Genetics, Heritability, Neuroimaging, and The Endocrine System

Purpose: Studying human genetics helps researchers understand the underlying biological basis of different behaviors, thoughts, and reactions.
Key questions:
- Why do two people infected by the same disease have different outcomes?
- Are there genetic components to psychological disorders, such as depression?
- How are genetic diseases passed through family lines?
Historical reference: Charles Darwin explored the concept of inheritance of traits throughout generations in his theory of evolution through natural selection.

Natural selection: The organisms that are better suited for their environment will survive and reproduce, while those that are poorly suited for their environment will die off.
Characteristics and behaviors that impact survival and reproduction:
- Those that help protect against predators.
- Those that increase access to food.
- Those that help to keep offspring alive.
Darwin quote: “It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most adaptable to change.”

Genetic variation = the genetic difference between individuals.
Role:
- Contributes to a species’ adaptation to its environment.
Basic origin: Begins when an egg (containing 23 chromosomes) is fertilized by a sperm (containing 23 chromosomes).
Chromosome: long strand of genetic information known as DNA.
DNA: deoxyribonucleic acid; a helix-shaped molecule made of nucleotide base pairs.
In each chromosome, sequences of DNA make up genes.
Gene: a sequence of DNA that controls or partially controls physical characteristics known as traits (eye color, hair color, etc.).
Alleles: different possible variations of a gene; a gene may have multiple alleles.
Example: A gene coding for hair color may have several alleles, influencing the observed hair color differences.
Note: Offspring inherit a combination of alleles from both parents, contributing to individual variation.

Most inheritable traits are polygenic = controlled by more than one gene.
Alleles can be dominant or recessive.
Genotype-phenotype relationships:
- 1+ dominant allele present (Aa or AA) → corresponding dominant phenotype is expressed.
- 1 recessive allele present (i.e., the genotype Aa) → dominant phenotype is expressed.
- Both recessive alleles present (aa) → corresponding recessive phenotype is expressed.
Terminology:
- Heterozygous = consisting of two different alleles (Aa).
- Homozygous = consisting of two identical alleles (AA / aa).
Credit: B4FA

A Punnett square is a tool used to predict how genes will interact in the production of offspring.
Notation: Capital letter represents the dominant allele; lowercase letter represents the recessive allele.
Example context: cleft chin trait.
- Dominant allele: B (cleft chin).
- Recessive allele: b (no cleft chin).
- Offspring phenotype expectation:
- If a pair contains at least one B, you can expect a cleft chin phenotype.
- A smooth chin phenotype appears only when there are two copies of the recessive allele, bb.

Nature and nurture work together like puzzle pieces; environment and genes interact to form individuals.
Approaches to gene–environment interplay:
- Range of reaction: genes set the boundaries within which we can operate; environment interacts with genes to determine where within that range we will fall.
- Genetic–environmental correlation: genes affect our environment, and our environment influences the expression of our genes.
- Epigenetics: study of gene–environment interactions such as how the same genotype leads to different phenotypes.

Neuroimaging techniques involve imaging the brain to study structure or function.
Techniques covered:
- CT Scan (Computed Tomography)
- PET Scan (Positron Emission Tomography)
- MRI (Magnetic Resonance Imaging)
- fMRI (Functional MRI)
- EEG (Electroencephalography)

Involves X-rays and creates an image through X-rays passing through varying densities within the brain.
Focus: shows structure, not function.
Applications: can be used to show brain tumors.
Example depiction: Healthy brain vs. brain tumor image (left vs. right lobes).

PET Scan shows activity in different parts of the brain (i.e., function).
How it works: involves injecting individuals with a mildly radioactive substance and monitoring changes in blood flow to different regions of the brain.
Source note: OpenStax ancillary resource; attribution information is provided with the resource.

MRI: uses magnetic fields to produce a picture of the tissue being imaged; structural imaging.
fMRI: shows changes in metabolic activity over time; functional imaging.
Visualization: an image from an fMRI represents activity over time; a single frame is shown as an example.
Credit: modification of work by Kim J, Matthews NL, Park S.

Involves recording the electrical activity of the brain via electrodes on the scalp.
Method: caps with electrodes; tracks amplitude and frequency of brainwaves.
Key characteristics:
- High temporal resolution (excellent timing)
- Low spatial resolution (less precise localization)
Cap-based electrode recording setup enables precise timing analysis of brain activities.
Credit: SMI Eye Tracking

The endocrine system consists of glands that produce hormones to regulate normal body functions.
Hypothalamus: links the nervous system and endocrine system by controlling the pituitary gland.
Pituitary gland: serves as the master gland, controlling the secretions of all other glands.
Thyroid: secretes thyroxine which regulates growth, metabolism, and appetite.
Adrenal glands: secrete hormones involved in the stress response.
Gonads: secrete sex hormones, which are important for successful reproduction, and regulate sexual motivation and behavior.
Pancreas: secretes hormones that regulate blood sugar.