Class 10 Notes

Class 10 Science Notes Chapter 9 (Chapter 9) – Examplar Problems (English) Book

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Philoid

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Examplar Problems (English)
Alright class, let's begin our preparation for Chapter 9, 'Heredity and Evolution', from the NCERT Class 10 Science Exemplar. This chapter is crucial not just for your board exams but also forms a fundamental part of biology sections in various government exams. Pay close attention to the concepts, definitions, and examples.

Chapter 9: Heredity and Evolution - Detailed Notes

1. Introduction: Heredity and Variation

  • Heredity: The transmission of characters (or traits) from parents to their offspring. It ensures the continuity of species characteristics.
  • Variation: The differences observed among individuals of a species, and also between offspring and their parents. Variations are the raw material for evolution.
  • Genetics: The branch of biology that deals with heredity and variation.

2. Accumulation of Variation during Reproduction

  • Variations arise due to inaccuracies during DNA copying (mutations) and during sexual reproduction.
  • Asexual Reproduction: Variations are fewer and arise mainly due to small errors in DNA replication. Offspring are largely identical to the parent (clones), but minor variations accumulate over generations.
  • Sexual Reproduction: Variations are much greater due to:
    • Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.
    • Independent Assortment: Random segregation of homologous chromosomes during meiosis.
    • Random Fertilization: Fusion of any sperm with any egg.
  • Significance of Variation: Increases the chances of survival of a species in a changing environment. Some individuals with favourable variations are better adapted to survive and reproduce (Natural Selection).

3. Mendel's Contributions to Heredity

  • Gregor Johann Mendel (Father of Genetics): Conducted experiments on garden pea plants (Pisum sativum) to understand the principles of inheritance.

  • Reasons for Choosing Pea Plants:

    • Easy to grow and maintain.
    • Short life cycle.
    • Produce a large number of offspring.
    • Have several easily distinguishable contrasting characters (e.g., Tall/Dwarf stem, Round/Wrinkled seeds, Violet/White flowers).
    • Naturally self-pollinating but can be easily cross-pollinated manually.

  • Key Terms:

    • Gene: A segment of DNA that codes for a specific protein, controlling a particular trait. Mendel called them 'factors'.
    • Alleles: Different forms of the same gene (e.g., 'T' for tallness and 't' for dwarfness are alleles of the gene for height).
    • Dominant Allele: An allele that expresses itself in the phenotype even in the presence of a recessive allele (usually represented by a capital letter, e.g., 'T').
    • Recessive Allele: An allele that expresses itself in the phenotype only when present in a homozygous state (both alleles are recessive) (usually represented by a small letter, e.g., 't').
    • Genotype: The genetic makeup of an individual for a particular trait (e.g., TT, Tt, tt).
    • Phenotype: The observable physical or biochemical characteristic of an individual (e.g., Tall, Dwarf).
    • Homozygous: Having two identical alleles for a trait (e.g., TT or tt). Also called pure-bred.
    • Heterozygous: Having two different alleles for a trait (e.g., Tt). Also called hybrid.
    • F1 Generation (First Filial): The first generation of offspring produced from a cross between two parental types.
    • F2 Generation (Second Filial): The generation produced by selfing (self-pollination) or interbreeding individuals of the F1 generation.

  • Monohybrid Cross: A cross involving only one pair of contrasting characters.

    • Experiment: Mendel crossed pure-bred tall (TT) pea plants with pure-bred dwarf (tt) pea plants.
    • F1 Generation: All plants were tall (Genotype Tt). This showed that 'T' (tallness) is dominant over 't' (dwarfness).
    • F2 Generation (Selfing F1): When F1 (Tt) plants were self-pollinated, the offspring were tall and dwarf in the ratio 3:1 (Phenotypic ratio). The genotypes were TT, Tt, and tt in the ratio 1:2:1 (Genotypic ratio).
    • Law of Dominance: When parents with pure contrasting traits are crossed, only one form of the trait appears in the next generation (F1). This trait is the dominant trait.
    • Law of Segregation (or Purity of Gametes): During gamete formation, the two alleles for a character separate (segregate) from each other, so that each gamete receives only one allele for that character. Alleles remain distinct and do not blend.

  • Dihybrid Cross: A cross involving two pairs of contrasting characters simultaneously.

    • Experiment: Mendel crossed pea plants with round yellow seeds (RRYY - dominant) with plants having wrinkled green seeds (rryy - recessive).
    • F1 Generation: All plants had round yellow seeds (RrYy).
    • F2 Generation (Selfing F1): When F1 (RrYy) plants were self-pollinated, four types of phenotypes appeared in the ratio 9:3:3:1.
      • 9 Round Yellow
      • 3 Round Green
      • 3 Wrinkled Yellow
      • 1 Wrinkled Green
    • Law of Independent Assortment: During inheritance of two or more characters, the alleles for each character assort (separate) independently of the alleles for other characters during gamete formation. (Note: This law applies mainly to genes located on different chromosomes or far apart on the same chromosome).

4. How are Traits Expressed? (Mechanism of Heredity)

  • DNA (Deoxyribonucleic Acid): The molecule carrying genetic information in the chromosomes found within the nucleus of a cell.
  • Gene: A specific segment of DNA.
  • Protein Synthesis: The sequence of nucleotides in a gene provides the information (code) for synthesizing a specific protein.
  • Proteins and Traits: Proteins act as enzymes, structural components, or hormones, controlling various metabolic pathways and cellular functions, which ultimately determine the phenotype or trait (e.g., a gene codes for an enzyme that produces a pigment, leading to flower colour).
  • Allelic Variation: Different alleles of a gene may produce slightly different proteins or affect the amount of protein produced, leading to variations in traits (e.g., allele 'T' might produce an efficient growth hormone leading to tallness, while allele 't' might produce a less efficient version leading to dwarfness).

5. Sex Determination

  • The process by which the sex of an individual is established.
  • Humans: Sex is determined genetically.
    • Humans have 23 pairs of chromosomes (46 total).
    • 22 pairs are autosomes (common to both sexes).
    • 1 pair is sex chromosomes (allosomes).
    • Females: Have two identical X chromosomes (XX). All eggs produced contain one X chromosome.
    • Males: Have one X and one shorter Y chromosome (XY). Sperm produced are of two types: 50% carry X and 50% carry Y.
    • Determination: If an X-carrying sperm fertilizes the egg (X), the offspring is female (XX). If a Y-carrying sperm fertilizes the egg (X), the offspring is male (XY).
    • Therefore, the father (male) is responsible for determining the sex of the child.
  • Other Organisms: Sex determination mechanisms vary (e.g., ZW system in birds, environmental factors in some reptiles like turtles).

6. Evolution

  • Definition: The gradual change in the inherited characteristics of biological populations over successive generations. It leads to the origin of new species from pre-existing ones.
  • Acquired vs. Inherited Traits:
    • Acquired Traits: Characteristics developed by an individual during its lifetime due to environmental influences or use/disuse of organs (e.g., muscle development through exercise, learning). These traits affect somatic (body) cells and are not passed on to offspring as they do not change the DNA of germ cells (sperms/eggs).
    • Inherited Traits: Characteristics determined by the genetic makeup (DNA) of an individual, passed from parents to offspring via germ cells. Evolution acts only on inherited traits.

7. Mechanisms of Evolution

  • Natural Selection: Proposed by Charles Darwin. The process by which organisms with variations better suited (adapted) to their environment survive, reproduce more successfully, and pass on these advantageous traits to their offspring. Over time, this leads to a change in the characteristics of the population. (Example: Industrial melanism in peppered moths).
  • Genetic Drift: Random changes in the frequency of alleles (gene variants) in a small population, purely by chance, rather than by natural selection. It can lead to the loss of some alleles and the fixation of others, potentially reducing genetic variation. More pronounced in small, isolated populations. (Example: Founder effect, Bottleneck effect).
  • Gene Flow: The movement of genes (alleles) between populations through migration and interbreeding. It can introduce new alleles or alter existing allele frequencies, increasing genetic variation within a population but reducing differences between populations.
  • Mutation: Sudden, random changes in the DNA sequence. Mutations are the ultimate source of new genetic variation upon which other evolutionary mechanisms can act.

8. Speciation

  • The evolutionary process by which new biological species arise.
  • Key Factor: Reproductive Isolation: The inability of individuals from different populations to interbreed and produce fertile offspring.
  • Factors Leading to Speciation:
    • Geographical Isolation: Physical separation of populations (e.g., by mountains, rivers) prevents gene flow. Over time, isolated populations may accumulate different variations due to different selection pressures, genetic drift, and mutations, eventually becoming reproductively isolated.
    • Genetic Drift & Natural Selection: Acting differently on isolated populations can lead to significant genetic divergence.
    • Changes in Chromosome Number: Can lead to instant reproductive isolation (common in plants).

9. Evidence for Evolution

  • Homologous Organs: Organs in different species that have a similar basic structure and developmental origin but may perform different functions. They indicate common ancestry. (Example: Forelimbs of humans, bats, whales, and cheetahs - same basic bone structure, different functions like grasping, flying, swimming, running).
  • Analogous Organs: Organs in different species that have different basic structures and developmental origins but perform similar functions. They indicate convergent evolution (adaptation to similar environments), not close common ancestry. (Example: Wings of birds and wings of insects - different structure, similar function of flying).
  • Fossils: Preserved remains, traces, or impressions of organisms that lived in the past.
    • Provide direct evidence of past life forms.
    • Show evolutionary transitions (e.g., Archaeopteryx - link between reptiles and birds).
    • Help reconstruct evolutionary lineages.
    • Fossil Dating:
      • Relative Dating: Determining the relative age by position in rock layers (older fossils are generally in deeper layers).
      • Absolute Dating (Radiometric Dating): Determining the actual age using the decay rate of radioactive isotopes (e.g., Carbon-14 dating for relatively recent fossils, Potassium-Argon dating for older ones).
  • Embryology: Similarities in early embryonic development among related species suggest common ancestry.
  • Biogeography: The geographical distribution of species provides clues about their evolutionary history and dispersal.
  • Molecular Evidence: Similarities and differences in DNA sequences and protein structures between species reflect their evolutionary relatedness. More closely related species have more similar DNA.

10. Evolution by Stages

  • Complex organs (like the eye) did not evolve in a single step but through a series of gradual changes over generations, each stage conferring some survival advantage. Example: Rudimentary light-sensitive spots in flatworms evolved gradually into complex eyes found in vertebrates.
  • Structures evolved for one function can later be adapted for a different function (e.g., feathers initially likely evolved for insulation or display, later adapted for flight).

11. Evolution Should Not Be Equated with 'Progress'

  • Evolution leads to increased complexity in some lineages, but also to simpler forms in others (e.g., parasites often become simpler than their free-living ancestors).
  • Evolution produces diversity adapted to specific environments, not necessarily a linear progression towards a 'higher' or 'better' form. Bacteria are simple but incredibly successful. Humans are one outcome of evolution, not the pinnacle.

12. Human Evolution

  • Humans (Homo sapiens) evolved from primate ancestors.
  • Evidence points to Africa as the origin of modern humans.
  • Key features of human evolution include bipedal locomotion, large brain size, tool use, and language.
  • All humans belong to a single species (Homo sapiens) despite variations in appearance (skin colour, hair type etc.), which are superficial adaptations to different environments. Genetic differences between human populations are very small.

Multiple Choice Questions (MCQs)

  1. Which of the following statements is INCORRECT regarding variations?
    (a) Variations are essential for the survival of species over time.

    • (b) Variations are introduced only during sexual reproduction.
      (c) Variations arise due to inaccuracies in DNA copying.
      (d) The degree of variation is higher in sexual reproduction compared to asexual reproduction.

  2. In Mendel's dihybrid cross between pea plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy), the F2 generation showed four phenotypes. What fraction of the F2 offspring were homozygous for both recessive traits (wrinkled and green)?
    (a) 1/4
    (b) 3/16

    • (c) 1/16
      (d) 9/16

  3. A cross between a tall pea plant (TT) and a short pea plant (tt) resulted in progeny that were all tall plants because:
    (a) Tallness is a recessive trait.
    (b) Shortness is a dominant trait.

    • (c) Tallness is a dominant trait.
      (d) Height of pea plant is not governed by gene ‘T’ or ‘t’.

  4. The genotype of offspring produced from a cross between a parent with genotype RrYy and another with rryy would NOT include:
    (a) RrYy
    (b) Rryy
    (c) rrYy

    • (d) RRYY

  5. Which of the following correctly represents homologous organs?
    (a) Wings of a butterfly and wings of a bat.
    (b) Flippers of a dolphin and fins of a fish.

    • (c) Forelimbs of a frog and forelimbs of a lizard.
      (d) Potato and sweet potato.

  6. In human males, all the chromosomes are paired perfectly except one. This/these unpaired chromosome is/are:
    (i) large chromosome
    (ii) small chromosome
    (iii) Y-chromosome
    (iv) X-chromosome
    (a) (i) and (ii)
    (b) (iii) only

    • (c) (iii) and (iv)
      (d) (ii) and (iv)

  7. The process where characteristics are transmitted from parent to offspring is called:
    (a) Variation

    • (b) Heredity
      (c) Speciation
      (d) Evolution

  8. Which concept explains the random change in allele frequency in a small population?
    (a) Natural Selection
    (b) Gene Flow

    • (c) Genetic Drift
      (d) Mutation

  9. Fossils are generally found in:
    (a) Igneous rocks

    • (b) Sedimentary rocks
      (c) Metamorphic rocks
      (d) Volcanic ash

  10. The theory of evolution by natural selection was proposed by:
    (a) Gregor Mendel
    (b) Jean-Baptiste Lamarck

    • (c) Charles Darwin
      (d) Hugo de Vries

Answer Key for MCQs:

  1. (b)
  2. (c)
  3. (c)
  4. (d)
  5. (c)
  6. (c) - The X and Y chromosomes are different in size and gene content, hence considered an 'unpaired' pair in a sense, although they do pair during meiosis.
  7. (b)
  8. (c)
  9. (b)
  10. (c)

Remember to correlate these notes with the specific problems given in your NCERT Exemplar book. Understanding the application of these concepts is key. Good luck with your preparation!

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