Unit VII lays the scientific foundation for understanding how traits are passed from one generation to the next and how life on Earth has evolved through time. This section combines the classical genetics of Gregor Mendel with modern molecular biology and the compelling evidence for evolution. As one of the most conceptually rich and frequently tested areas in the NEET exam, this unit demands thorough conceptual clarity and regular practice. It includes two crucial chapters:
- Chapter 5: Principles of Inheritance and Variation
- Chapter 6: Molecular Basis of Inheritance
Both chapters provide deep insight into fundamental biological mechanisms and are often the source of multi-layered MCQs in NEET.
Chapter 5: Principles of Inheritance and Variation
Deep Dive Into Key Concepts:
- Mendel’s Laws of Inheritance
- Mendel, the father of genetics, used pea plants (Pisum sativum) to discover how traits are transmitted.
- Three laws emerged from his experiments:
- Law of Dominance: One allele can mask the presence of another.
- Law of Segregation: Alleles separate during gamete formation, ensuring purity.
- Law of Independent Assortment: Traits segregate independently unless linked.
- Monohybrid cross (3:1 ratio) and dihybrid cross (9:3:3:1 ratio) reveal how different alleles interact.
- Probability and Punnett squares are used to predict genetic outcomes.
- Chromosomal Theory of Inheritance
- Sutton and Boveri demonstrated that chromosomes are the carriers of genes.
- Morgan’s experiments on Drosophila highlighted linkage (genes close together on a chromosome tend to be inherited together) and recombination (new gene combinations due to crossing over).
- Linkage maps quantify recombination frequency and help locate genes.
- Sex Determination Mechanisms
- Genetic systems vary across species:
- Humans: XX (female), XY (male)
- Insects: XO (male), XX (female)
- Birds: ZW (female), ZZ (male)
- Environmental factors influence sex in some reptiles and fishes.
- Genetic systems vary across species:
- Mutation and Genetic Disorders
- Mutations are abrupt heritable changes in DNA.
- Types include point mutations (e.g., sickle cell anemia from a single base change) and chromosomal abnormalities (e.g., duplication, deletion, inversion).
- Mendelian disorders: Include Thalassemia, cystic fibrosis, and color blindness (X-linked recessive).
- Chromosomal disorders: Include Turner’s syndrome (XO), Klinefelter’s syndrome (XXY), and Down syndrome (trisomy 21).
- Pedigree analysis is used to track inheritance patterns.
Chapter 6: Molecular Basis of Inheritance
Comprehensive Concepts:
- DNA Structure and Packaging
- DNA (Deoxyribonucleic acid) consists of two strands forming a right-handed double helix.
- Components: phosphate, deoxyribose sugar, nitrogenous bases (A, T, G, C).
- Base pairing: A-T (2 H-bonds), G-C (3 H-bonds).
- Forms of DNA:
- A-DNA: Compact, right-handed
- B-DNA: Most common, right-handed
- Z-DNA: Left-handed and zigzag
- DNA wraps around histones to form nucleosomes, creating chromatin structure.
- DNA Replication
- A semi-conservative process: each daughter molecule retains one old strand and one new strand.
- Meselson and Stahl validated this using nitrogen isotopes in E. coli.
- Key enzymes:
- Helicase: unwinds DNA
- DNA polymerase: adds nucleotides
- Ligase: seals gaps between Okazaki fragments
- Primase: lays down RNA primer
- Transcription and Translation
- Transcription: DNA to mRNA, occurs in the nucleus, requires RNA polymerase.
- Translation: mRNA to protein, takes place in ribosomes using tRNA and rRNA.
- Genetic code: Triplet, universal, non-overlapping, degenerate.
- Codons specify amino acids (AUG = Methionine, start codon).
- Regulation of Gene Expression
- Lac Operon in E. coli is a model for prokaryotic gene regulation.
- Components: promoter, operator, structural genes (z, y, a), and repressor.
- Inducible system: lactose acts as an inducer to deactivate the repressor.
- Human Genome Project and DNA Fingerprinting
- HGP sequenced the entire human genome: 3.3 billion base pairs, 20,000-25,000 genes.
- Opened new fields like genomics and personalized medicine.
- DNA fingerprinting uses polymorphisms (VNTRs) to identify individuals.
- Applied in forensics, paternity testing, biodiversity studies.
Essential Diagrams and Visual Aids:
- Punnett squares for mono- and dihybrid crosses
- Detailed DNA double helix model
- DNA replication fork with enzymes
- Central Dogma flowchart: DNA → RNA → Protein
- Operon model highlighting regulatory sequences
NEET Practice MCQs (Expanded Set):
- Which principle allows the reappearance of recessive traits in offspring?
Answer: A. Law of Segregation - Which enzyme facilitates joining of Okazaki fragments?
Answer: C. Ligase - Down syndrome is characterized by:
Answer: B. Trisomy of chromosome 21 - Which codon initiates protein synthesis?
Answer: A. AUG - Which of the following is not a dominant X-linked disorder?
Answer: D. None - What role does tRNA play in translation?
Answer: C. Transports amino acids to ribosome - What organism did Morgan use to study linkage?
Answer: Drosophila melanogaster - How many base pairs are in the human genome?
Answer: Approximately 3.3 billion
FAQs (Expanded):
What is linkage and how does it affect inheritance?
Linkage is the tendency of genes located close together on the same chromosome to be inherited together. It can reduce recombination frequency and lead to deviations from Mendelian ratios.
Why is the Lac Operon important?
It demonstrates how prokaryotes can regulate gene expression based on environmental conditions, ensuring energy conservation and efficiency.
How do DNA and RNA differ functionally?
DNA stores genetic information; RNA plays various roles including messenger (mRNA), structural (rRNA), and transporter (tRNA) roles in protein synthesis.
How does DNA fingerprinting work?
It detects individual-specific DNA sequences using techniques like gel electrophoresis and hybridization. VNTR analysis allows for precise identity verification.
What are examples of Mendelian vs. chromosomal disorders?
- Mendelian: Thalassemia, hemophilia, cystic fibrosis
- Chromosomal: Down syndrome, Turner syndrome, Klinefelter syndrome
NEET Tips for Genetics and Evolution:
- Solve 50+ MCQs from each chapter for pattern recognition.
- Review previous years’ NEET questions from this unit.
- Use mnemonics for genetic code and chromosomal anomalies.
- Revise flowcharts and concept maps regularly.
- Practice diagram labeling for operons and DNA replication.
Conclusion:
Genetics and Evolution lie at the heart of understanding how life functions and changes. This unit integrates Mendelian theory with cutting-edge molecular biology and evolutionary biology. With frequent appearances in NEET, mastering this unit gives students a strong edge. Use NCERT as your base, enhance with diagrams, practice-based learning, and stay conceptually strong. Your grasp over this unit could be the difference-maker in your NEET score.
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