BC03 — Genetics & Evolution

📖 BC03 · CDS General Knowledge — Biology ★ High Priority

Genetics explains why children resemble their parents; evolution explains why species change over time. These two ideas together answer the deepest question in biology: where does biological diversity come from? CDS tests Mendel's laws, DNA structure, and Darwin's theory with direct factual questions.

📌 CDS Focus: Mendel's law of segregation = most important law; dominant masks recessive; phenotype ratio in monohybrid cross = 3:1; genotype ratio = 1:2:1; DNA = double helix (Watson & Crick, 1953); adenine pairs with thymine (A-T), guanine pairs with cytosine (G-C); Darwin's "survival of the fittest"; homologous organs = evidence for common ancestry.

PART 1 — MENDEL'S LAWS

1. Mendel's Laws of Inheritance

Gregor Mendel (1822–1884) worked with pea plants (Pisum sativum) and discovered the mathematical rules of inheritance. His three laws apply to all sexually reproducing organisms.

Fig. 1 — Monohybrid Cross (Tt × Tt): Phenotype and Genotype Ratios

Three Laws of Mendel — One Line Each:

● Law of Dominance: When two contrasting traits cross, only the dominant trait appears in F1 (first filial generation). The recessive trait disappears (but is not lost).
● Law of Segregation (most important): The two alleles of a gene separate during gamete formation — each gamete carries only one allele for each trait.
● Law of Independent Assortment: Genes for different traits are distributed to gametes independently of each other (applies to genes on different chromosomes).

PART 2 — DNA & EVOLUTION

2. DNA Structure & Central Dogma

Fig. 2 — DNA Structure and Base Pairing Rules

Central Dogma of Molecular Biology (Francis Crick, 1958):

DNA → RNA → Protein

● Transcription: DNA is copied into mRNA in the nucleus
● Translation: mRNA is read by ribosomes; tRNA brings amino acids; protein chain forms
● RNA types: mRNA (messenger — carries code); tRNA (transfer — brings amino acids); rRNA (ribosomal — structural component of ribosome)

3. Darwin's Theory of Evolution

Key ConceptExplanationExample OverproductionAll organisms reproduce more offspring than can surviveA frog lays thousands of eggs; most do not survive VariationIndividuals in a population differ in heritable traitsSome moths darker, some lighter Struggle for existenceCompetition for limited resources (food, space, mates)Predation, disease, competition for food Natural selectionIndividuals with favourable variations survive and reproduce morePeppered moth (industrial melanism) Survival of the fittestCoined by Herbert Spencer; means "best adapted to environment"Dark moths survived soot-covered trees Homologous organsSame structure, different function — evidence for common ancestryHuman arm, whale flipper, bat wing, horse forelimb Analogous organsDifferent structure, similar function — convergent evolutionButterfly wing and bird wing

📝 CDS PYQs — Genetics & Evolution

Q1. In a monohybrid cross Tt × Tt, the phenotype ratio of offspring is: CDS PYQ

(a) 1:1(b) 1:2:1(c) 3:1(d) 9:3:3:1

✔ Answer: (c) 3:1

When two heterozygotes (Tt × Tt) cross: TT, Tt, Tt, tt offspring — 3 show the dominant phenotype (Tall): 1 shows recessive phenotype (Dwarf). The phenotype ratio is 3:1. The genotype ratio is 1(TT):2(Tt):1(tt). The 9:3:3:1 ratio is for a dihybrid cross (two traits). This is Mendel's most tested result in CDS.

Q2. Which scientist proposed the double helix model of DNA? CDS PYQ

(a) Mendel and Darwin(b) Watson and Crick(c) Hershey and Chase(d) Griffith and Avery

✔ Answer: (b) Watson and Crick

James Watson and Francis Crick proposed the double helix model in 1953, based on X-ray diffraction data from Rosalind Franklin. They won the Nobel Prize in 1962. Key rules: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). The two strands are antiparallel. This discovery explained how DNA replicates and carries genetic information.

Q3. Organs that have the same fundamental structure but different functions are called: CDS PYQ

(a) Analogous organs(b) Vestigial organs(c) Homologous organs(d) Rudimentary organs

✔ Answer: (c) Homologous organs

Homologous organs have the same basic structure (same bones, same embryonic origin) but perform different functions — evidence of common ancestry. Example: human arm (grasping), whale flipper (swimming), bat wing (flying), horse forelimb (running) — all built from the same five-digit bone plan. Analogous organs (butterfly and bird wings) have different structure but same function — convergent evolution, NOT evidence of common ancestry.

🧠 Quick Memory Chart — BC03

📈 Mendel's Laws

Dominance: dominant masks recessive
Segregation: alleles separate in gametes
Independent assortment: unlinked genes
Monohybrid F2: 3:1 phenotype
Dihybrid F2: 9:3:3:1 phenotype

📊 DNA Facts

Watson & Crick (1953): double helix
A—T (2 bonds); G—C (3 bonds)
3 components: sugar + phosphate + base
Central dogma: DNA→RNA→Protein
mRNA, tRNA, rRNA = 3 RNA types

🌾 Evolution

Darwin: natural selection (1859)
Survival of fittest = best adapted
Homologous: same structure, diff function
Analogous: diff structure, same function
Fossils = direct evidence of evolution

📝 Practice Exercise

E1. In DNA, if Adenine content is 30%, what is the Guanine content?

(a) 30%(b) 20%(c) 40%(d) 10%

E2. Blood group in humans is an example of:

(a) Incomplete dominance(b) Codominance(c) Sex-linked inheritance(d) Epistasis

E3. Which of the following is correct about Mendel's Law of Segregation?

(a) Two different genes always move together(b) Alleles of a gene separate during gamete formation(c) Recessive alleles never appear in F2(d) All offspring resemble the dominant parent

Answers:
E1 → (b) 20% [A pairs with T; if A=30%, then T=30%. A+T+G+C=100%; G+C=40%; G=C, so G=20%] | E2 → (b) Codominance [both A and B alleles are expressed equally; AB blood group shows both A and B antigens] | E3 → (b) Alleles separate during gamete formation [the paired alleles (Tt) segregate so each gamete gets one allele: T or t]

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