Autosomal recessive inheritance Study Guide

📖 Core Concepts Dominance – one allele masks the effect of another at the same locus; the masking allele is dominant, the hidden one is recessive. Genotype vs. Phenotype – genotype = allele makeup (e.g., RR, Rr, rr); phenotype = observable trait. Allele Notation – uppercase = dominant (R), lowercase = recessive (r). Homozygous – two identical alleles (RR or rr). Heterozygous – two different alleles (Rr). Inheritance Patterns – autosomal dominant, autosomal recessive, X‑linked dominant/recessive, Y‑linked (male‑only). Genetic Interactions – epistasis (one gene masks another), pleiotropy (one gene → many traits), polygenic inheritance (many genes → continuous trait). --- 📌 Must Remember Complete dominance → heterozygote phenotype = dominant phenotype; F₂ ratio = 3:1 (phenotype), 1:2:1 (genotype). Incomplete dominance → heterozygote phenotype is intermediate (e.g., red × white → pink). Co‑dominance → both alleles fully expressed (e.g., IA + IB → AB blood type). Overdominance → heterozygote has greater fitness than either homozygote. Autosomal dominant = trait appears in heterozygotes; autosomal recessive = trait appears only in homozygotes. X‑linked recessive diseases usually manifest in males (XY) because they have only one X. Punnett square sizes – monohybrid 2 × 2, dihybrid 4 × 4. Dihybrid complete‑dominance phenotypic ratio = 9:3:3:1. Epistasis can change classic ratios (e.g., recessive epistasis → 9:3:4). --- 🔄 Key Processes Set up a monohybrid cross Write parental genotypes. List possible gametes (one per allele). Fill a 2 × 2 grid; combine gametes to get offspring genotypes. Calculate F₂ ratios for complete dominance Cross heterozygotes (Aa × Aa). Use Punnett square to count genotypes → 1 AA : 2 Aa : 1 aa. Convert to phenotypes → 3 dominant : 1 recessive. Dihybrid cross (independent assortment) Determine gamete types for each parent (e.g., AB, Ab, aB, ab). Populate 4 × 4 grid, tally genotype combos, then collapse to phenotype ratio 9:3:3:1. Sex‑linked trait prediction For X‑linked recessive: male genotype = XⁿY, female = XⁿXⁿ. Build separate squares for each sex’s gametes (Xⁿ or Xⁿⁿ). --- 🔍 Key Comparisons Complete dominance vs. Incomplete dominance Complete: heterozygote = dominant phenotype. Incomplete: heterozygote = intermediate phenotype. Co‑dominance vs. Complete dominance Co‑dominance: both alleles visible simultaneously (AB blood). Complete: only one allele visible (RR or Rr → round). Autosomal vs. X‑linked recessive Autosomal: both sexes need two recessive alleles. X‑linked: males need only one recessive allele; females need two. Epistasis vs. Simple dominance Epistasis: gene at one locus masks another locus. Dominance: masking occurs within the same locus. --- ⚠️ Common Misunderstandings “Dominant = more powerful” – dominance only describes expression, not functional superiority. “Y‑linked traits are dominant/recessive” – there’s no second copy, so dominance terminology doesn’t apply. Confusing genotype with phenotype – Rr (heterozygous) still shows the dominant round phenotype, not a “mixed” trait. Assuming all heterozygotes are 50% dominant/50% recessive – only true for incomplete dominance, not for complete or co‑dominance. --- 🧠 Mental Models / Intuition Masking Model – imagine the dominant allele “covers” the recessive allele like a paint coat; the underlying allele is still there but invisible. Allele “Traffic Light” – dominant = green (go), recessive = red (stop); in co‑dominance both lights are on (yellow) giving a blend. Two‑Lane Highway for Independent Assortment – each gene travels on its own lane; the crossing of lanes creates the 9:3:3:1 pattern. --- 🚩 Exceptions & Edge Cases Overdominance – heterozygote advantage (e.g., sickle‑cell trait conferring malaria resistance). Ambidirectional dominance – each allele may be dominant over the other for different traits. Epistatic modifications – recessive epistasis can collapse the 9:3:3:1 ratio to 9:3:4 (e.g., Labrador coat color). --- 📍 When to Use Which Complete dominance → use simple 3:1 (phenotype) & 1:2:1 (genotype) calculations. Incomplete dominance → treat heterozygote as a distinct phenotype; expect a 1:2:1 phenotypic ratio. Co‑dominance → count heterozygotes as a third phenotype (e.g., AB) in ratios. Sex‑linked analysis → draw separate Punnett squares for male and female gametes; apply X/Y chromosome rules. Epistasis → first determine which locus is epistatic, then apply modified ratio (e.g., 9:3:4 for recessive epistasis). --- 👀 Patterns to Recognize 3:1 phenotypic ratio → classic complete dominance monohybrid. 1:2:1 genotypic ratio → heterozygote frequency in F₂ of a single‑gene cross. 9:3:3:1 ratio → two independent, completely dominant genes. Intermediate phenotype → incomplete dominance. Two distinct traits in one individual → co‑dominance (AB blood, sickle‑cell trait). Sex‑specific expression → X‑linked trait, especially when males are affected but females are carriers. --- 🗂️ Exam Traps Choosing 3:1 vs. 1:2:1 – 3:1 applies to phenotypes; 1:2:1 applies to genotypes. Misreading co‑dominance as incomplete dominance – co‑dominance shows both parental phenotypes, not a blend. Assuming Y‑linked traits follow dominance – they never have a dominant/recessive relationship. Applying autosomal ratios to sex‑linked problems – ignore the extra X/Y differences and you’ll get wrong probabilities. Forgetting epistasis – classic 9:3:3:1 may be altered; look for “missing” phenotype classes (e.g., yellow Labrador). Overlooking heterozygote advantage – a heterozygote may be more fit (overdominance), which changes expected fitness but not Mendelian ratios.