Chapter 9: Heredity and Evolution

Answer: (c) TtWW

Explanation:
Let's analyze step by step:

Traits:
• Tall (T) = dominant, Short (t) = recessive
• Violet flowers (W) = dominant, White flowers (w) = recessive

Observations in progeny (F1 generation):
1. All plants have violet flowers → Violet is dominant and tall parent is homozygous for violet (WW)
2. Almost half are short → Tall parent must be heterozygous for height (Tt)

Cross: Tall violet (TtWW) × Short white (ttww)
Gametes from tall parent: TW, tW
Gametes from short parent: tw only

Progeny: TtWw (tall violet) and ttWw (short violet) in 1:1 ratio
Exactly matches observation: all violet, half tall, half short.

Other options:
(a) TTWW × ttww → all TtWw (all tall violet) ❌
(b) TTww × ttww → all Ttww (all tall white) ❌
(d) TtWw × ttww → four types in equal ratio ❌

Answer: (d) all of the above

Explanation:
Homologous organs have similar basic structure and embryonic origin but different functions, indicating common ancestry.

Analysis of each option:
1. Our arm and dog's fore-leg: Both have humerus, radius, ulna, carpals, metacarpals, and phalanges (same bone structure) but different functions (grasping vs walking).
2. Our teeth and elephant's tusks: Both are modified teeth (incisors) but serve different functions (chewing vs defense/digging).
3. Potato (tuber) and grass runners (stolon): Both are modified stems for vegetative propagation but look different.

Contrast with analogous organs: Similar function but different structure/origin (e.g., wings of birds and insects).

Answer: (a) a Chinese school-boy

Explanation:
Evolutionary closeness is determined by:
1. Common ancestry (recent common ancestor)
2. Genetic similarity
3. Morphological and physiological similarities

Order of closeness to humans:
1. Chinese school-boy: Same species (Homo sapiens) → 99.9% genetic similarity
2. Chimpanzee: Different species but same family (Hominidae) → ~98% genetic similarity
3. Spider: Different phylum (Arthropoda) → very distant
4. Bacterium: Different domain (Prokaryota) → most distant

All humans regardless of ethnicity/geography share the same evolutionary lineage from Africa ~200,000 years ago. Race is a social construct with no biological basis for species separation.

Answer:

No, we cannot determine dominance/recessiveness from this information alone.

Reason:
The observation only shows correlation, not inheritance pattern. Several possibilities exist:

Scenario 1 (Light eyes recessive):
If light eyes (l) are recessive and dark eyes (L) dominant:
• Both parents could be heterozygous (Ll) or homozygous recessive (ll)
• ll × ll → all ll (light-eyed children)
• Ll × Ll → 25% ll (light-eyed children possible)

Scenario 2 (Light eyes dominant):
If light eyes (L) are dominant and dark eyes (l) recessive:
• Parents could be homozygous dominant (LL) or heterozygous (Ll)
• LL × LL → all LL (light-eyed children)
• Ll × Ll → 75% light-eyed children possible

To determine dominance: We need:
1. Cross between pure-breeding light and dark-eyed parents
2. Observation of F1 and F2 generations
3. Statistical ratios in progeny

Note: Eye color inheritance is actually polygenic (multiple genes), not simple Mendelian, but the principle remains: single-generation observation is insufficient.

Answer:

Evolution and classification are fundamentally interconnected:

How Classification Reflects Evolution	How Evolution Informs Classification
Hierarchy shows evolutionary relationships: Kingdom → Phylum → Class → Order → Family → Genus → Species represents increasing closeness of common ancestry	Common ancestry determines grouping: Organisms sharing more recent common ancestors are placed in same/similar taxa
Homologous structures = same taxon: Similar anatomical structures indicate common origin (e.g., vertebrate limbs)	Evolutionary trees = classification maps: Phylogenetic trees visually represent both evolutionary history and classification
Fossil records connect classification: Transitional fossils show links between different classified groups	DNA sequencing confirms/reveals relationships: Molecular phylogeny uses genetic similarities to refine classification
Embryological similarities: Similar embryonic development patterns in related classified groups	Speciation events = new species in classification: When evolution produces reproductive isolation, new species are recognized

Modern classification (phylogenetic system) is explicitly based on evolutionary relationships, unlike older artificial systems based only on observable traits.

Answer:

Aspect	Homologous Organs	Analogous Organs
Definition	Organs with similar basic structure and embryonic origin but different functions	Organs with similar functions but different structure and embryonic origin
Evolutionary Significance	Indicate common ancestry (divergent evolution)	Indicate adaptation to similar environment (convergent evolution)
Genetic Basis	Similar genes/developmental pathways	Different genes/developmental pathways
Examples	• Forelimbs: Human arm, bat wing, whale flipper, horse leg (all have humerus, radius, ulna, carpals, metacarpals, phalanges) • Plant stems: Potato tuber, ginger rhizome, onion bulb, cactus spine • Mouthparts: Butterfly proboscis and mosquito proboscis	• Wings: Bird wing (feathers on forearm) vs insect wing (chitinous membrane) vs bat wing (skin stretched between fingers) • Eyes: Octopus eye (from skin) vs vertebrate eye (from brain) • Fins: Fish fin (bony rays) vs penguin flipper (modified limb) • Sweet potato (root) vs potato (stem): Both store food but different origins
Visual Clue	Look different but have similar bone/structural plan	Look/function similar but have different structural plans

Memory tip: "Homologous = same origin, different job; Analogous = same job, different origin."

Answer:

Project: Determining Dominant Coat Color in Dogs

Objective: To identify whether black coat color or brown coat color is dominant in a specific dog breed.

Materials Needed:
• Pure-breeding black dogs (if available)
• Pure-breeding brown dogs (if available)
• Records of parent and offspring coat colors
• Camera for documentation
• Notebook for data recording

Procedure (Mendelian Approach):
Step 1: Establish pure lines
• Identify dogs that consistently produce only black or only brown puppies when bred with similar-colored mates for multiple generations.

Step 2: Perform cross 1 (Parental generation)
• Cross pure-breeding black dog with pure-breeding brown dog.
• Record coat colors of all F1 puppies.

Step 3: Analyze F1 results
• If all F1 are black → black is dominant, brown is recessive.
• If all F1 are brown → brown is dominant, black is recessive.
• If mixed → incomplete dominance or other pattern.

Step 4: Perform cross 2 (F1 self-cross)
• Breed F1 dogs with each other.
• Record coat colors of F2 puppies.
• Expected ratio if simple dominance: 3:1 (dominant:recessive).

Step 5: Perform test cross
• Cross F1 black dogs with pure-breeding brown dogs.
• Expected ratio if black dominant: 1:1 (black:brown).

Data Recording:
• Maintain pedigree charts
• Photograph parents and offspring
• Record litter sizes and colors
• Note any exceptions or variations

Expected Outcomes & Interpretation:
• Black dominant: F1 all black; F2 3 black:1 brown; test cross 1:1
• Brown dominant: F1 all brown; F2 3 brown:1 black; test cross 1:1
• Incomplete dominance: F1 all intermediate (gray); F2 1:2:1

Real-world considerations: Dog coat color is often controlled by multiple genes (polygenic), so results may not follow simple Mendelian ratios. The project should focus on a specific breed to minimize genetic variability.

Answer:

Fossils are crucial evidence for evolutionary relationships because they:

Role of Fossils	How It Helps Determine Evolutionary Relationships	Examples
Provide Historical Record	Show organisms that lived in past, documenting life's history over billions of years	Stromatolites (3.5 billion years old) show early life
Reveal Transitional Forms	Show intermediate characteristics between different groups, indicating evolutionary links	Archaeopteryx (dinosaur-bird), Tiktaalik (fish-amphibian)
Establish Chronological Sequence	Deeper fossils = older; shows progression from simple to complex forms over time	Fish → amphibians → reptiles → mammals in rock layers
Show Anatomical Changes	Document gradual modifications in structures over generations	Horse evolution: small multi-toed → large single-hoofed
Indicate Geographical Distribution	Show how species spread/migrated, explaining current distributions	Similar fossils in South America and Africa support continental drift
Provide Absolute Dating	Radioisotope dating gives actual ages, creating evolutionary timelines	Carbon-14 dating of human fossils
Document Extinctions	Show species that no longer exist, indicating evolutionary dead ends or mass extinctions	Dinosaur fossils show dominance and extinction

Limitations: Fossil record is incomplete (rare conditions for preservation), but even partial records provide compelling evidence for evolution and help reconstruct phylogenetic trees.

Answer:

Evidence for chemical evolution (abiogenesis) - life from non-living matter:

Evidence Type	Description	Significance
Miller-Urey Experiment (1953)	Simulated early Earth conditions (methane, ammonia, hydrogen, water vapor, electric sparks). Produced amino acids, sugars, nitrogen bases - building blocks of life.	Showed organic molecules can form from inorganic precursors under plausible early Earth conditions.
Chemical Analysis of Meteorites	Carbonaceous chondrites (meteorites) contain amino acids, purines, pyrimidines - same as Earth life.	Suggests organic molecules form in space; life's building blocks may have extraterrestrial origin or be universal.
Deep-Sea Hydrothermal Vent Research	Vents provide energy, minerals, and protected environments where organic synthesis occurs.	Alternative to "primordial soup" theory; vents could be life's cradle.
Formation of Microspheres & Coacervates	When proteins/lipids are shaken, they form membrane-bound spheres resembling primitive cells.	Shows how protocells (pre-cells) could form spontaneously.
RNA World Hypothesis Evidence	RNA can store information AND catalyze reactions; ribozymes (RNA enzymes) exist.	Suggests RNA preceded DNA/proteins as first self-replicating molecule.
Oldest Fossils	Stromatolites (3.5 billion years old) and microfossils show life existed soon after Earth cooled.	Indicates rapid emergence of life once conditions permitted.
Universal Genetic Code	All life uses same DNA→RNA→protein system with minor variations.	Suggests single origin event from which all life descended.

Current understanding: Life likely arose through gradual chemical evolution: inorganic → simple organic → complex organic → self-replicating molecules → protocells → true cells. The exact steps are still being researched.

Answer:

Part 1: Why sexual reproduction produces more variations:

Mechanism in Sexual Reproduction	How It Creates Variation
Genetic Recombination	Crossing over during meiosis shuffles genes between homologous chromosomes
Independent Assortment	Random alignment of chromosomes during meiosis creates 2n possible gamete combinations (n=haploid number)
Fertilization (Random Fusion)	Any sperm can fertilize any egg → astronomical number of possible zygotes
Diploidy	Two copies of each gene allow recessive traits to be preserved and reappear in new combinations
Mutation Accumulation	Mutations from both parents combine in offspring

Contrast with asexual reproduction:
• Only source of variation = DNA copying errors (mutations)
• Mutations accumulate slowly
• All offspring are clones (except for rare mutations)
• No gene mixing/recombination

Part 2: Evolutionary consequences for sexually reproducing organisms:

Evolutionary Advantage	Explanation
Faster Adaptation	New gene combinations tested every generation → rapid response to environmental changes
Disease Resistance	Genetic diversity prevents pathogens from wiping out entire population
Elimination of Harmful Mutations	Recombination can separate harmful mutations from beneficial genes
Evolution of Complex Traits	Allows accumulation of multiple beneficial mutations in same lineage
Speciation	Genetic diversity provides raw material for reproductive isolation and new species formation
Long-term Survival	Populations withstand environmental fluctuations better

Trade-off: Sexual reproduction is energetically costly and requires finding mates, but the variation benefits outweigh costs in changing environments.

Answer:

Equal genetic contribution is ensured through specific biological mechanisms:

Mechanism	How It Ensures Equal Contribution
Meiosis (Gamete Formation)	Both parents produce haploid gametes (sperm/egg) with exactly half the chromosome number (n). During meiosis I, chromosome pairs separate randomly.
Fertilization	Fusion of haploid sperm (n) and haploid egg (n) restores diploid number (2n) in zygote. One set from father, one from mother.
Chromosome Inheritance	Autosomes (22 pairs in humans): One from each parent for each pair. Sex chromosomes: X from mother, X or Y from father.
DNA Content	Each gamete contributes ~3 billion base pairs of DNA in humans. Mitochondrial DNA is exception (maternal only).
Gene Copy Number	For each gene (except sex-linked), offspring inherit one allele from mother and one from father.
Random Assortment	Which parental chromosome goes to which gamete is random, but exactly 50% of chromosomes come from each parent.

Visual Example (Humans):
• Parent cells: 46 chromosomes (23 pairs) each
• Gametes: 23 chromosomes each (after meiosis)
• Zygote: 46 chromosomes (23 from mother + 23 from father)

Exception to equal contribution:
1. Mitochondrial DNA: Inherited only from mother (in egg cytoplasm)
2. Cytoplasmic factors: Egg contributes cytoplasm, sperm contributes almost none
3. Genomic imprinting: Some genes expressed differently depending on parental origin

Significance: Equal contribution ensures genetic diversity, prevents doubling of DNA each generation, and allows for Mendelian inheritance patterns.

Answer:

No, I do not fully agree with this statement. While advantageous variations are more likely to survive, other variations can also persist in populations.

Type of Variation	Survival in Population	Reason/Mechanism
Advantageous Variations	Most likely to survive and spread	Natural selection favors traits that increase survival/reproduction (e.g., antibiotic resistance in bacteria)
Neutral Variations	Can survive and persist	No effect on fitness; not selected for or against (e.g., attached vs free earlobes in humans)
Slightly Disadvantageous Variations	May survive in certain conditions	If disadvantage is small and population is large, may persist through genetic drift
Recessive Harmful Variations	Can survive in heterozygotes	Carriers don't show symptoms; eliminated only when homozygous (e.g., sickle cell trait in malaria zones)
Variations with Context-Dependent Effects	Survive in some environments	Traits advantageous in one environment may be neutral/disadvantageous in another (e.g., dark fur in cold vs hot climates)
Variations Preserved by Genetic Drift	Can survive by chance	In small populations, random events can fix neutral or slightly harmful traits (founder effect, bottleneck)

Key Concepts:
1. Natural Selection: Acts on variations, but not all variations are subject to selection.
2. Genetic Drift: Random changes in gene frequencies, especially in small populations.
3. Neutral Theory: Many molecular variations are neutral and evolve by drift, not selection.
4. Balancing Selection: Maintains variation (e.g., heterozygote advantage in sickle cell anemia).

Example: The beetle example from NCERT:
• Green beetles survive better (advantageous) → natural selection
• Blue beetles survive by chance after elephant stampede (neutral/random) → genetic drift

Conclusion: While advantageous variations have higher survival probability, variations can persist through other mechanisms too. Evolution is driven by both natural selection AND genetic drift.