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Behavioral Genetics and Neural Communication

Behavioral Genetics: The Heritability of Behavior and Mental Processes

Introduction to Behavioral Genetics

  • Definition: The study of the heritability of behavior and mental processes.
  • Scope: Examines how ways of behaving and thinking are inherited from parents through genes.
  • Note: While called "behavioral genetics," it applies to both behavior and mental processes.

Biological Background (Review, Not Exam Material)

  • Cells: The human body is composed of numerous cells.
  • Nucleus: A major part of almost every cell, containing chromosomes.
  • Chromosomes:
    • 23 pairs (total 46 chromosomes) are found in the nucleus of nearly every cell.
    • Rod-like, long, and skinny structures.
    • Composed of DNA.
  • DNA (Deoxyribonucleic Acid):
    • A spiral structure, often called a double helix.
    • Contains hereditary information, dictating what characteristics will be inherited.
  • Genes:
    • Strands or sections of DNA.
    • Code for the characteristics inherited from parents.
    • Determine inheritance by manufacturing proteins, which in turn produce the chemicals making up the body.
  • Gene Variations:
    • All humans possess the same basic genes.
    • Each gene can have two or more variations (alleles).
    • Different versions of genes can produce different characteristics (e.g., varying eye colors from the same gene for eye color).

Genotype vs. Phenotype (Essential Exam Material)

  • Genotype: Refers specifically to the genes a person inherits from their parents.
  • Phenotype: Refers to a person's actual, real-world characteristics, or traits; how their genes are expressed.
  • Interaction: Individuals with the same genotype can exhibit different phenotypes.
    • Genes do not dictate a concrete, fixed amount of a characteristic.
  • Range of Reaction (or Range of Possibilities):
    • Genes provide a range of possible amounts for a particular characteristic.
    • For example, genes for height might indicate a range between 5 feet 7 inches tall and 6 feet tall, rather than an exact height like 5 feet 11 rac{1}{8} inches.
  • Gene-Environment Interaction: Our surroundings, or environment, interact with our genes to determine our phenotype.
    • Example (Height & Nutrition):
      • Genes might provide a height range for an individual from 5 feet 7 inches to 6 feet.
      • Good Nutrition (wealthy environment): Leads to an individual growing towards the taller end of their range (e.g., 5 feet 11 inches or 6 feet), as the environment provides necessary nutrients for growth.
      • Poor Nutrition (impoverished environment): Leads to an individual growing towards the shorter end of their range (e.g., 5 feet 7 inches or 5 feet 8 inches), as the body doesn't receive adequate nutrients despite the genetic potential for taller growth.

Measuring Genetic vs. Environmental Influence

  • Psychologists aim to quantify how much influence genotype versus environment has on phenotype.
  • Challenge: Directly manipulating genes or major environmental factors for causal experiments is often impossible or unethical.
    • Independent variables like genes cannot be manipulated (e.g., altering a person's genes).
    • Major environmental factors (e.g., upbringing, parents, neighborhood) are difficult to manipulate experimentally.
  • Solution: Natural Experiments: Studies that utilize naturally occurring groups, effectively having an experimental and control group without direct manipulation.

Methods for Studying Behavioral Genetics

1. Twin Studies

  • Utilize two natural types of twins:
    • Identical (Monozygotic) Twins:
      • Result from one sperm fertilizing one egg, with the resulting embryo dividing into two.
      • Originate from the same sperm and egg, which carry genetic material.
      • Are 100 ext{%} genetically identical, sharing the same genotype.
    • Fraternal (Dizygotic) Twins:
      • Result from two different eggs being released and fertilized simultaneously by two different sperm.
      • Are not genetically identical; they are as genetically similar as non-twin siblings born at different times.
  • Environmental Control: Both identical and fraternal twins typically share the same amount of environmental factors:
    • Grow up in the same womb.
    • Grow up in the same household with the same parents.
    • Experience similar influences, live in the same neighborhood, attend the same schools.
    • Therefore, environment is naturally controlled across these two groups.
  • Methodology: Compare identical twins to fraternal twins on a trait of interest.
  • Interpretation:
    • If identical twins are similar on a trait (e.g., similar height) but fraternal twins are not similar on that trait (e.g., dissimilar height), then the trait is considered highly influenced by genetics.

2. Adoption Studies

  • Focus on children adopted at birth, who have two distinct sets of parents:
    • Biological Parents:
      • Contribute only genotype (sperm and egg providing genes).
      • Do not contribute to the child's environment (child is given up at birth).
    • Adopted Parents:
      • Do not contribute to the child's genotype (not biologically related).
      • Do contribute the child's environment (determine upbringing, home life).
  • Methodology: Compare how similar a child is to their biological parents versus their adopted parents on a specific trait.
  • Interpretation:
    • If a child is more similar to their biological parents on a trait, it suggests the trait is fairly genetic.
    • If a child is more similar to their adopted parents on a trait, it suggests the trait is more due to the environment.
  • Example (Schizophrenia):
    • Children with biological parents who have schizophrenia are more likely to develop schizophrenia themselves.
    • However, children with adoptive parents who have schizophrenia are no more likely to develop the disorder.
    • Conclusion: Schizophrenia is primarily due to genetic factors (contributed by biological parents), not environmental factors (contributed by adopted parents).

Neural Communication (Laying Groundwork for Future Topics)

  • Electrical Charge of Neurons:
    • A neuron in a resting state has an electrical charge of approximately ext{-}70 millivolts ( ext{-}70 ext{ mV}).
    • This charge is maintained by differences in the concentration of ions (positively or negatively charged particles) inside and outside the cell.
      • Outside the cell: High concentration of positively charged Na^+ (sodium) ions and negatively charged Cl^- (chloride) ions (sodium chloride/salt).
      • Inside the cell: Lower concentration of positively charged K^+ (potassium) ions and a higher quantity of negatively charged protein ions.
  • Ion Channels:
    • Protein structures embedded in the cell membrane (which separates inside from outside).
    • Act as gates, controlling the flow of ions into and out of the cell.
    • Normally held closed, maintaining the cell's electrical potential.
    • Can open upon receiving a signal from a neighboring neuron.
  • Synapse: The connection point where one neuron communicates with another.
  • Neurotransmitter (NT): A chemical signal released by a neuron into the synapse.
  • Receptor: A type of protein embedded in the membrane of the receiving neuron.
    • NTs bind to receptors (like a key fitting a lock), initiating biochemical events.
    • This binding can open ion channels.
  • Excitatory Postsynaptic Potential (EPSP):
    • Occurs when NT binding opens Na^+ channels, allowing positive Na^+ ions to flow into the cell.
    • Causes a small, localized increase in positive charge around the channel.
  • Inhibitory Postsynaptic Potential (IPSP):
    • Occurs when a part of the cell becomes more negative (e.g., due to influx of negatively charged ions).
  • Action Potential (AP):
    • Trigger: If enough EPSPs accumulate, causing the membrane potential at the axon hillock to reach a threshold of excitation ( ext{-}55 ext{ mV} to ext{-}40 ext{ mV} typically).
    • A brief, significant reversal in membrane polarity.
    • Process: A rapid increase in membrane permeability to Na^+ (depolarization), immediately followed by a brief increase in permeability to K^+ (repolarization).
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