functional plasticity allows the brain to move functions from damaged areas to auxiliary networks (Neural Compensation Hypothesis).

Sex Hormones and Behavior

Sex hormones (steroids derived from cholesterol) regulate reproductive systems and secondary sexual characteristics. Organization effects are permanent (gestational/puberty periods), while activation effects are reversible (e.g., aggression, courtship).

• Testosterone: Influences competition and relationship status. Levels drop following reproductive success to favor pair bonding.
• Oxytocin and Vasopressin: Dimorphic hormones influencing bonding and social behavior.

Visual Perception and Transduction

Environmental stimuli are transduced into action potentials. The retina contains photoreceptors, while the fovea provides high-acuity vision.

Photoreceptor Function

Rhodopsin (11-cis retinal + opsin) absorbs light and bleaches into all-trans + opsin. In light, photoreceptors hyperpolarize and decrease glutamate release.

Visual Processing

Bipolar and ganglion cells utilize center-surround organization to detect edges. Muller cells act as fiber optics to focus light on specific cones.

Binocular Cues: Retinal disparity allows for depth perception, processed by disparity-selective binocular cells.