Class 10 Chapter 8 Long Qa Science Ncert

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Extra 20 long-answer questions from the Chapter:8 Heredity and Evolution chapter of Class 10 CBSE Science:

1. What were Mendel’s experiments on pea plants? Explain his conclusions.

Answer:
Mendel performed experiments on pea plants (Pisum sativum) to study the inheritance of traits.

Mendel’s Experiment

Monohybrid Cross:

Studied one trait at a time (e.g., height).

Crossed

tall (TT) × dwarf (

)

→ All

tall (Tt) in F1 generation

Self-crossed F1 (Tt × Tt)

→ F2 generation:

tall :

1 dwarf

(3:1 phenotypic ratio).

Dihybrid Cross:

Studied two traits at a time (e.g., seed

colour

& shape).

Crossed

yellow-round × green-wrinkled

→ F1:

All yellow-round

Self-crossed F1

→ F2 ratio:

9:3:3:1

(Mendel's Law of Independent Assortment).

Mendel’s Conclusions

Law of Dominance

: A dominant trait masks a recessive trait.

Law of Segregation

: Alleles separate during gamete formation.

Law of Independent Assortment

: Traits are inherited independently.

2. What is sex determination? Explain how it occurs in humans.

Answer:
Sex determination is the process that decides whether an individual is male or female.

Mechanism in Humans

Humans have

23 pairs of chromosomes (46 total)

22 pairs are autosomes

, and

1 pair is sex chromosomes (XX or XY)

Females have

chromosomes, and males have

chromosomes.

During reproduction:

Mother contributes X

(always).

Father contributes either X or Y

If sperm carries

→ Baby is

female (XX)

If sperm carries

→ Baby is

male (XY)

Thus,

fathers determine the sex

of the child.

3. Explain the difference between acquired and inherited traits with examples.

Answer:

Feature	Acquired Traits	Inherited Traits
Definition	Traits developed during an organism’s lifetime.	Traits passed from parents to offspring.
Cause	Environmental influence or lifestyle.	Genetic information (DNA).
Effect on DNA	No change in DNA.	Change in DNA inherited by offspring.
Example	Learning a skill, bodybuilder’s muscles.	Eye colour, height, skin colour.

Mendel’s experiments showed that only inherited traits pass to the next generation.

Evolution-Based Questions

4. What is evolution? Explain Darwin’s Theory of Natural Selection.

Answer:
Evolution is the gradual change in organisms over generations, leading to new species.

Darwin’s Theory of Natural Selection

Overproduction

: Organisms produce more offspring than can survive.

Variation

: Individuals in a species have natural variations.

Survival of the Fittest

: Organisms with beneficial traits survive and reproduce.

Inheritance

Favourable

traits are passed to the next generation.

Speciation

: Over time, new species form due to accumulated changes.

Example:

Giraffes

: Short-necked giraffes died due to a lack of food, while long-necked giraffes survived and reproduced.

5. What is speciation? How does it occur?

Answer:
Speciation is the formation of a new species due to evolutionary changes.

Causes of Speciation

Genetic Variation

: Differences in DNA among individuals.

Natural Selection

Favorable

traits lead to survival.

Geographical Isolation

: A population is separated (e.g., mountains, rivers).

Reproductive Isolation

: Different groups

stop

interbreeding.

Mutation

: Random DNA changes can lead to new species.

Example: Darwin’s finches developed different beak shapes due to different food sources.

6. Explain the role of fossils in tracing evolutionary history.

Answer:
Fossils are preserved remains of ancient organisms.

How Fossils Support Evolution

Provide Evidence of Extinct Species

(e.g., Dinosaurs).

Show Transitional Forms

(e.g., Archaeopteryx - link between reptiles & birds).

Reveal Structural Changes Over Time

(e.g., Horse evolution from small to large body).

Show Common Ancestry

(e.g., Whale fossils show hind limb bones, proving they evolved from land animals).

7. Differentiate between homologous, analogous, and vestigial organs with examples.

Answer:

Type of Organ	Structure	Function	Example
Homologous	Similar	Different	Human arm & bat wing
Analogous	Different	Similar	Wings of birds & insects
Vestigial	Present but functionless	None	Appendix in humans, hind limb bones in whales

Homologous organs suggest common ancestry, while analogous organs show convergent evolution.

8. What is genetic drift? How does it affect evolution?

Answer:
Genetic drift is a random change in allele frequency in a population.

Effects on Evolution

Reduces Genetic Diversity

: Some traits disappear randomly.

Can Lead to Speciation

: If a small group gets isolated, they evolve separately.

Example - Bottleneck Effect

: A natural disaster kills most of a population, leaving only a few survivors with limited genetic diversity.

9. What are transitional fossils? Give examples.

Answer:
Transitional fossils show intermediate traits between ancient and modern species.

Examples

Archaeopteryx

(Reptile & Bird features).

Tiktaalik

(Fish & Amphibian features).

These fossils prove gradual evolutionary changes.

10. How do embryological studies support the theory of evolution?

Answer:

Early embryos of different species look similar.

Example:

Human and fish embryos both have

gill slits

in early stages.

This suggests a

common ancestor

11. Explain Lamarck’s theory of evolution and why it was rejected.

Answer:

Lamarck proposed that

traits acquired during life

(e.g., bodybuilder’s muscles) are inherited.

Rejected because

DNA does not change due to lifestyle

Darwin’s theory of

natural selection

was accepted instead.

12. How does industrial melanism provide evidence for evolution?

Answer:

Before industrialization

: White moths were common as they camouflaged on

light-coloured

trees.

After industrialization

: Black moths became more common as trees became dark due to pollution.

This shows

natural selection

in action.

13. How does artificial selection differ from natural selection? Explain with examples.

Answer:
Artificial selection is a process where humans selectively breed organisms to enhance specific traits, while natural selection occurs naturally based on survival advantages.

Feature	Artificial Selection	Natural Selection
Selection	By humans	By nature
Purpose	Improve specific traits	Survival advantage
Speed	Faster	Slow & gradual
Example	Selective breeding of high-yield crops, dairy cows	Evolution of giraffes with long necks

Thus, artificial selection helps humans in agriculture, while natural selection drives evolution.

14. Describe the evolution of modern humans.

Answer:
Human evolution occurred millions of years ago through several species:

Australopithecus

(4 million years ago) → First bipedal species.

Homo habilis

(2 million years ago) → Used tools.

Homo erectus

(1.5 million years ago) → Used fire, better hunting skills.

Neanderthals

(400,000 years ago) → Lived in caves, made weapons.

Homo sapiens (Modern Humans)

(200,000 years ago) → Advanced tools, languages, civilizations.

Fossils and DNA studies confirm that modern humans evolved from a common ancestor with chimpanzees.

15. What is the importance of variation in organisms? How does it help in evolution?

Answer:
Variation refers to the differences in traits among individuals of a species.

Importance of Variation

Survival in Changing Environments

→ Some organisms can adapt to climate change or diseases.

Basis of Evolution

→ Beneficial traits accumulate over generations.

Prevents Extinction

→ A genetically diverse population is more likely to survive.

Example:

Bacteria developing antibiotic resistance

is a result of genetic variation.

16. What are fossils? How do they help in studying evolution?

Answer:
Fossils are preserved remains of ancient organisms found in rocks.

How Fossils Help in Evolution

Show Extinct Species

→ Dinosaurs, Mammoths.

Provide Transitional Links

→

Archaeopteryx

(link between reptiles & birds).

Age Determination

→ Using

carbon dating

& rock layers.

Prove Gradual Changes

→ Horse evolution (small to large size).

Fossil records confirm that life has evolved over millions of years.

17. How does comparative anatomy provide evidence for evolution?

Answer:
Comparative anatomy studies similarities and differences in the structure of different species.

Types of Anatomical Evidence

Homologous Organs

→ Same structure, different functions (e.g., human arm & whale flipper).

Analogous Organs

→ Different structure, same function (e.g., wings of birds & insects).

Vestigial Organs

→ No longer functional but present (e.g., appendix in humans).

These similarities suggest that organisms evolved from common ancestors.

18. Explain the concept of ‘Survival of the Fittest’ with an example.

Answer:
Survival of the Fittest is a key concept of Darwin’s Theory of Natural Selection, where only organisms best adapted to their environment survive and reproduce.

Example: Peppered Moths

Before industrialization:

Light-

colored

moths

camouflaged on trees and survived.

After industrialization:

Dark-

colored

moths

became common as soot-darkened trees.

Over time, the population of dark moths increased due to

natural selection

This proves that nature selects beneficial traits for survival.

19. What is embryological evidence of evolution? Explain with an example.

Answer:
Embryological evidence compares the early development of different species to show common ancestry.

Example: Human & Fish Embryos

In early stages, both have

gill slits

and

tail-like structures

Later, human embryos develop lungs, while fish embryos develop gills.

This suggests that

humans and fish share a common ancestor

Comparative embryology supports Darwin’s theory that all vertebrates evolved from a common ancestor.

20. How does genetic drift differ from natural selection? Explain with examples.

Answer:

Feature	Genetic Drift	Natural Selection
Definition	Random change in gene frequency	Favourable traits are selected
Cause	Chance events (disasters)	Environmental adaptation
Effect	Can eliminate traits randomly	Leads to evolution
Example	Bottleneck effect (cheetah population decline)	Giraffes evolving long necks

Genetic drift occurs by random chance, whereas natural selection favours adaptive traits.