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Three-Point Crosses and Chromosome Interference

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1,262prerequisites beneath it

Genetic Mapping and Recombination Frequency Dihybrid Crosses and Independent Assortment→→Linkage Disequilibrium and Evolutionary Dynamics

Core Idea

Three-point crosses involve three linked loci and reveal non-independence of crossovers: crossovers are not randomly distributed. Interference is the reduction in probability of a second crossover near a first crossover; coefficient of coincidence measures observed vs. expected double crossovers. This reveals the physical basis of recombination.

How It's Best Learned

Analyze a three-point testcross progeny: identify parental, single-crossover, and double-crossover classes. Calculate map distances between adjacent genes, then predict frequencies and detect interference. Compare with data to understand the mechanics of crossing over.

Common Misconceptions

Assuming crossovers occur independently; interference shows they are not independent.
Not recognizing that interference can vary along the chromosome and be sex-specific.
Thinking double-crossover classes are the most frequent when they are the rarest class, allowing easy identification.

Explainer

You already know from genetic mapping that recombination frequency between two loci estimates the genetic distance between them, and that linked genes produce fewer recombinant offspring than expected under independent assortment. A three-point cross extends this logic by tracking three linked genes simultaneously, and it reveals something two-point crosses cannot: crossovers along a chromosome are not independent events. A crossover in one interval changes the probability of a crossover occurring nearby.

The experimental setup is straightforward. You cross an organism heterozygous at three linked loci (ABC/abc) to a homozygous recessive tester (abc/abc), then classify every offspring by its combination of phenotypes. With three genes, there are eight possible phenotype classes that fall into four categories: parentals (the two most frequent classes, matching the original chromosome arrangements), single crossovers in interval I (recombination between genes A and B), single crossovers in interval II (recombination between genes B and C), and double crossovers (recombination in both intervals simultaneously). The double crossover class is always the rarest, and this is your entry point for analysis — comparing the two rarest classes to the parentals immediately tells you which gene is in the middle, because a double crossover reverses only the middle gene's allele relative to the parentals.

Once you have identified gene order and counted each class, you calculate map distances for each interval by summing all crossover events in that interval (singles plus doubles) and dividing by total offspring. The critical insight comes next: if crossovers in interval I and interval II were truly independent, the expected frequency of double crossovers would simply be the product of the two single-crossover frequencies. But when you compare expected doubles to observed doubles, you almost always find fewer doubles than predicted. This deficit is interference — a physical phenomenon in which the occurrence of one crossover suppresses additional crossovers nearby.

Interference is quantified through the coefficient of coincidence (c.o.c.), defined as observed double crossovers divided by expected double crossovers. Interference itself equals 1 minus the coefficient of coincidence. A coefficient of coincidence of 0.4 means you observed only 40% of the expected doubles, giving interference of 0.6 — a 60% reduction. Complete interference (interference = 1) means no double crossovers occur at all; no interference (interference = 0) means crossovers are fully independent. This is not just a statistical curiosity — it reflects the physical mechanics of recombination, where the protein machinery involved in one crossover event physically inhibits formation of another crossover within a certain chromosomal distance.

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → One-to-One Correspondence → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Making 10 as an Addition Strategy → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals in Polar Coordinates → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → The Measurement Problem → Interpretations of Quantum Mechanics → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → Translation: Initiation and Elongation → Post-Translational Modifications → Proteasomal Degradation and Ubiquitin-Mediated Marking → Cell Cycle Regulation and Checkpoints → Mitosis → Cytokinesis → Meiosis → Chromosomal Theory of Inheritance → Mendelian Genetics → Dominance, Recessiveness, and Allelic Interactions → Monohybrid Crosses and Mendel's Law of Segregation → Dihybrid Crosses and Independent Assortment → Genetic Mapping and Linkage → Genetic Mapping and Recombination Frequency → Three-Point Crosses and Chromosome Interference

Longest path: 225 steps · 1262 total prerequisite topics

Prerequisites (2)

Genetic Mapping and Recombination Frequencyhard Dihybrid Crosses and Independent Assortmentsoft

Leads To (1)

Linkage Disequilibrium and Evolutionary Dynamicssoft