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 $11\frac{1}{8}$ 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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