IB Biology Topic 3: Genetics - Comprehensive Study Notes

Introduction to Genetics and Heredity

• Etymology: The term "Genetics" is derived from the word "Genesis," meaning "to generate," or the concept of "coming into being" and "developing the self."

• Definition of Genetics: It is the formal study of heredity, specifically focusing on the transmission of characteristics (genes) to future generations.

• Scope of the Field:     * It encompasses the passage of genetic information between generations, resulting in the diversity and variability of traits within a species.     * It investigates molecular processes and mechanisms involving DNA, genes, and chromosomes that facilitate organismal functioning and inheritance.     * It provides insights into evolutionary drivers, such as genetic mutations, adaptations, and natural selection, which shape biological diversity over long periods.

Importance of Genetics in Society

• News and Media: Genetics informs the public regarding breakthroughs in research, biotechnology, and ethical debates. Key topics include genetic discoveries, cloning, and gene editing.

• Medicine:     * Personalized Medicine: Genetics allows for diagnostics tailored to an individual’s genetic makeup.     * Risk Assessment: Identifying predispositions to specific diseases through genetic testing and analysis allows for more effective healthcare strategies.

• Epigenetics: This subfield explores how environmental factors influence gene expression without altering the DNA sequence. It is vital for studying cancer, aging, developmental disorders, and lifestyle interventions.

• Agriculture: Genetic engineering enables the creation of Genetically Modified (GM) crops engineered for resistance against pests, diseases, and environmental stressors, thereby increasing yields and food security.

• Pest Management: Genetic strategies, such as using sterile insects or GMOs, reduce pest populations and the reliance on chemical pesticides.

• Bioinformatics: DNA sequence analysis and bioinformatics are tools used to decipher the genetic code, identify genes, and understand their functions in genomics and proteomics.

• Therapeutic Applications: Gene editing and gene therapy involve modifying or replacing faulty genes to treat previously incurable genetic disorders.

Essential Genetic Vocabulary

• Allele: A specific form or variation of a gene; typically, there are two possible variations.

• Phenotype: The observable physical appearance or trait of an individual.

• Genotype: The specific combination of alleles an individual possesses for a given gene, often represented by alphabetic abbreviations.

• Homozygous: A condition where the two alleles for a specific trait are identical.

• Heterozygous: A condition where the two alleles for a specific trait are different.

• Dominant: An allele that is physically expressed in the phenotype of a heterozygous individual.

• Recessive: An allele that is masked and not physically seen in the phenotype of a heterozygous individual.

Specific Human Genetic Disorders

• Osteogenesis Imperfecta:     * Inheritance: Dominant mutation.     * Symptoms: Multiple bone fractures, decreased mobility, and blue or grayish-blue tinted sclera (the white of the eye).     * Complications: Hearing loss caused by deformities in the small bones of the middle ear.

• Marfan Syndrome:     * Inheritance: Dominant mutation (requires only one allele to manifest).     * Physical Traits: Affected individuals typically have long limbs.     * Clinical Implications: Weakens the aorta, leading to easy bleeding and heart complications.     * Notable Case: Flo Hyman, who died due to heart complications related to the disorder.

• Sickle Cell Anemia:     * Inheritance: Recessive mutation (requires $2$ mutated alleles).     * Mechanism: Blood cells become sickled rather than smooth and round, causing them to get stuck in capillaries.     * Demographics: Most common in individuals of African descent.

• Cystic Fibrosis:     * Inheritance: Recessive mutation located on Chromosome #$7$.     * Mechanism: Causes thick mucus to accumulate in the lungs, resulting in respiratory infections.     * Demographics: Common in those of European descent.     * Notable Case: Gunnar Esiason. Genetic testing is available for this condition.

• Xeroderma Pigmentosum (XP):     * Inheritance: Recessive mutation (requires $2$ alleles).     * Symptoms: Extreme sensitivity to UV light.     * Complications: Development of many small tumors resembling freckles, which are actually skin cancer.

Chromosome Structure and Karyotyping

• Chromosome Composition: A chromosome consists of approximately $15\%$ DNA, $10\%$ RNA, and $75\%$ Protein.

• Centromere Positions:     * Metacentric: Centromere is located in the middle.     * Submetacentric: Centromere is slightly off-center.     * Acrocentric: Centromere is significantly off-center.     * Telocentric: Centromere is at the very top.

• Karyotype Definition: An organized visual array of a cell's complete set of chromosomes, including both autosomes (non-sex chromosomes) and sex chromosomes (X and Y).

• Utility of Karyotyping:     * Determines gender and overall chromosomal health.     * Identifies structural abnormalities and numerical issues, such as Trisomy $21$ (Down Syndrome).     * Used in prenatal screening and cancer diagnosis.

• Karyotype Preparation Techniques:     * Colchicine: Used to destroy spindle fibers to ensure an unobstructed view of chromosomes.     * Hypotonic Salt Solution: Used to increase cell volume.     * Dropping from Height: Uses force to splinter the cells apart and reveal chromosomes.

• Homologous Chromosomes: Chromosomes that share the same length, centromere position, banding pattern, and genetic loci.

• FISH Technique: Stands for Fluorescent In Situ Hybridization. It uses fluorescent DNA probes to target specific chromosomal locations; results are viewed as colored signals under a fluorescent microscope.

The Cell Cycle and Mitosis

• Interphase: Composed of the $G_1$, $S$, and $G_2$ phases.     * $G_1$ (Gap 1): The cell grows larger, performs metabolic tasks, and synthesizes proteins/organelles needed for DNA synthesis. It serves as a checkpoint for health and preparation.     * $S$ (S-phase): The cell performs DNA replication. Double-stranded DNA is unwound and complementary nucleotides are added. Includes quality control mechanisms to repair replication errors.     * $G_2$ (Gap 2): Continued growth and preparation for mitosis. Checkpoints ensure DNA replication was accurate before proceeding.

• $M$ Phase: Consists of Nuclear Division (Mitosis) and Cytoplasmic Division (Cytokinesis).     * Mitotic Spindle: Microtubule structure that attaches to centromeres to segregate sister chromatids.

• Stages of Mitosis:     * Prophase ($60\%$ of duration): Chromatin condenses into visible chromosomes, spindle fibers form, and the nuclear envelope breaks down.     * Prometaphase: Nuclear envelope disassembles completely, allowing spindle fibers access to distinct chromatids connected at the centromere.     * Metaphase ($5\%$ of duration): Chromosomes align at the equatorial metaphase plate. A checkpoint ensures proper alignment before proceeding.     * Anaphase ($5\%$ of duration): Sister chromatids are pulled apart as the centromere divides. Now, there are $2$ chromatids total per original centromere position (moving toward poles).     * Telophase ($30\%$ of duration): Chromatids move to poles, nuclear envelopes reform, and chromosomes decondense. There is only $1$ chromatid per centromere.

• Cytokinesis: The final division of cytoplasm.     * Animal Cells: Formation of a cleavage furrow (contractile ring of proteins).     * Plant Cells: Formation of a cell plate at the center.

• Somatic Cells: Cells that do not produce gametes.

Meiosis and Genetic Variation

• Definition: Cell division in a diploid cell producing haploid gametes.

• Two Divisions:     * Reductional Division (Meiosis I): Chromosome number is halved.     * Equational Division (Meiosis II): Chromosome number remains the same from the start of the phase to the end.

• Stages of Meiosis I:     * Prophase I: Chromosomes condense and pair with partners. Crossing over (recombination) occurs, where alleles are switched between overlapping chromosomes to create variation.     * Metaphase I: Homologue pairs align at the metaphase plate.     * Anaphase I: Homologues separate to opposite ends, but sister chromatids remain attached.     * Telophase I: Cytokinesis begins, resulting in $2$ haploid daughter cells.

• Stages of Meiosis II:     * Prophase II: Centrosomes move apart; spindles form.     * Metaphase II: Chromosomes line up individually.     * Anaphase II: Sister chromatids separate.     * Telophase II: Nuclear membranes reform; cytokinesis produces $4$ haploid cells (sperm or egg).

• Biological Significance:     * Maintains a constant chromosome number after fertilization.     * Provides variation via crossing over and Independent Assortment (genes separating independently during gamete development).     * Meiosis allows for $2^{23}$ possible combinations of chromosomes.

Human Inheritance and Population Genetics

• Gametes: Normal human gametes contain $23$ chromosomes.

• Sex Determination: The father's sperm determines the sex. The egg always carries an X chromosome; sperm can carry either X ($XX = \text{female}$) or Y ($XY = \text{male}$).

• Prenatal Determination: Methods include ultrasounds, bloodwork, and sperm sorting.

• Twins:     * Monozygotic: Identical; result of $1$ sperm and $1$ egg where the embryo divides.     * Dizygotic: Fraternal; result of $2$ eggs fertilized by $2$ different sperm. These run in families.

• Siblings: Full siblings share roughly $50\%$ of their genes.

• Carriers: Individuals with one normal allele and one recessive disease allele (e.g., $Ff$ for Cystic Fibrosis). If both parents are carriers, there is a $\frac{1}{4}$ or $25\%$ chance the child will have the disorder.

• Mutation and Evolution:     * Mutation: An incorrect copy or change in the DNA sequence.     * Role: Mutations serve as a form of adaptation. If a mutation is beneficial, the organism is favored for reproduction, allowing the mutation to spread. Mutations are essential for species growth and environmental adaptation.     * Environment: Factors like diet and sun can influence phenotype (height, weight, skin color) regardless of genetics.

Genetic Modification (GMOs) and Biotechnology

• GMO Definition: Any organism whose genetic material has been altered via genetic engineering.

• vs. Traditional Breeding: Breeding requires sexual compatibility and transfers both desirable and undesirable traits. Genetic engineering allows direct insertion of specific genes without sexual reproduction.

• Desired Traits: Resistance to pests/drought, improved harvest/storage (e.g., Flavr Savr Tomato), and improved nutrition.

• Specific Examples:     * Bt Corn: Contains a gene from the soil bacteria Bacillus thuringiensis to resist the corn borer insect.     * Rainbow Papaya: Resistant to the ringspot virus.     * Golden Rice (Version 2): Contains the Psy gene from corn to produce beta-carotene, which the body converts to Vitamin A.

• GMO Challenges and Concerns:     * Golden Rice: Faced low enthusiasm and marketing difficulties due to its color. Blessed by the Pope; approved in Canada, Australia, NZ, and the US in $2018$. Bangladesh announced potential cultivation in $2019$.     * Environmental Risks: Potential harm to food webs and "wild" gene competition via cross-pollination.     * Socio-Economic: Seeds are patented by companies (e.g., Monsanto). Impacted by laws making it illegal to save or replant seeds, leading to decreased genetic diversity.     * Legal: Over $60$ countries restrict or ban GMOs (Japan, Australia, parts of Europe).     * Labeling: The $2017$ National Bioengineered Food Disclosure Standard led to USDA labels released in $2018$. Compliance became mandatory by January $1$, $2022$.

Advanced Genetic Technologies

• Genetic Pest Management:     * Gene Drive: A genetic element passed to progeny at a higher frequency than Mendelian expectation. Used to reduce pest fertility or disease transmission.     * New World Screwworm (C. hominivorax): Currently managed by releasing sterile flies. A potential "male-only" line involves a female-lethal gene suppressed by tetracycline in the lab.     * Red Flour Beetle: Uses the "Medea" selfish genetic element for gene drives.

• RNA Interference (RNAi):     * Nobel Prize: Awarded to Fire and Mello.     * Mechanism: Double-stranded RNA ($dsRNA$) is cut by the enzyme Dicer into small RNAs ($miRNA$ or $siRNA$). These complex with the protein Argonaut to form RISC (RNA Induced Silencing Complex). RISC paired with mRNA either inhibits translation or degrades the mRNA.     * Applications: Studying gene function, managing macular degeneration, cancer therapy, and Smartstax Corn (targets rootworm genes).

• CRISPR-Cas9 System:     * Origin: Prokaryotic mechanism to cleave invading phage DNA.     * Palindrome: A DNA sequence that reads the same $5'$ to $3'$ on both strands (e.g., $5'–GAATTC–3'$).     * Function: Guide RNA binds to the Cas9 enzyme, directing it to cut target DNA. Modified Cas9 can activate transcription, silence genes, or attach fluorescent tags.     * Medical Trials: Targeted treatments for Sickle Cell Anemia, Beta-thalassemia, and various cancers.

Forensic and Diagnostic Applications

• Polymerase Chain Reaction (PCR):     1. Denaturation: Heating to $95^{\circ}C$ to separate DNA strands.     2. Annealing: Primers bind at $50–65^{\circ}C$.     3. Extension: Taq polymerase adds nucleotides at $72^{\circ}C$.     * Limitations: Requires prior sequence knowledge, prone to contamination, and Taq lacks proofreading (error rate approx. $1$ in $20000$ bp).

• Gel Electrophoresis: Separates DNA fragments by size as they move toward a positive pole due to DNA's negative charge.

• Short Tandem Repeats (STRs): Polymorphic loci used for identification.     * CODIS: Combined DNA Index System; FBI-funded database using $20$ loci.     * Forensics: A suspect is excluded if their profile does not match perfectly at all loci.     * Paternity Logic: A child must receive one allele from each parent at every locus.

• The Innocence Project: A non-profit dedicated to exonerating the wrongfully convicted using DNA evidence, reforming the criminal justice system to address flawed forensics or eyewitness misidentification, and raising public awareness.