Genetics Basics: DNA, RNA, Mendel's Laws, Blood Groups, Genetic Disorders

Genetics is a high-scoring science topic in Kerala PSC exams. Questions focus on DNA/RNA basics, Mendel’s laws, blood groups, and genetic disorders. Expect 2-3 questions per paper.

1. DNA (Deoxyribonucleic Acid)

Aspect	Detail
Full form	Deoxyribonucleic Acid
Discovered by	Friedrich Miescher (1869) — called it “nuclein”
Structure elucidated by	Watson and Crick (1953) — Double Helix model
X-ray crystallography by	Rosalind Franklin and Maurice Wilkins
Shape	Double helix (two anti-parallel strands)
Sugar	Deoxyribose
Bases	Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
Base pairing	A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds)
Location	Nucleus (mainly), mitochondria, chloroplasts
Function	Carries genetic information; heredity
Replication	Semi-conservative (Meselson and Stahl experiment, 1958)

Chargaff’s Rules

• A = T (equal amounts)
• G = C (equal amounts)
• Purines (A+G) = Pyrimidines (T+C)

2. RNA (Ribonucleic Acid)

Aspect	Detail
Full form	Ribonucleic Acid
Sugar	Ribose
Bases	Adenine (A), Guanine (G), Cytosine (C), Uracil (U) — no Thymine
Structure	Usually single-stranded
Location	Nucleus and cytoplasm

Types of RNA

Type	Function
mRNA (messenger)	Carries genetic code from DNA to ribosome
tRNA (transfer)	Brings amino acids to ribosome during translation
rRNA (ribosomal)	Structural component of ribosomes

DNA vs RNA Comparison

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded	Single-stranded (usually)
Location	Nucleus	Nucleus + Cytoplasm
Function	Stores genetic information	Protein synthesis
Stability	More stable	Less stable

3. Central Dogma of Molecular Biology

DNA → (Transcription) → RNA → (Translation) → Protein

Process	What happens	Enzyme
Replication	DNA makes copy of itself	DNA Polymerase
Transcription	DNA → mRNA	RNA Polymerase
Translation	mRNA → Protein	Ribosome (with tRNA)

4. Mendel’s Laws of Heredity

Gregor Johann Mendel (1822–1884) — “Father of Genetics” — experimented with garden pea (Pisum sativum).

Why Pea Plant?

• Short life cycle, many generations per year
• Clear contrasting characters (7 pairs)
• Self-pollination possible
• Easy cross-pollination

Mendel’s Three Laws

Law	Statement	Example
Law of Dominance	In a heterozygote, one allele (dominant) masks the other (recessive)	Tall (Tt) appears tall, not short
Law of Segregation (Law of Purity of Gametes)	Two alleles of a gene separate during gamete formation; each gamete gets one allele	Tt produces T and t gametes equally
Law of Independent Assortment	Genes on different chromosomes are inherited independently of each other	Seed colour and seed shape inherited separately

Key Genetic Terms

Term	Meaning
Gene	Unit of heredity; segment of DNA
Allele	Alternative forms of a gene (e.g., T and t)
Genotype	Genetic makeup (TT, Tt, tt)
Phenotype	Physical appearance (Tall, Short)
Homozygous	Both alleles same (TT or tt)
Heterozygous	Different alleles (Tt)
Dominant	Expressed in heterozygous condition
Recessive	Expressed only in homozygous condition

Monohybrid Cross Ratio

• F2 Genotypic ratio: 1 TT : 2 Tt : 1 tt
• F2 Phenotypic ratio: 3 Tall : 1 Short (3:1)

Dihybrid Cross Ratio

• F2 Phenotypic ratio: 9:3:3:1

5. Blood Groups (ABO System)

Blood Group	Antigen on RBC	Antibody in Plasma	Genotype
A	A	Anti-B	I^A I^A or I^A i
B	B	Anti-A	I^B I^B or I^B i
AB	Both A and B	None	I^A I^B
O	None	Both Anti-A and Anti-B	ii

Blood Transfusion Rules

Blood Group	Can Donate To	Can Receive From
A	A, AB	A, O
B	B, AB	B, O
AB	AB only	A, B, AB, O (Universal Recipient)
O	A, B, AB, O (Universal Donor)	O only

Rh Factor

• Rh+ (has D antigen) — approximately 85% of population
• Rh- (lacks D antigen)
• Erythroblastosis fetalis: Rh- mother carrying Rh+ foetus (second pregnancy onwards)

Blood Group Inheritance (PSC Favourite)

Parents	Possible Children
A x B	A, B, AB, O (all possible)
A x A	A, O
B x B	B, O
AB x O	A, B (never AB or O)
O x O	O only

6. Genetic Disorders

Common Genetic Disorders

Disorder	Type	Chromosome	Key Features
Sickle Cell Anaemia	Autosomal recessive	Chromosome 11	Sickle-shaped RBCs, HbS
Colour Blindness	X-linked recessive	X chromosome	Cannot distinguish red-green
Haemophilia	X-linked recessive	X chromosome	Blood fails to clot
Down Syndrome	Chromosomal (trisomy)	Extra chromosome 21 (47 total)	Intellectual disability, flat face
Turner Syndrome	Chromosomal	44 + XO (45 total)	Female, short stature, infertile
Klinefelter Syndrome	Chromosomal	44 + XXY (47 total)	Male, infertile, tall
Phenylketonuria (PKU)	Autosomal recessive	Chromosome 12	Cannot metabolise phenylalanine
Thalassemia	Autosomal recessive	Chromosome 11/16	Defective haemoglobin synthesis

Key Points for PSC

• Colour blindness and haemophilia are X-linked recessive — more common in males
• Carrier females can pass these to sons
• Down Syndrome = Trisomy 21 (most common chromosomal disorder)

7. Biotechnology Basics

Term	Definition
Biotechnology	Use of living organisms/biological systems in technology and industry
Genetic Engineering	Direct manipulation of an organism’s genes using recombinant DNA technology
Recombinant DNA	DNA formed by combining DNA from two different sources
Restriction enzymes	”Molecular scissors” — cut DNA at specific sequences
Ligase	”Molecular glue” — joins DNA fragments
Vector	Vehicle to carry foreign DNA into host (e.g., plasmid)
PCR	Polymerase Chain Reaction — amplifies DNA copies (invented by Kary Mullis)
Gene therapy	Replacing defective genes with functional ones
DNA fingerprinting	Identification technique using variable DNA sequences (developed by Alec Jeffreys)

8. GMO (Genetically Modified Organisms)

Aspect	Detail
Definition	Organisms whose DNA has been altered using genetic engineering
Bt Cotton	First GM crop approved in India (2002); contains Bacillus thuringiensis gene for pest resistance
Golden Rice	GM rice with beta-carotene (Vitamin A) — not approved in India
GM Mustard (DMH-11)	Approved for environmental release in India (2022)
Regulator in India	Genetic Engineering Appraisal Committee (GEAC) under MoEFCC
Concerns	Biodiversity loss, allergenicity, gene flow to wild species

9. Human Genome Project

Aspect	Detail
Duration	1990–2003
Led by	USA (NIH and DOE), international collaboration
Human genes	Approximately 20,000–25,000 genes
Total base pairs	Approximately 3.2 billion
Significance	Complete map of human DNA

10. PSC Quick Recall Table

Question	Answer
Father of Genetics?	Gregor Mendel
DNA model proposed by?	Watson and Crick (1953)
DNA base not in RNA?	Thymine (replaced by Uracil)
Universal donor blood group?	O
Universal recipient?	AB
Down Syndrome chromosome?	Trisomy 21
First GM crop in India?	Bt Cotton (2002)
PCR invented by?	Kary Mullis
Haemophilia is which type of disorder?	X-linked recessive
Mendel’s F2 monohybrid ratio?	3:1 (phenotypic)
DNA replication is?	Semi-conservative
Restriction enzymes are called?	Molecular scissors
Rh factor named after?	Rhesus monkey
GEAC full form?	Genetic Engineering Appraisal Committee