Genetics & Evolution —UG Short Notes-A Quick Revision

 Genetics & Evolution —UG Short Notes

How to use: read once for concepts, then revise only the tables, ratios and ⚠ traps.

PART 1 — CLASSICAL GENETICS

1. Key terms

Term	Meaning
Gene / Allele	Unit of heredity / alternative form of a gene
Locus	Position of a gene on a chromosome
Genotype / Phenotype	Genetic makeup / observable trait
Homozygous / Heterozygous	Same alleles / different alleles
Dominant	Expressed in heterozygote
Genome / Gene pool	All genes of an organism / all alleles in a population

2. Mendel (Father of Genetics)

•       Material: garden pea (Pisum sativum). Published 1865–66; rediscovered 1900 (de Vries, Correns, Tschermak).

•       7 contrasting pairs (dominant / recessive): seed shape round / wrinkled; seed colour yellow / green; flower colour violet / white; pod shape inflated / constricted; pod colour green / yellow; flower position axial / terminal; stem tall / dwarf.

Law	Statement
Dominance	One allele masks the other in the heterozygote
Segregation	Two alleles separate in gametes; each gamete gets one (purity of gametes)
Independent assortment	Genes on different chromosomes assort independently (dihybrid cross)

 

Cross	Ratio
Monohybrid F2 phenotype	3 : 1
Monohybrid F2 genotype	1 : 2 : 1
Test cross (unknown × homozygous recessive)	1 : 1
Dihybrid F2 phenotype	9 : 3 : 3 : 1
Dihybrid test cross	1 : 1 : 1 : 1

•       Back cross = F1 × either parent. Test cross is a back cross with the recessive parent.

•       Product rule: P(A and B) = P(A) × P(B). Sum rule: P(A or B) = P(A) + P(B) for exclusive events.

•       Gamete types for n heterozygous independent genes = 2ⁿ (AaBbCc → 8).

3. Deviations from Mendelism

Concept	Key point	Example
Incomplete dominance	Intermediate F1; F2 = 1 : 2 : 1	Snapdragon / Mirabilis (red × white → pink)
Codominance	Both alleles expressed in heterozygote	AB blood group (IᴬIᴮ)
Multiple alleles	>2 alleles in population; an individual still has only 2	ABO: Iᴬ, Iᴮ, i
Pleiotropy	One gene → many traits	PKU, sickle-cell
Polygenic	Many genes → one trait (continuous variation)	Skin colour, height

 

Blood group	Genotype	Antigen on RBC	Plasma antibody
A	IᴬIᴬ, Iᴬi	A	anti-B
B	IᴮIᴮ, Iᴮi	B	anti-A
AB	IᴬIᴮ	A and B	none (universal recipient)
O	ii	none	anti-A, anti-B (universal donor)

⚠ Iᴬ = Iᴮ > i. AB is codominance, not incomplete dominance.

4. Chromosomal theory, linkage, crossing over

•       Chromosomal theory: Sutton & Boveri (1902). Genes lie on chromosomes. Morgan proved it in Drosophila.

•       Linkage: genes on the same chromosome are inherited together. Tightly linked = low recombination.

•       Crossing over: between non-sister chromatids of homologues, at pachytene. Chiasmata are visible at diplotene.

•       Recombination frequency (RF) = recombinants / total progeny × 100. Maximum 50%. 1% RF ≈ 1 map unit (cM).

•       Sturtevant used RF to map genes (distance ∝ RF).

•       Coupling (cis) = AB/ab. Repulsion (trans) = Ab/aB.

•       Morgan: yellow body / white eye were tightly linked (1.3% RF). White eye / miniature wing were loosely linked (37.2% RF).

5. Sex determination

Type	Male	Female	Example
XX–XY	XY	XX	Humans, Drosophila (sperm decides sex)
XX–XO	XO	XX	Grasshopper
ZZ–ZW	ZZ	ZW	Birds (egg decides sex)
Haplodiploidy	Drone: 16 chromosomes (unfertilised egg)	Queen/worker: 32 (fertilised)	Honey bee

•       Humans: 46 chromosomes = 22 pairs autosomes + 1 pair sex chromosomes. Gametes have 23.

•       Y carries SRY (male-determining gene).

6. Sex-linked inheritance

•       Males are hemizygous. X-linked recessive traits show up more in males.

•       X-linked genes pass father → daughter only. Never father → son. Daughter → grandson is possible (criss-cross inheritance).

•       Colour blindness: red or green cone defect. About 8% of males, 0.4% of females.

•       Hemophilia (A: factor VIII, B: factor IX): an affected daughter needs a carrier mother and an affected father. This is very rare. Royal disease: Queen Victoria was a carrier.

•       X-inactivation: Barr bodies = number of X − 1.

XY	XX	XXY	XXX	X0
0	1	1	2	0

•       Pedigree symbols: □ male, ○ female, shaded = affected, horizontal line = mating, double line = consanguineous.

7. Mendelian disorders

Disorder	Inheritance	Key fact
Hemophilia	X-linked recessive	Clotting-protein defect
Colour blindness	X-linked recessive	Cone defect
Sickle-cell anaemia	Autosomal recessive	β-globin point mutation: GAG→GUG, Glu→Val at position 6 (HbS). Qualitative defect. Heterozygote resists malaria
Thalassaemia	Autosomal recessive	Reduced globin synthesis (quantitative). α: 2 genes on chr 16. β: chr 11
Phenylketonuria	Autosomal recessive	Phenylalanine hydroxylase missing → mental retardation
Cystic fibrosis	Autosomal recessive	CFTR chloride channel (chr 7)
Albinism	Autosomal recessive	Tyrosinase defect
Huntington / Myotonic dystrophy	Autosomal dominant	Late-onset neurological / muscular

8. Chromosomal disorders

•       Aneuploidy: gain or loss of single chromosomes (monosomy 2n−1, trisomy 2n+1). Caused by nondisjunction.

•       Polyploidy: extra whole sets (common in plants).

•       Structural changes: Deletion, Duplication, Inversion, Translocation (DDIT).

Syndrome	Karyotype	Features
Down	Trisomy 21 (47)	Short, round head, furrowed tongue, palm crease, mental retardation. Described by Langdon Down
Edwards	Trisomy 18	Severe malformations
Patau	Trisomy 13	Severe malformations
Turner	45, X0	Female, sterile, short, ovaries underdeveloped, no Barr body
Klinefelter	47, XXY	Male, tall, gynaecomastia, sterile, 1 Barr body

PART 2 — MOLECULAR BASIS OF INHERITANCE

9. Search for genetic material

Scientist(s)	Year	Experiment and conclusion
Griffith	1928	Streptococcus pneumoniae. Heat-killed S + live R killed mice → transforming principle
Avery, MacLeod, McCarty	1944	Protease and RNase did not stop transformation. DNase did → DNA is the transforming principle
Hershey & Chase	1952	T2 phage in E. coli. ³²P labels DNA, ³⁵S labels protein. ³²P entered the cell → DNA is genetic material
Meselson & Stahl	1958	¹⁵N/¹⁴N, CsCl gradient. Gen 1 all hybrid; gen 2 half hybrid, half light → semiconservative replication

•       RNA is the genetic material in some viruses (TMV, retroviruses).

•       S strain: smooth, capsulated, virulent. R strain: rough, non-capsulated, non-virulent.

10. DNA and RNA

•       Nucleotide = phosphate + pentose sugar + base. Nucleoside = sugar + base.

•       Purines: A, G (two rings). Pyrimidines: C, T, U (one ring).

•       DNA has deoxyribose and thymine. RNA has ribose and uracil.

•       Chargaff: A = T, G = C, so purines = pyrimidines. H-bonds: A=T 2, G≡C 3.

•       Watson & Crick, 1953. Antiparallel, right-handed. Sugar-phosphate backbone outside, bases inside.

•       B-DNA: 10 bp per turn, pitch 3.4 nm, 0.34 nm between bp, diameter 2 nm. Phosphodiester bond joins nucleotides.

•       Human haploid genome ≈ 3.2 × 10⁹ bp. E. coli ≈ 4.6 × 10⁶ bp.

•       RNA types: mRNA (message), tRNA (adapter, clover-leaf), rRNA (ribosome; structural and catalytic).

11. DNA packaging

•       Prokaryotes: DNA is supercoiled in the nucleoid (no histones).

•       Eukaryotes: histones are basic (rich in lysine and arginine). The octamer is 2 × (H2A, H2B, H3, H4).

•       Nucleosome: DNA wrapped on the octamer. About 146 bp wrap the core; NCERT gives about 200 bp per nucleosome. H1 binds linker DNA.

•       Chromatin: beads-on-string → higher-order fibre. Euchromatin is light-staining and active. Heterochromatin is dense and inactive.

12. DNA replication

•       Semiconservative. Starts at ori. DNA polymerase works only 5′→3′.

Enzyme	Role
Helicase	Unwinds the helix
SSB proteins	Keep strands apart
Topoisomerase	Relieves supercoiling
Primase	Makes RNA primer
DNA polymerase	Adds nucleotides 5′→3′
DNA ligase	Joins Okazaki fragments

•       Leading strand: continuous. Lagging strand: discontinuous (Okazaki fragments).

•       Nucleotides are added as deoxynucleoside triphosphates, which supply the energy.

13. Transcription

•       Central dogma (Crick): DNA → RNA → protein. Temin & Baltimore found reverse transcription (RNA → DNA).

•       Transcription unit = promoter + structural gene + terminator.

•       The template strand is 3′→5′. The RNA is made 5′→3′ and matches the coding strand (U for T).

•       Bacteria: one RNA polymerase (σ factor starts, ρ factor terminates).

•       Eukaryotes: Pol I → rRNA (28S, 18S, 5.8S). Pol II → hnRNA/mRNA. Pol III → tRNA, 5S rRNA, snRNA.

•       hnRNA processing: splicing (introns out, exons joined), capping (5′ methyl-guanosine), tailing (poly-A, 200–300 A at 3′).

14. Genetic code

•       64 codons: 61 sense + 3 stop (UAA, UAG, UGA). AUG is the start codon (Met). UGG (Trp) is the only other single-codon amino acid.

•       Leu, Ser and Arg have 6 codons each.

•       Triplet, degenerate, non-overlapping, comma-less, unambiguous, nearly universal, colinear.

•       Nirenberg (cell-free system), Khorana (synthetic RNA), Ochoa (polynucleotide phosphorylase). Wobble is at the 3rd base.

•       Mutations: point (silent, missense, nonsense) and frameshift (insertion or deletion not a multiple of 3).

15. Translation

•       Ribosomes: prokaryotic 70S (50S + 30S); eukaryotic 80S (60S + 40S).

•       Charging: aminoacyl-tRNA synthetase attaches the amino acid at the 3′ CCA end of tRNA.

•       Steps: initiation (initiator tRNA at AUG) → elongation (peptidyl transferase, a ribozyme: 23S rRNA in bacteria) → termination (release factor at a stop codon).

•       Sites: A (entry) → P (peptide) → E (exit). UTRs flank the coding region.

•       Polysome: many ribosomes on one mRNA.

16. Lac operon (Jacob & Monod)

•       Genes: i (repressor), promoter, operator, z (β-galactosidase), y (permease), a (transacetylase).

•       No lactose: repressor binds operator → OFF.

•       Lactose or allolactose (inducer) binds repressor → operator free → RNA polymerase transcribes z, y, a → ON.

•       Inducible and negative control. Glucose also affects expression.

17. Human Genome Project (1990–2003)

•       13-year international project. Methods: ESTs and sequence annotation. Vectors: BAC, YAC. Sanger sequencing.

•       Facts: ~3 × 10⁹ bp. NCERT: ~30,000 genes (modern estimate about 20,000). Average gene 3,000 bp. Dystrophin is the largest gene (2.4 million bp).

•       99.9% of the sequence is the same in all people. Less than 2% codes for protein. Much of the genome is repeats.

•       Chromosome 1 has the most genes (2,968). Y has the fewest (231). About 1.4 million SNPs were found.

•       Bioinformatics: computational analysis of sequence data.

18. DNA fingerprinting (Alec Jeffreys)

•       Based on VNTRs (variable number tandem repeats) in satellite DNA. Modern forensics uses STRs.

•       Steps: isolate DNA → restriction digestion → gel electrophoresis → Southern blot → hybridise with labelled VNTR probe → autoradiography.

•       Uses: forensics, paternity, identifying remains, population studies.

PART 3 — EVOLUTION

19. Origin of life

•       Earth formed about 4.5 billion years ago (bya). Life appeared roughly 4 bya, first as non-cellular aggregates, then single-celled anaerobes.

•       Spontaneous generation was disproved by Louis Pasteur (biogenesis). Panspermia: life came from space.

•       Oparin–Haldane: chemical evolution. Inorganic → simple organic → complex organic → protocells. The early atmosphere was reducing (CH₄, NH₃, H₂, water vapour) with little free O₂. Energy came from lightning, UV and volcanic heat.

•       Miller (1953): CH₄ + NH₃ + H₂ + water vapour at 800 °C with electric sparks gave amino acids. Later versions gave sugars, bases and fats. It showed abiotic synthesis, not the creation of life.

•       RNA-world idea: RNA was the first self-replicating molecule.

20. Evidence for evolution

Evidence	Key point
Fossils	Age shown by radioactive dating; show gradual change
Homologous organs	Same origin, different function → divergent evolution. Forelimbs of human, bat, whale, cheetah; Bougainvillea thorn and Cucurbita tendril
Analogous organs	Same function, different origin → convergent evolution. Bird and butterfly wings; octopus and mammal eye; penguin and dolphin flippers; potato (stem) and sweet potato (root)
Vestigial organs	Reduced, functionless remnants (coccyx, appendix)
Embryology	Vertebrate embryos show gill slits. Haeckel's recapitulation was disproved by von Baer
Biogeography	Distribution of organisms (Australian marsupials)
Molecular	Similar DNA and protein sequences
Adaptive evidence	Industrial melanism; antibiotic and pesticide resistance; artificial selection

21. Theories of evolution

Theory	Idea
Lamarck (1809)	Use and disuse; inheritance of acquired characters (giraffe neck). Rejected
Darwin (1859)	On the Origin of Species after the HMS Beagle voyage (Galapagos). Natural selection on small, heritable variations. Influenced by Malthus; Wallace reached the same idea
Hugo de Vries	Mutation theory. Large, sudden single-step changes (saltation), seen in Oenothera
Modern synthetic theory	Mutation + recombination + natural selection + gene flow + genetic drift + isolation

•       Darwin's key ideas: variation, overproduction, struggle for existence, survival of the fittest, branching descent.

•       Fitness = reproductive success.

•       Evolution happens in populations, not in individuals.

22. Natural selection

Type	Favours	Result
Stabilising	Mean phenotype	Narrower peak
Directional	One extreme	Peak shifts
Disruptive	Both extremes	Two peaks

•       Industrial melanism (peppered moth, Biston betularia): dark moths increased in polluted woods. Natural selection in action.

23. Population genetics

•       Evolution = change in allele frequency over generations.

•       Forces that change allele frequency: mutation, recombination, gene flow (migration), genetic drift, natural selection.

•       Genetic drift: random change, strongest in small populations. Founder effect and bottleneck effect are both kinds of drift.

•       Mutation creates new alleles. Recombination makes new combinations of existing alleles.

Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1.

•       Equilibrium needs: large population, random mating, no mutation, no migration, no selection, no drift.

•       Example: q = 0.2 → p = 0.8. Then AA = p² = 0.64, Aa = 2pq = 0.32, aa = q² = 0.04.

24. Patterns of evolution

•       Adaptive radiation: one ancestor splits into forms for different niches. Darwin's finches (Galapagos, beak variation); Australian marsupials. It is a type of divergent evolution.

•       Convergent evolution: different ancestry, similar adaptation. Placental wolf and Tasmanian wolf (marsupial); shark and dolphin.

25. Human evolution

Form	Time	Key facts
Dryopithecus	~15 mya	Ape-like
Ramapithecus	~15 mya	More man-like
Australopithecus	~2 mya	East African grasslands, upright, stone weapons, fruit-eater. "Lucy" is A. afarensis (~3.2 mya)
Homo habilis	~2 mya	First human-like; brain 650–800 cc; probably no meat
Homo erectus	~1.5 mya	Brain ~900 cc; meat-eater; Java man
Neanderthal	1,00,000–40,000 yrs ago	Brain ~1,400 cc; near East and Central Asia; used hides; buried dead
Homo sapiens	75,000–10,000 yrs ago	Arose in Africa, then spread. Cave art ~18,000 yrs ago. Agriculture ~10,000 yrs ago

•       Brain size trend: 650 → 900 → 1400 cc.

•       ⚠ Humans did not evolve from modern chimpanzees. Both share a common ancestor. Human evolution is branching, not a straight line.

PART 4 — QUICK REVISION

26. Scientists and dates

Scientist	Contribution	Year
Mendel	Laws of inheritance	1865–66 (rediscovered 1900)
Sutton & Boveri	Chromosomal theory	1902
Morgan, Sturtevant	Linkage, gene mapping	1910s
Griffith	Transformation	1928
Avery, MacLeod, McCarty	DNA is the transforming principle	1944
Hershey & Chase	DNA is genetic material	1952
Watson & Crick	Double helix	1953
Miller	Abiotic synthesis of amino acids	1953
Meselson & Stahl	Semiconservative replication	1958
Jacob & Monod	Lac operon	1961
Nirenberg, Khorana, Ochoa	Genetic code	1960s
Jeffreys	DNA fingerprinting	1985
HGP	Human genome sequencing	1990–2003

27. Do not confuse

A	B
Incomplete dominance: intermediate	Codominance: both expressed
Test cross: with homozygous recessive	Back cross: with either parent
Mutation: new allele	Recombination: new combination
Monosomy: 2n−1	Trisomy: 2n+1
Homologous: divergent evolution	Analogous: convergent evolution
Gene flow: migration	Genetic drift: random sampling
Founder: few colonisers	Bottleneck: population crash
Codon: on mRNA	Anticodon: on tRNA
Transcription: DNA → RNA	Translation: RNA → protein
Leading: continuous	Lagging: discontinuous
Sickle-cell: structural defect	Thalassaemia: quantity defect

28. Top traps

1.      Dominant does not mean more common. Recessive does not mean inferior.

2.      Crossing over: pachytene. Chiasmata: diplotene.

3.      X-linked genes never pass father → son.

4.      Barr bodies = X − 1. Turner has none.

5.      DNA polymerase works 5′→3′ only.

6.      AUG is both start codon and methionine.

7.      70S = 50S + 30S. 80S = 60S + 40S.

8.      Hershey–Chase used ³²P for DNA and ³⁵S for protein.

9.      Mutation is not always harmful.

10.   Genetic drift matters most in small populations.

11.   Hardy–Weinberg means no evolution is happening.

12.   Mendelian phenotype ratios: 3 : 1, 9 : 3 : 3 : 1. Incomplete dominance F2 is 1 : 2 : 1.

29. Quick drill

Question	Answer
Dihybrid test-cross ratio?	1 : 1 : 1 : 1
Which gene codes for the lac repressor?	i
Female heterogametic sex?	Birds (ZW)
Amino acid replaced in sickle-cell?	Glu → Val (position 6, β-chain)
Okazaki fragments are joined by?	DNA ligase
Number of stop codons?	3
Max recombination frequency?	50%
Karyotype of Turner syndrome?	45, X0
Hardy–Weinberg when q = 0.1: heterozygotes?	2pq = 0.18
Who proposed the mutation theory?	Hugo de Vries
Largest-brained Neanderthal figure?	~1,400 cc