PRELIMS

Genetics and Heredity

Genetics is the study of the transmission of hereditary characters from one generation to the next. Gregor Mendel is the 'Father of Genetics'.

Mendel's Laws of Inheritance

Based on experiments with the Pea plant (Pisum sativum).

1. Law of Paired Unit: Dominant traits (e.g., Tallness) express themselves, while recessive traits (e.g., Dwarfness) are masked.
2. Law of Dominance: Offspring only show dominant characters in the generation.
3. Law of Segregation: Traits separate in the generation.
4. Law of Independent Assortment: Different pairs of characters segregate independently in dihybrid crosses.

DNA vs. RNA

DNA (Deoxyribonucleic Acid)

• Sugar: Deoxyribose
• Bases: Adenine, Thymine, Cytosine, Guanine
• Structure: Double-stranded (Helix)
• Location: Mostly in Nucleus (Some in Mitochondria)
• Function: Genetic information carrier

RNA (Ribonucleic Acid)

• Sugar: Ribose
• Bases: Adenine, Uracil, Cytosine, Guanine
• Structure: Single-stranded
• Location: Nucleus and Cytoplasm
• Function: Protein synthesis

RNA Types: mRNA (Messenger), rRNA (Ribosomal), tRNA (Transfer)

Central Dogma: Replication, Transcription, Translation

• Replication: DNA makes a copy of itself before cell division (semi-conservative — each new DNA has one old and one new strand).
• Transcription: DNA's genetic code is copied into mRNA (occurs in the nucleus).
• Translation: mRNA sequence is decoded at the Ribosome to synthesize a protein, with tRNA bringing specific amino acids.
• Flow: DNA → RNA → Protein (proposed by Francis Crick).

Human Sex Determination

• Male: XY (Determines the sex of the child).
• Female: XX.
• Mechanism: Sperm carrying 'X' results in a female (XX). Sperm carrying 'Y' results in a male (XY).

Chromosomal and Genetic Disorders

Klinefelter Syndrome (XXY or XYY - Male)

• Extra 'X' or 'Y'; sterile; masculine with feminine features

Turner's Syndrome (XO - Female)

• Single 'X'; rudimentary ovaries; sterile

Down's Syndrome (Trisomy 21 - Autosomal)

• Mentally retarded; Mangolism; protruding eyes

Patau's Syndrome (Trisomy 13 - Autosomal)

• Mentally retarded; cut lip

Sickle Cell Anaemia

• RBC destruction; Change in 11th autosomal chromosome

Phenylketonuria

• Metabolic error; Change in 12th autosomal chromosome; mental retardation

Haemophilia (Sex-linked)

• Failure of blood clotting

Color Blindness (Sex-linked)

• Failure to distinguish Red and Green

Key Genetic Terms

• Linkage: Exception to Mendel's law (Morgan); genes on same chromosome inherited together.
• Mutation: Sudden heritable change in gene (Hugo de Vries).
• Cloning: Producing identical organisms (e.g., Dolly sheep).
• Totipotency: Ability of a single cell to grow into a complete plant.
• Genome: All genes in a haploid cell.

Confused Pair: Gene Mutation vs Chromosomal Mutation

• Gene (Point) Mutation: Change within a single gene's DNA sequence (e.g., substitution, insertion, deletion of bases) — e.g., Sickle Cell Anaemia.
• Chromosomal Mutation: Change in the structure or number of whole chromosomes (e.g., deletion, duplication, translocation of chromosome segments, or aneuploidy) — e.g., Down's Syndrome (Trisomy 21).

UPSC Relevance

Genetics is a core pillar for Science & Technology (GS-III).

• Disorders: Understanding the chromosomal cause of conditions like Down's Syndrome is essential for health-related questions.
• DNA Technology: Structural understanding (A-T, G-C base pairing) is foundational for Crispr-Cas9/Cloning discussions.
• Sex Linkage: Knowing that Haemophilia and Color Blindness are sex-linked explains their higher prevalence in males.