Chapter 27 · Class 11-12 & NEET
Principles of Inheritance & Variation
59 topics in 18 parts
Key ideas
Conceptual Foundation & Core Theory
Genetics and Molecular Inheritance form the bedrock of modern biology, establishing how chemical information stored in polymeric deoxyribonucleic acid (DNA) is faithfully replicated, transmitted across generations, and expressed as cellular proteins via the Central Dogma.
1. The Molecular Blueprint: DNA Structure & Replication
According to the Watson-Crick double helix model (1953):
- DNA consists of two antiparallel polynucleotide chains ($5' \to 3'$ and $3' \to 5'$) with a sugar-phosphate backbone outside and purine-pyrimidine base pairs projected inward.
- Adenine pairs with Thymine via two hydrogen bonds ($A=T$); Guanine pairs with Cytosine via three hydrogen bonds ($G\equiv C$). Hence, purines equal pyrimidines: $\frac{A+G}{T+C} = 1$ (Chargaff's Rule).
- Semiconservative Replication (Meselson & Stahl, 1958): Each daughter duplex retains one conserved parental strand and one newly synthesized strand. Catalyzed by DNA-dependent DNA polymerase which adds dNTPs exclusively in the $5' \to 3'$ direction. Consequently, the leading strand is synthesized continuously towards the replication fork, whereas the lagging strand is synthesized discontinuously as Okazaki fragments, later spliced by DNA ligase.
2. Mendelian Genetics & Allelic Interactions
Gregor Mendel deduced fundamental laws of transmission genetics through garden pea (Pisum sativum) hybridization experiments:
- Law of Segregation (Purity of Gametes): Alleles of a gene separate during meiosis such that each gamete receives only one allele. This law has no biological exceptions in sexual organisms.
- Law of Independent Assortment: Alleles of different unlinked gene pairs assort independently during gamete formation, generating an $F_2$ phenotypic ratio of $9:3:3:1$ in a dihybrid cross. (Note: Genes located close together on the same chromosome exhibit linkage, discovered by T.H. Morgan in Drosophila, violating independent assortment).
- Non-Mendelian Ratios: Incomplete dominance (e.g. Mirabilis jalapa, $1:2:1$ red, pink, white) and Codominance (e.g. ABO blood group $I^A, I^B, i$).
Key Scientific Principles, Pathways & Diagnostic Points
// Molecular & Genetic Ratios & Governing Laws
1. Chargaff's Equivalence Rule (dsDNA):
%A = %T and %G = %C => (A + G) = (T + C)
If DNA has 20% Cytosine → G = 20%, so G + C = 40% → A + T = 60% → A = 30%, T = 30%
2. Mendelian F2 Dihybrid Cross (RrYy x RrYy):
Phenotypic Ratio : 9 (Round Yellow) : 3 (Round Green) : 3 (Wrinkled Yellow) : 1 (Wrinkled Green)
Genotypic Ratio : 1:2:1:2:4:2:1:2:1 (9 distinct genotypes)
3. Lac Operon Regulatory Dynamics:
- Absence of Inducer (Lactose) : Lac Repressor (i gene) binds Lac Operator (O) → Blocks RNA Polymerase
- Presence of Inducer (Allolactose): Binds Repressor protein → Inactivates it → RNA Pol transcribes:
* lacZ (β-Galactosidase)
* lacY (Permease)
* lacA (Transacetylase)Core Genetic Processes: Replication vs Transcription vs Translation
| Feature / Parameter | Replication | Transcription | Translation |
|---|---|---|---|
| Template Used | Both strands of genomic dsDNA | Single antisense (3' to 5') template strand | Mature mRNA transcript (reading 5' to 3') |
| Key Catalytic Enzyme | DNA Polymerase III / I + Primase | DNA-dependent RNA Polymerase | Ribosome (23S/28S rRNA peptidyl transferase) |
| Substrates Consumed | Deoxyribonucleoside triphosphates (dNTPs) | Ribonucleoside triphosphates (rNTPs: ATP, UTP, GTP, CTP) | Aminoacyl-tRNA complexes + GTP/ATP |
| Polymerization Direction | Strictly 5' → 3' | Strictly 5' → 3' | N-terminus to C-terminus of polypeptide chain |
NEET Problem: Molecular Chargaff Calculation
NEET Problem: A sample of double-stranded DNA contains 340 Adenine nucleotides, representing 34% of the total base pairs. Calculate the number of Cytosine bases present in this DNA molecule.
Step-by-step Solution:
1. By Chargaff’s Rule in dsDNA: $%A = %T$. Therefore, $%T = 34\%$.
2. Total percentage of Adenine + Thymine: $34\% + 34\% = 68\%$.
3. Remaining percentage belonging to Guanine + Cytosine: $100\% - 68\% = 32\%$.
4. Because $%G = \%C$, the proportion of Cytosine is: $\frac{32\%}{2} = 16\%$.
5. If $34\%$ equals 340 bases, the total number of bases is: $\frac{340}{0.34} = 1000$ nucleotides.
6. Cytosine count: $16\% \text{ of } 1000 = 160$ Cytosine bases.
Answer: 160 Cytosine bases.
Conceptual Foundation & Core Theory
The Human Circulatory System employs closed double circulation comprising pulmonary and systemic circuits. The myogenic human heart contracts autonomously under impulses generated by the Sinoatrial (SA) node, commonly termed the natural pacemaker.
1. The Cardiac Cycle & Volume Dynamics
A single heartbeat corresponds to one cardiac cycle lasting approximately 0.8 seconds at a typical resting heart rate of 72 beats per minute:
- Joint Diastole (0.4 s): All four chambers are in a relaxed state. The tricuspid and bicuspid (mitral) AV valves remain open, allowing blood from the pulmonary veins and vena cava to flow passively into the left and right ventricles respectively.
- Atrial Systole (0.1 s): The SA node fires an action potential causing both atria to contract simultaneously, boosting ventricular blood filling by roughly 30%.
- Ventricular Systole (0.3 s): The impulse conducts through the AV node, Bundle of His, and Purkinje fibers to depolarize the ventricular myocardium. Ventricular pressure rises sharply, immediately shutting the AV valves (producing the first heart sound 'LUB'). When intraventricular pressure exceeds arterial pressures, the semilunar valves open, ejecting ~70 mL of blood per ventricle (Stroke Volume).
- Ventricular Diastole: As ventricles relax, falling intraventricular pressure causes arterial backflow that snaps shut the semilunar valves (producing the second heart sound 'DUB').
2. Standard Electrocardiogram (ECG) Waves
A clinical ECG graphically records electrical voltage changes across cardiac tissue over time using surface leads (Lead I, II, III):
- P Wave: Represents electrical depolarization of both atria initiated by the SA node.
- QRS Complex: Represents rapid depolarization of both ventricles immediately preceding ventricular systole. The number of QRS complexes per minute directly determines the heart rate.
- T Wave: Represents ventricular repolarization (return of ventricles from excited to normal polarized resting state). The end of the T wave marks the end of mechanical ventricular systole.
Key Scientific Principles, Pathways & Diagnostic Points
// Hemodynamic Formulas & Quantitative Relationships
1. Stroke Volume (SV) = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
SV = 120 mL - 50 mL = ~70 mL per beat
2. Cardiac Output (CO) = Stroke Volume × Heart Rate (HR)
CO = 70 mL × 72 beats/min = ~5,040 mL/min ≈ 5 Litres/min
3. Cardiac Cycle Duration = 60 seconds / Heart Rate
For 72 bpm → Duration = 60 / 72 = 0.8 seconds:
- Atrial Systole = 0.1 s | Atrial Diastole = 0.7 s
- Ventricular Systole = 0.3 s | Ventricular Diastole = 0.5 s
- Complete Joint Diastole = 0.4 sECG Waves & Associated Mechanical Cardiac Events
| ECG Waveform | Electrical Event | Mechanical Consequence | Clinical Pathology Indication |
|---|---|---|---|
| P Wave | Atrial depolarization (SA node fire) | Atrial contraction (completes ventricular filling) | Enlarged P wave indicates atrial hypertrophy / mitral stenosis |
| Q-R-S Complex | Ventricular depolarization (Purkinje network) | Ventricular isovolumetric contraction & rapid ejection | Broadened QRS indicates bundle branch block / myocardial infarction |
| T Wave | Ventricular repolarization | Ventricular relaxation (closure of semilunar valves) | Flat or inverted T wave indicates cardiac ischemia / hypokalemia |
| P-R Interval | AV conduction transit time (~0.12–0.20 s) | Ensures atria empty before ventricles contract | Prolonged PR indicates first-degree AV heart block |
NEET Problem: Cardiac Output Calculation under Exercise
NEET Problem: During vigorous athletic exercise, an athlete’s heart rate increases to 150 beats/min and end-diastolic volume increases to 140 mL while end-systolic volume drops to 40 mL. Calculate their cardiac output.
Step-by-step Solution:
1. Calculate Stroke Volume: $SV = EDV - ESV = 140\text{ mL} - 40\text{ mL} = 100\text{ mL}$.
2. Calculate Cardiac Output: $CO = SV \times HR = 100\text{ mL/beat} \times 150\text{ beats/min} = 15,000\text{ mL/min} = 15\text{ L/min}$.
Result: The athlete’s cardiac output expands threefold from 5 L/min to 15 L/min.
Checkpoint
1. During DNA replication in E. coli, which enzyme unwinds the double helix at the replication fork by breaking hydrogen bonds between complementary bases?
2. In a standard clinical electrocardiogram (ECG), which feature directly correlates with ventricular depolarization and can be counted to determine an individual's heart rate?
All 59 topics in Principles of Inheritance & Variation
Mendel's Experiments
- Contrasting Traits in Garden Pea
- True-Breeding Pea Lines
- Selection of Pisum sativum & Experimental Method
Mendelian Laws
- Law of Dominance & Law of Segregation
- Monohybrid Ratios & Factor Concept
- Dominant & Recessive Allele Expression
Monohybrid Cross
- Punnett Squares & Phenotypic & Genotypic Ratios
- F1 & F2 Progeny Analysis
- Inheritance of a Single Gene
- Test Cross & Backcross
Dihybrid Cross
- Law of Independent Assortment & 9:3:3:1 Ratio
- Inheritance of Two Genes
- Dihybrid Gamete Formation & Genotypic Ratios
- Gene Interaction & Epistasis
Incomplete Dominance
- Antirrhinum & Mirabilis Inheritance
- Intermediate Phenotypes & 1:2:1 Ratio
- Heterozygous Phenotypic Expression
Codominance
- Codominant Alleles & Phenotypic Expression
- RBC Surface Antigens & Codominant Expression
- HbA & HbS Carrier Expression
Multiple Alleles
- Multiple Allelism & Population Allele Count
- ABO Blood-Group Alleles
- Genotype Count Formula n(n+1)/2
ABO Blood Groups
- Genotypes & Phenotypes
- IA, IB & i Allele Interaction
- Transfusion Compatibility & Phenotype Determination
Polygenic Inheritance
- Quantitative Traits & Additive Gene Action
- Human Skin Colour & Height Inheritance
- Continuous Variation & Phenotypic Ratios
Pleiotropy
- Single Gene With Multiple Phenotypic Effects
- Phenylketonuria Symptoms & Metabolic Block
- Starch Grain Size & Seed Shape in Peas
Chromosomal Theory
- Sutton-Boveri Principle & Chromosome Behaviour
- Parallelism Between Chromosomes & Genes
- Experimental Verification in Drosophila melanogaster
Linkage & Crossing Over
- Recombination Frequency & Gene Mapping
- Morgan's Drosophila Experiments
- Complete & Incomplete Linkage
Sex Determination
- XX-XY, XX-XO & Haplodiploid Systems
- Human & Drosophila Systems
- Bird & Insect Systems
Mutation
- Point Mutation, Frameshift & Mutagens
- Sickle Cell Base Substitution (GAG to GUG)
- Physical & Chemical Mutagens in Plants
Pedigree Analysis
- Pedigree Symbols & Inheritance Patterns
- Autosomal & Sex-Linked Pedigrees
- Dominant & Recessive Inheritance in Families
Genetic Disorders
- Sickle Cell Anaemia & Thalassemia
- Phenylketonuria & Autosomal Recessive Disorders
- Alpha & Beta Thalassemia
- Chromosomal Disorders
- Down's, Klinefelter's & Turner's Syndromes
- Aneuploidy & Polyploidy
Sex-Linked Inheritance
- Haemophilia & Colour Blindness
- X-Linked Recessive Inheritance & Carrier Females
- Criss-Cross Inheritance
Genetics Terminology
- Alleles, Genes, Genotypes & Phenotypes
- Homozygous & Heterozygous States
- Ploidy & Related Terms