Mendelian inheritance Study Guide

📖 Core Concepts Mendelian inheritance – transmission of a trait controlled by a single gene with discrete alleles; foundation of classical genetics. Allele – alternative form of a gene at a specific locus on a chromosome. Genotype – the complete set of alleles an organism carries; Phenotype – the observable trait that results from the genotype plus environment. Homozygous (AA or aa) – two identical alleles; Heterozygous (Aa) – two different alleles. Law of Dominance – in a heterozygote the dominant allele masks the recessive allele’s effect. Law of Segregation – the two alleles for a gene separate into different gametes during meiosis (1 : 2 : 1 genotypic ratio in F₂). Law of Independent Assortment – alleles of unlinked genes sort into gametes independently (9 : 3 : 3 : 1 dihybrid F₂ ratio). Incomplete (partial) dominance – heterozygote phenotype is intermediate between the two homozygotes. 📌 Must Remember F₁ generation from a true‑breeding dominant × recessive cross: all show the dominant phenotype. Test cross = back‑cross F₁ to recessive parent → reveals hidden recessive alleles. F₂ phenotypic ratio (single‑gene) = 3 : 1 (dominant : recessive). F₂ genotypic ratio = $1:2:1$ ($AA:Aa:aa$). Dihybrid F₂ phenotypic ratio = $9:3:3:1$ (dominant–dominant, dominant–recessive, recessive–dominant, recessive–recessive). True Mendelian trait = single‑locus, two‑allele, dominant‑recessive pattern. Non‑Mendelian traits involve multiple alleles, polygenic control, linkage, or epigenetics. 🔄 Key Processes Set up a monohybrid cross Write parental genotypes (e.g., $AA \times aa$). Form gametes (all $A$ from the dominant parent, all $a$ from the recessive). Fill a $2 \times 2$ Punnett square → F₁ all $Aa$ (dominant phenotype). Self‑fertilize F₁ (segregation) Gametes: $A$ and $a$ from each $Aa$ parent. $2 \times 2$ Punnett square → $AA$, $Aa$, $aa$ in 1 : 2 : 1 ratio. Dihybrid cross (independent assortment) Parental genotypes $AaBb \times AaBb$. List four possible gametes per parent ($AB$, $Ab$, $aB$, $ab$). Fill a $4 \times 4$ Punnett square → $9:3:3:1$ phenotypic ratio. Test cross Cross $Aa$ (or $AaBb$) to homozygous recessive ($aa$ or $aabb$). Offspring phenotypes directly reveal the genotype of the heterozygous parent. 🔍 Key Comparisons Dominant vs. Recessive allele – Dominant allele masks recessive in heterozygote; recessive only expressed in homozygous recessive. Complete dominance vs. Incomplete dominance – Complete: heterozygote phenotype = dominant phenotype; Incomplete: heterozygote phenotype is intermediate. Mendelian vs. Non‑Mendelian trait – Mendelian: single gene, two alleles, predictable ratios; Non‑Mendelian: multiple alleles, polygenic, linked, or epigenetically regulated → ratios deviate. ⚠️ Common Misunderstandings “3:1 ratio always applies” – Only for single‑gene traits with complete dominance in the F₂ generation. “All heterozygotes look the same” – False for incomplete dominance or co‑dominance. “Independent assortment = random” – Only true for genes on different chromosomes or far apart on the same chromosome (unlinked). 🧠 Mental Models / Intuition “Allele suitcase” – Each parent carries two “suitcases” (alleles) but only one is packed into each gamete; think of shuffling a deck (segregation) and then dealing two independent decks (independent assortment). “Dominance as a spotlight” – The dominant allele shines bright enough to hide the recessive “lamp” unless both lamps are off (homozygous recessive). 🚩 Exceptions & Edge Cases Linked genes – Do not follow the 9:3:3:1 ratio; recombination frequency alters expected proportions. Polygenic traits – Produce a continuum of phenotypes, not discrete ratios. Epigenetic effects – Can mimic or mask Mendelian patterns without changing DNA sequence. 📍 When to Use Which Punnett square – Quick visual for one or two genes, when parents’ genotypes are known. Test cross – Use to determine the unknown genotype of a heterozygote (or to confirm segregation). Pedigree chart – Best for tracking inheritance across multiple generations, especially for sex‑linked or dominant/recessive traits in families. 👀 Patterns to Recognize 3:1 phenotypic pattern → single‑gene, complete dominance. 1:2:1 genotypic pattern → segregation in F₂ of a heterozygous cross. 9:3:3:1 phenotypic pattern → dihybrid cross of two heterozygotes with unlinked genes. Intermediate phenotype → incomplete dominance (look for “blended” traits). 🗂️ Exam Traps Distractor: “2:1 ratio” – Appears in some textbooks for allele frequencies but not a Mendelian phenotypic ratio. Choosing 9:3:1 – A common mis‑read of the dihybrid ratio; the correct is 9:3:3:1. Assuming linkage is always present – If the question does not mention linkage, treat genes as independent. Confusing genotype with phenotype – Remember $AA$ and $Aa$ share the same dominant phenotype; only $aa$ shows recessive. --- All content derived directly from the provided outline.