Chapter 9 — Heredity and Evolution
Welcome to HSLC Guru! This complete study guide for Class 10 Science Chapter 9 — Heredity and Evolution follows the latest ASSEB (Assam State School Education Board) syllabus. Here you will find a clear summary, full textbook question-answers, additional MCQs, fill in the blanks, true/false statements and a useful glossary — all designed to help you score top marks in your HSLC Science examination.
Chapter Summary
Heredity is the transmission of characters or traits from parents to their offspring. The science that studies heredity and variation is called genetics. The unit of heredity is the gene, which is a segment of DNA located on chromosomes. Variations among individuals are essential for the survival of species and form the raw material for evolution. Variations may be inheritable when they occur in germ cells, but acquired variations in body cells are not passed on to the next generation.
Gregor Johann Mendel, the Father of Genetics, conducted experiments on garden pea (Pisum sativum) and proposed the laws of inheritance. In a monohybrid cross between a tall (TT) and a dwarf (tt) plant, the F1 generation was all tall (Tt), and the F2 generation showed a phenotypic ratio of 3:1 (3 tall : 1 dwarf). In a dihybrid cross involving two pairs of contrasting traits (e.g., round-yellow × wrinkled-green seeds), the F2 generation showed a phenotypic ratio of 9:3:3:1. Traits which appear in F1 are called dominant traits, while those that remain hidden are called recessive traits.
In humans, sex determination is governed by sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The mother always contributes an X chromosome to the offspring, while the father may contribute either X (resulting in a girl) or Y (resulting in a boy). Thus, the sex of a child is determined by the father’s chromosome contribution.
Evolution is the gradual change in living organisms over generations. Charles Darwin proposed the theory of Natural Selection, which states that organisms better adapted to their environment survive, reproduce and pass on favourable traits. Acquired traits (developed during one’s lifetime) are not inherited, while inherited traits (encoded in DNA) pass to offspring. Speciation is the process of formation of new species through reproductive isolation, genetic drift, and natural selection. Evidences of evolution include homologous organs (similar origin, different functions — e.g., forelimbs of mammals), analogous organs (different origin, similar functions — e.g., wings of bird and insect), vestigial organs (e.g., human appendix, wisdom teeth), and fossils. Human evolution traces from Australopithecus through Homo habilis, Homo erectus to Homo sapiens.
Textbook Questions and Answers
1-Mark Questions
Q1. Who is known as the Father of Genetics?
Answer: Gregor Johann Mendel is known as the Father of Genetics.
Q2. What is a gene?
Answer: A gene is the basic unit of heredity. It is a segment of DNA on a chromosome that codes for a particular character.
Q3. What is the phenotypic ratio of Mendel’s monohybrid cross in the F2 generation?
Answer: The phenotypic ratio is 3:1 (three dominant : one recessive).
Q4. What is the phenotypic ratio of Mendel’s dihybrid cross in F2?
Answer: The phenotypic ratio is 9:3:3:1.
Q5. Which chromosomes determine the male sex in humans?
Answer: The XY chromosomes determine the male sex in humans.
Q6. Who proposed the theory of Natural Selection?
Answer: Charles Darwin proposed the theory of Natural Selection.
Q7. Give one example of a vestigial organ in humans.
Answer: The vermiform appendix is a vestigial organ in humans.
Q8. What plant did Mendel use for his experiments?
Answer: Mendel used the garden pea plant (Pisum sativum) for his experiments.
Q9. Define species.
Answer: A species is a group of similar organisms that can interbreed and produce fertile offspring.
Q10. What are fossils?
Answer: Fossils are the preserved remains or impressions of dead organisms found in rocks; they provide evidence of evolution.
2-3 Marks Questions
Q1. Differentiate between dominant and recessive traits.
Answer: A dominant trait is the one that expresses itself even in the presence of one allele (e.g., tallness in pea plants represented by T). A recessive trait expresses itself only when both alleles are recessive (e.g., dwarfness, tt). In F1, only the dominant trait is visible while the recessive trait remains hidden but reappears in F2.
Q2. Distinguish between acquired and inherited traits.
Answer: Acquired traits are characters developed by an organism during its lifetime due to environmental influences (e.g., learning to swim, muscular body of a wrestler). They cannot be passed to offspring as they do not affect DNA. Inherited traits are encoded in genes and transferred from parents to children (e.g., eye colour, blood group).
Q3. Explain sex determination in humans.
Answer: Humans have 23 pairs of chromosomes — 22 pairs of autosomes and 1 pair of sex chromosomes. Females are XX (homogametic) and produce only X-bearing eggs. Males are XY (heterogametic) and produce two types of sperm: 50% X-bearing and 50% Y-bearing. If an X-sperm fertilizes the egg → XX (girl); if a Y-sperm fertilizes → XY (boy). Therefore, the sex of the child is determined by the father.
Q4. What are homologous and analogous organs? Give one example each.
Answer: Homologous organs have the same basic structure and origin but perform different functions — e.g., forelimbs of humans, whales, bats and birds. Analogous organs have different structures and origin but perform similar functions — e.g., wings of a bird and wings of an insect. Homologous organs indicate common ancestry, whereas analogous organs indicate convergent evolution.
Q5. What is speciation? Mention any two factors causing it.
Answer: Speciation is the process by which new species are formed from pre-existing ones through accumulation of variations and reproductive isolation. Two key factors are: (i) Geographical isolation — physical barriers like rivers and mountains that prevent interbreeding, and (ii) Genetic drift — random change in gene frequencies in a small population.
Q6. Why are vestigial organs considered evidence of evolution?
Answer: Vestigial organs are reduced, non-functional organs that were once functional in ancestors. Their presence (e.g., appendix, wisdom teeth, body hair in humans) suggests that humans evolved from ancestors who actively used these organs. They thus provide anatomical evidence supporting the theory of evolution.
5-6 Marks Questions
Q1. Describe Mendel’s monohybrid cross with a suitable example and ratio.
Answer: Mendel crossed a pure tall pea plant (TT) with a pure dwarf plant (tt). All F1 plants were tall (Tt), showing that tallness is dominant over dwarfness. When F1 plants were self-pollinated, the F2 generation produced both tall and dwarf plants in the ratio 3:1 (1 TT : 2 Tt : 1 tt). The genotypic ratio was 1:2:1 and phenotypic ratio 3:1. This proved Mendel’s Law of Segregation — alleles separate during gamete formation. The reappearance of dwarfness in F2 also showed that recessive traits are not lost but only masked in F1.
Q2. Explain Mendel’s dihybrid cross. Why is the F2 ratio 9:3:3:1?
Answer: Mendel crossed pea plants with two pairs of contrasting traits — round-yellow (RRYY) × wrinkled-green (rryy) seeds. F1 plants were all round-yellow (RrYy). On self-pollination, F2 produced four phenotypes in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. The appearance of new combinations (round-green and wrinkled-yellow) shows that the two pairs of traits assort independently. This is Mendel’s Law of Independent Assortment, which states that during gamete formation, alleles of different genes segregate independently of one another.
Q3. What is evolution? Discuss Darwin’s theory of Natural Selection.
Answer: Evolution is the gradual change in inheritable characteristics of a population over successive generations. Charles Darwin’s Theory of Natural Selection is based on the following ideas: (i) Organisms produce more offspring than can survive. (ii) Individuals show variations within a population. (iii) Resources like food and space are limited, leading to a struggle for existence. (iv) Individuals with favourable variations are better adapted and have higher chances of survival of the fittest. (v) These individuals reproduce and transmit their useful traits to the next generation. Over many generations, accumulated favourable variations lead to the formation of new species.
Q4. Discuss the various evidences of evolution.
Answer: The major evidences of evolution are: (i) Morphological / Anatomical evidence: Homologous organs (e.g., forelimbs of mammals) suggest common ancestry. (ii) Analogous organs: Show convergent evolution (e.g., wings of birds and insects). (iii) Vestigial organs: Reduced organs without function (e.g., appendix in humans) showing relationship with ancestors. (iv) Fossil evidence: Preserved remains of organisms in rocks (e.g., Archaeopteryx linking reptiles and birds). (v) Embryological evidence: Embryos of vertebrates show similar early stages. (vi) Connecting links: Organisms like Archaeopteryx and lungfish bridge two groups.
Q5. Briefly describe human evolution.
Answer: Human evolution started in Africa around 5–7 million years ago. The major stages are: (i) Australopithecus (~4 million years ago) — could walk upright, used simple tools. (ii) Homo habilis (‘handy man’) — first to make and use stone tools. (iii) Homo erectus — walked fully upright, used fire, had larger brain. (iv) Homo neanderthalensis — buried their dead and used clothing. (v) Homo sapiens (modern humans) — developed language, art, agriculture and culture. Studies of DNA and fossils confirm that all modern humans share a common African origin and migrated to other continents.
Additional Multiple Choice Questions (MCQs)
Q1. The unit of heredity is —
(a) Cell (b) Gene (c) Tissue (d) Nucleus
Answer: (b) Gene
Q2. The phenotypic ratio of a monohybrid cross is —
(a) 1:2:1 (b) 9:3:3:1 (c) 3:1 (d) 2:1
Answer: (c) 3:1
Q3. The phenotypic ratio of a dihybrid cross is —
(a) 3:1 (b) 9:3:3:1 (c) 1:1 (d) 1:2:1
Answer: (b) 9:3:3:1
Q4. Sex of a human child is determined by —
(a) Mother (b) Father (c) Both (d) Doctor
Answer: (b) Father
Q5. Which of the following is a vestigial organ in humans?
(a) Heart (b) Appendix (c) Liver (d) Kidney
Answer: (b) Appendix
Q6. Wings of a bird and wings of an insect are —
(a) Homologous (b) Analogous (c) Vestigial (d) None
Answer: (b) Analogous
Q7. Forelimbs of human, whale and bat are examples of —
(a) Analogous (b) Homologous (c) Vestigial (d) Fossils
Answer: (b) Homologous organs
Q8. The theory of Natural Selection was given by —
(a) Mendel (b) Darwin (c) Lamarck (d) Watson
Answer: (b) Darwin
Q9. Archaeopteryx is a connecting link between —
(a) Fish and Amphibian (b) Reptile and Bird (c) Bird and Mammal (d) Reptile and Mammal
Answer: (b) Reptile and Bird
Q10. Which is an inherited trait?
(a) Knowledge of swimming (b) Body weight (c) Eye colour (d) Scar from accident
Answer: (c) Eye colour
Fill in the Blanks
Q1. The branch of biology dealing with heredity and variation is called __________.
Answer: Genetics
Q2. Mendel performed his experiments on __________ plant.
Answer: Garden pea (Pisum sativum)
Q3. The chromosomes of a normal human female are __________.
Answer: XX
Q4. Wings of a butterfly and wings of a bat are __________ organs.
Answer: Analogous
Q5. The preserved remains of organisms found in rocks are called __________.
Answer: Fossils
True or False
Q1. Acquired traits are inheritable.
Answer: False
Q2. Mendel is called the Father of Genetics.
Answer: True
Q3. Sex of a child is determined by the mother.
Answer: False
Q4. Homologous organs indicate common ancestry.
Answer: True
Q5. Vestigial organs are fully functional organs.
Answer: False
Glossary
| Term | Meaning |
|---|---|
| Heredity | Transmission of characters from parents to offspring. |
| Genetics | Branch of biology dealing with heredity and variation. |
| Gene | Unit of heredity; segment of DNA on a chromosome. |
| Allele | Alternative form of a gene (e.g., T and t). |
| Dominant Trait | Trait that expresses itself in the presence of one allele. |
| Recessive Trait | Trait that expresses only when both alleles are recessive. |
| Monohybrid Cross | Cross involving one pair of contrasting traits (ratio 3:1). |
| Dihybrid Cross | Cross involving two pairs of traits (ratio 9:3:3:1). |
| Genotype | Genetic constitution of an organism (e.g., TT, Tt, tt). |
| Phenotype | Observable physical appearance (e.g., tall or dwarf). |
| Sex Chromosomes | Chromosomes that determine sex (XX female, XY male). |
| Evolution | Gradual change in organisms over generations. |
| Natural Selection | Process by which fittest organisms survive and reproduce. |
| Speciation | Process of formation of new species. |
| Homologous Organs | Same origin, different functions; show common ancestry. |
| Analogous Organs | Different origin, similar functions; show convergent evolution. |
| Vestigial Organs | Reduced, non-functional organs (e.g., appendix). |
| Fossils | Preserved remains of ancient organisms in rocks. |
| Acquired Trait | Trait gained during life; not inherited. |
| Inherited Trait | Trait passed from parents to offspring through genes. |
Important Differences (Quick Compare)
| Feature | Homologous Organs | Analogous Organs |
|---|---|---|
| Origin | Same embryonic origin | Different embryonic origin |
| Structure | Similar internal structure | Different internal structure |
| Function | Different functions | Similar functions |
| Example | Forelimbs of human, bat, whale | Wings of bird and insect |
| Indicates | Common ancestry (divergent evolution) | Convergent evolution |
| Feature | Acquired Traits | Inherited Traits |
|---|---|---|
| Origin | Developed during lifetime | Received from parents through genes |
| DNA Change | No change in DNA | Encoded in DNA |
| Transmission | Not passed to offspring | Passed to offspring |
| Example | Muscular body, scar mark | Eye colour, blood group |
Quick Revision Points
1. Heredity = inheritance of traits; Variation = differences between individuals.
2. Mendel proposed three laws — Law of Dominance, Law of Segregation, and Law of Independent Assortment.
3. Genotype = genetic makeup (TT, Tt, tt); Phenotype = visible appearance (tall/dwarf).
4. Humans have 23 pairs of chromosomes — 22 autosomes + 1 sex chromosome pair.
5. Acquired traits cannot be inherited because they do not bring changes in DNA of germ cells.
6. Speciation requires reproductive isolation + genetic variation + natural selection.
7. Modern man is scientifically known as Homo sapiens.
8. Variations occur in DNA copying during reproduction; they form the raw material of evolution.
9. Charles Darwin wrote the famous book “On the Origin of Species” (1859).
10. Fossils are dated by two methods — relative dating (depth in rocks) and carbon dating (using radioactive carbon-14).
11. Evolution is not a ‘progress’ from lower to higher forms — it is simply branching of populations through diversification.
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