Genetics Study Guide

📖 Core Concepts Genetics – study of genes, genetic variation, and heredity. Gene – a DNA segment that encodes a functional product (protein or RNA). Allele – alternative version of a gene; organisms are diploid (two alleles per locus). Genotype vs. Phenotype – genotype = complete set of alleles; phenotype = observable traits. Mendel’s Laws Law of Segregation: each parent passes one allele per gene to offspring. Law of Independent Assortment: genes on different chromosomes (or far apart) sort independently. DNA Structure – double helix of nucleotides; base‑pairing: A↔T, C↔G. Replication is semi‑conservative. Central Dogma – DNA → RNA (transcription) → Protein (translation). Codons = 3‑nt units. Genetic Linkage & Recombination – genes close together tend to be inherited together; crossing‑over creates new allele combinations. Epistasis – one gene masks/modifies the effect of another. Mutation Types – point (substitution), insertion/deletion, chromosomal (duplication, inversion, translocation). Heritability – proportion of phenotypic variance attributable to genetic variance in a given environment. Natural Selection – differential reproductive success of genotypes; drives adaptation and evolution. --- 📌 Must Remember Mendelian ratio (monohybrid cross, heterozygote × heterozygote): phenotype 3 dominant : 1 recessive; genotype 1 AA : 2 Aa : 1 aa. DNA base‑pair rule: A pairs with T, C pairs with G. Codon length: 3 nucleotides → 1 amino acid; 64 possible codons. PCR cycle: Denaturation → Annealing → Extension → exponential amplification (≈ 2ⁿ copies after n cycles). Linkage mapping: 1 % recombination ≈ 1 centimorgan (cM). Heritability (h²) formula: $$h^{2}= \frac{V{G}}{V{P}}$$ where \(V{G}\) = genetic variance, \(V{P}\) = phenotypic variance. Two‑hit hypothesis (tumor suppressors): both alleles must be inactivated for cancer to arise. Human Genome Project: completed reference human genome (≈ 3 × 10⁹ bp) in 2003. --- 🔄 Key Processes Meiosis & Recombination Prophase I: homologous chromosomes pair → crossing‑over. Metaphase I–Anaphase I: homologs separate (reduces chromosome number). Meiosis II: sister chromatids separate (produces 4 haploid gametes). Transcription → Translation Transcription: RNA polymerase binds promoter → synthesizes pre‑mRNA (5’→3’). RNA processing: intron removal, 5’ cap, poly‑A tail → mature mRNA. Translation: ribosome reads codons, tRNA brings amino acids, peptide chain elongates, termination at stop codon. PCR Denature (≈ 94 °C) → DNA strands separate. Anneal (≈ 55–65 °C) → primers bind. Extend (≈ 72 °C) → DNA polymerase synthesizes new strand. Repeat 25–35 cycles. Sanger Sequencing Incorporate dideoxynucleotides (ddNTP) → chain termination. Separate fragments by size → read sequence from gel or capillary electrophoresis. Cancer Development (simplified) Initiation: mutation in proto‑oncogene or tumor‑suppressor. Promotion: additional mutations disable growth‑inhibitory pathways. Progression: acquisition of hallmarks (e.g., limitless replication, invasion). --- 🔍 Key Comparisons Dominant vs. Recessive – dominant allele masks recessive phenotype in heterozygote. Codominance vs. Incomplete Dominance – codominance: both alleles expressed (e.g., AB blood type); incomplete dominance: blended phenotype (e.g., pink flowers from red × white). Sex‑linked vs. Autosomal – sex‑linked genes located on X/Y; inheritance patterns differ (e.g., hemophilia). Somatic vs. Germline Mutations – somatic affect only the individual (cancer); germline can be passed to offspring. PCR vs. Sanger Sequencing – PCR amplifies DNA exponentially; Sanger reads the exact base order of a single fragment. Linkage Analysis vs. GWAS – linkage maps rare, high‑penetrance variants in families; GWAS scans common variants across populations. --- ⚠️ Common Misunderstandings “Genes = traits.” Genes encode proteins; traits result from gene–environment interactions. All mutations are harmful. Most are neutral; a few are beneficial. Heritability = immutability. Heritability is population‑specific and changes with environment. Mendelian 3:1 ratio always appears. Only for single‑gene, complete dominance crosses; epistasis, linkage, or multiple alleles alter ratios. DNA → protein → DNA (reverse flow). The central dogma is unidirectional (except in retroviruses). --- 🧠 Mental Models / Intuition Gene as a recipe, allele as a variation of the recipe. Meiosis = shuffling a deck of cards – each gamete gets a random hand. PCR = photocopier for DNA – each cycle doubles the number of copies. Cancer as a car with broken brakes – tumor‑suppressor loss = brake failure; oncogene activation = stuck accelerator. Linkage map as a subway line – distance between stations (genes) predicts how often passengers (recombination) switch lines. --- 🚩 Exceptions & Edge Cases Incomplete dominance / codominance – deviate from simple dominant/recessive patterns. Epistasis – masks Mendelian ratios (e.g., color dilution). Mitochondrial inheritance – maternal only (e.g., mitochondrial diseases). Horizontal gene transfer in bacteria – can bypass Mendelian inheritance. Variable penetrance & expressivity – same genotype yields different phenotypes. Environmental modulation of heritability – high‑nutrient environment raises height heritability; low‑nutrient reduces it. --- 📍 When to Use Which Predict offspring ratios: use Punnett squares for simple monohybrid/dihybrid crosses; use pedigree analysis for multi‑generation inheritance patterns. Identify disease genes: start with linkage analysis for rare, high‑penetrance families; switch to GWAS for common, modest‑effect variants. Amplify DNA: choose PCR for rapid, specific amplification; use RT‑PCR when starting from RNA. Sequence a gene: Sanger for ≤ 1 kb, high accuracy; NGS for whole‑genome or large panels. Model organism selection: prioritize short generation time, genetic tractability, and relevance to the biological question (e.g., Drosophila for development, E. coli for metabolism). --- 👀 Patterns to Recognize 3:1 phenotype ratio → single‑gene, complete dominance. 1:2:1 genotype ratio → heterozygote cross, indicates segregation. Multiple bands on a gel → presence of restriction sites or polymorphisms. Two‑hit loss of tumor suppressor → look for both allele inactivation in cancer cases. High mutation frequency in UV‑exposed cells → signature C→T transitions at dipyrimidine sites. Co‑segregation of markers in families → suggests genetic linkage. --- 🗂️ Exam Traps Confusing genotype with phenotype – remember heterozygotes may look like homozygous dominant. Misreading pedigrees – carriers are often unshaded but have a “half‑filled” symbol; affected individuals are fully shaded. Assuming all traits are Mendelian – many human traits are polygenic or influenced by environment. Choosing the most common mutation type when the question specifies a disease (e.g., sickle‑cell is a missense point mutation, not a frameshift). Overlooking epigenetic regulation – gene expression can be silenced without DNA sequence change. Selecting “linkage” when the loci are far apart – distant loci recombine freely and behave as independent. ---