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This crash course in genetics covers topics such as simple genetic disorders, environmental influences, cancer, genetics and behavior, and factors influencing phenotype. It also explores the debate of nature vs. nurture and how genetics may influence susceptibility to certain behaviors and diseases.
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The Genetics of Health EPI 6181 Beth Lowcock 22 November, 2004
Where We’re Going… • A crash course in genetics • Simple genetic disorders • Environmental influences • Cancer • Genetics and behaviour • Some things to think about
Chromosomes • Humans have 23 pairs • 22 pairs of autosomes • 1 pair of sex chromosomes • Only gametes have one set of chromosomes • Condensed strands of DNA located within the nucleus of every cell
DNA • Double strand of nucleotides (A, T, C, G) form double helix • Regions of DNA that code for proteins are called “genes” • Proteins are polypeptides of amino acids
From DNA to Protein 3 DNA nucleotides 1 Codon 1 Amino Acid Mutation??
Proteins • More than 50% of dry weight in most cells • Drive and regulate all bodily processes Structural Storage Transport Receptors Contractile Defensive Hormonal Enzymatic PROTEIN TYPES Genes “act” via the proteins they code for
Factors Influencing Phenotype • Dominance • Polygenic Inheritance • Multiple genes involved • Environment The observable physical or physiological trait
Dominance • 2 copies of each gene • Allele – a particular form of a gene • Homozygous – 2 copies of same allele • Heterozygous – 2 different alleles • Dominant vs. Recessive AA Aa aa “carrier” of recessive trait
Recessive Genetic Disorders • Thousands of diseases or disorders are inherited as simple recessive traits (aa genotype) • Disorder occurs if recessive allele codes for an ineffective protein, or no protein at all • Cystic Fibrosis • 4% of Caucasians are carriers of gene • defective gene for chloride ion transport protein • Tay Sach’s disease • Defective enzyme doesn’t break down lipids in brain
Dominant Genetic Disorders • AA and Aa genotypes • Lethal dominant genes are rare – why? • People can’t “carry” the gene • Huntington’s Disease X-linked Genetic Disorders • Expressed mainly in males • Haemophilia
Polygenic Inheritance • Most traits don’t result from expression of only one gene – so for most traits we don’t see discrete variation • Many traits vary along a continuum • Normal distribution • Certain diseases may result from mutations of one or more genes involved in polygenic inheritance • A number of mutated genes linked to diabetes mellitus, asthma, heart disease, etc. • Complex, and gene interaction not well understood
Environment • Environment influences gene action in many ways • Proteins must use elements from the environment to build structures • Influences gene regulation (temperature, light, nutrients, toxins, stress, etc) • Demonstrated in genetically identical plants, twin studies
Nature Vs Nurture • Debate over relative contributions of genes and environment • Biological determinism wrong • Genes provide a range of phenotypic possibilities – Norm of Reaction • May be narrow (eye colour, CF) or broad (height, heart disease)
Cancer • A genetic disease of somatic cells (non sex cells) • “The class of disease characterized by rapid and uncontrolled proliferation of cells within a tissue” • Normal growth and maintenance requires cell division – involves DNA replication • Mutations may occur in genes that regulate this process • Spontaneous mutation, or environmental influence • Chemical carcinogens, UV light, Viruses Cancer is genetic, but is it inherited?
The “Breast Cancer Genes” • Genes identified • BRCA1 – normal gene functions as a transcriptional regulator and growth inhibitory protein • BRCA2 – also a tumor suppressor gene, exact function not well known • Mutation of these genes associated with 3-7x increased lifetime risk of breast cancer
5-10% of breast cancer is “hereditary” • You inherit genes that predispose you to cancer, not cancer itself • Just because you have the mutation doesn’t mean you’ll get cancer • Just because you don’t have the mutation doesn’t mean won’t get cancer Some people are genetically PREDISPOSED to cancer
Genetics and Behaviour • Evidence that genes do influencebehaviour • “Innate” behaviours we observe in many different species • Changes to biological structures, such as brain areas, may lead to change in behaviour • Behaviours of closely related species, e.g. humans and chimpanzees
How Genes May Influence Behaviour • Behaviours are highly complex traits, and likely involve many genes • Remember – Genes code for proteins Hormonal Receptor Contractile Structural Storage Transport Defensive Enzymatic TYPES
Example: Tobacco Use • Nicotine binds to nicotinic-acetylcholine receptors in the brain, which in turn modulate release of dopamine • The gene CHRNA4 codes for one subunit of the nicotine receptor • There are several different forms of this gene • Feng et al 2004 found a form of the gene to be protective against nicotine addiction Genetics may influence susceptibility to nicotine-addiction
Summary • Genes Proteins • Many factors influence phenotype • Dominance • Number of genes involved • ENVIRONMENT • There are many “simple” genetic disorders • Genes may PREDISPOSE people to certain health outcomes (e.g., cancer) or health related behaviours (e.g., smoking)
Some Things to Think About… • Genetics and SES health gradient • Is there a relationship? • Genetic quality • Evolution – survival of the fittest? • Implications for society • Is there such thing as free will? • Gene therapy • Designer babies • Are genes the problem?
How do psychosocial influences bring about biological change? The Systems Limbic Endocrine Immune
In order for the body to react to a psychosocial influence, we must first perceive the influence and develop an emotional response to it • Much of our discussion has centered around the emotional aspects of health • Mental stress • Social support – love or isolation • Emotions generated from health behaviours, such as relaxation from smoking, runner’s high • SES – worry, shame, helplessness
The Limbic System • Evidence from brain-damaged animals and people • Loosely defined • About the size of a walnut (larger in women than men)
Amygdala • Receives sensory information from other regions • Organization of emotional information • Role in memory (damage results in amnesia for non-procedural memories) • Connected to olfactory bulb – memory and scent • Stimulation causes aggression, damage leads to passivity and lack of reaction to fearful stimuli
Hippocampus • Involved in converting short-term memory (things in your mind) into long-term memory • Damage prevents formation of new memories
Thalamus • A relay station • Links “thinking brain” with sensory and emotional areas • Damage results in emotional apathy
Hypothalamus • Important in homeostatic regulation (the body’s thermostat) • Regulates drives (hunger, thirst, sex), autonomic nervous system (stress response), aggressive behaviour • Linked to pituitary gland (and thereby the endocrine system)
Together, these structures: • Set the emotional tone • Activity Negative tone • Inactivity Positive tone • Depression/mania, PMS • Regulate motivation and drive • Store emotional components of memory • Emotional memories help set emotional tone (cf. life events) • Facilitate bonding • Damage prevents animals from bonding with young
Limbic system connected to the frontal lobe (the “thinking brain”) • Frontal lobotomy to treat emotional disorders • Also connected to endocrine system • Hypothalamus connected to pituitary (“master gland”) Limbic system links “thinking brain” and endocrine system
The Endocrine System • Made up of all hormone-secreting cells in the body • Organs made up of hormone- secreting cells are endocrine GLANDS • Secrete directly into bloodstream (vs. exocrine glands)
Function of Endocrine System • Internal communication (complementary to nervous system) • Maintains homeostasis and long-term control • Regulates slower processes than nervous system (e.g., growth, stress response) Hormones are the messengers for this communication
Hormones • Def – “a substance produced by one tissue and transported to another tissue where it induces a specific physiological response” • More than 50 known human hormones • Grouped into 3 classes: • Peptides • Amines • Steroids
Peptide Hormones • Most hormones are peptide hormones • Peptides are short chains of amino acids Amine Hormones • Derivatives of the amino acid tyrosine
Steroid Hormones • Lipids derived from cholesterol • Include the sex hormones secreted by the gonads
Hormone Action • Hormones trigger actions in specific TARGET CELLS • Target cells have receptor molecules that hormones bind to • Lock and key analogy
Hormone Action • Binding changes the shape of the receptor, eliciting a chemical/physiological response • Receptors are proteins (form = function) • Action of steroid hormones occurs inside the cell, while peptide and amine hormones bind with receptors on the cellular membrane
Action of Steroid Hormones • Steroids can enter cell since they are lipids and can cross the cellular membrane
Action of Non-Steroid Hormones • Peptides and amines cannot cross the cell membrane • Bind to receptor on membrane, which causes chemical signal (second messenger) inside the target cell
Homeostasis and Regulation • Negative feedback regulates secretion of almost every hormone
The Endocrine System: • Allows for communication • between the brain and other • parts of the body • Allows for homeostasis and • regulation of bodily processes
The Immune System • The cells and tissues which enable us to mount a response to invading microorganisms, parasites and other foreign substances • General immunity – 1st line of defense • Specific immunity – 2nd line of defense
General Immunity • Physical barriers • Skin and mucous membranes • Inflammatory response • Damaged cells release histamine resulting in increased blood flow and temperature • Complement System • Proteins that cause pores in microorganism to open so that fluids and salts enter, causing cell to burst • Phagocytosis • Neutrophils are a type of white blood cell