Class 12 Notes

Class 12 Biology Notes Chapter 5 (Principle of Inheritance and Variation) – Examplar Problems Book

Philoid

Philoid

— 9 min read

Examplar Problems
Alright class, let's dive deep into Chapter 5: Principles of Inheritance and Variation. This is a cornerstone chapter for understanding genetics, a topic frequently tested in various government exams. We'll break down the concepts systematically.

Principles of Inheritance and Variation: Detailed Notes

1. Introduction:

  • Genetics: The branch of biology dealing with inheritance and variation of characters from parents to offspring.
  • Inheritance: The process by which characters (traits) are passed from one generation to the next. It is the basis of heredity.
  • Variation: The degree by which progeny differ from their parents and among themselves. Variation can be in morphology, physiology, cytology, behaviour, etc. It occurs due to recombination, mutation, and environmental effects.
  • Gregor Mendel (1822-1884): Known as the 'Father of Genetics' for his pioneering work on inheritance using garden peas (Pisum sativum).

2. Mendel's Laws of Inheritance:
Mendel conducted hybridization experiments on garden peas for seven years (1856-1863).

  • Why Pea Plant? Easy to cultivate, short life cycle, produces many offspring, has many contrasting traits, self-pollination possible, cross-pollination easy to perform.

  • Contrasting Traits Studied: Mendel selected 7 pairs of contrasting traits (e.g., Stem height: Tall/Dwarf; Seed shape: Round/Wrinkled; Seed colour: Yellow/Green).

  • Terminology:

    • Gene: Unit of inheritance, segment of DNA. Mendel called them 'factors'.
    • Alleles: Alternative forms of a gene (e.g., T and t for height). Usually two alleles for each gene.
    • Homozygous: Having two identical alleles for a trait (e.g., TT or tt).
    • Heterozygous: Having two different alleles for a trait (e.g., Tt).
    • Genotype: The genetic makeup of an individual (e.g., TT, Tt, tt).
    • Phenotype: The observable physical or biochemical characteristic of an individual (e.g., Tall, Dwarf). Determined by genotype and environment.
    • Dominant Allele: An allele that expresses itself in both homozygous (TT) and heterozygous (Tt) conditions.
    • Recessive Allele: An allele that expresses itself only in the homozygous condition (tt).
    • Monohybrid Cross: A cross involving inheritance of only one pair of contrasting characters (e.g., Tall x Dwarf).
    • Dihybrid Cross: A cross involving inheritance of two pairs of contrasting characters (e.g., Round Yellow x Wrinkled Green).
    • Punnett Square: A graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross. Developed by Reginald C. Punnett.
    • F1 Generation: First filial generation (offspring from the initial cross of parents).
    • F2 Generation: Second filial generation (offspring from selfing or intercrossing F1 individuals).

  • Mendel's Laws:

    • Law of Dominance:
      • Characters are controlled by discrete units called factors (genes).
      • Factors occur in pairs.
      • In a dissimilar pair of factors (heterozygous condition), one member of the pair dominates (dominant) the other (recessive).
      • Example: In a cross between Tall (TT) and Dwarf (tt) pea plants, the F1 generation (Tt) is phenotypically Tall because T is dominant over t.
      • Monohybrid Ratio (F2): Phenotypic ratio = 3 (Tall) : 1 (Dwarf); Genotypic ratio = 1 (TT) : 2 (Tt) : 1 (tt).
    • Law of Segregation (Purity of Gametes):
      • During gamete formation (meiosis), the two alleles of a pair segregate or separate from each other such that each gamete receives only one of the two factors.
      • Alleles do not blend and both characters are recovered as such in the F2 generation. Homozygous parents produce similar gametes; heterozygous parents produce different gametes in equal proportion.
      • This law is universally applicable.
    • Law of Independent Assortment:
      • Based on dihybrid crosses.
      • When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters during gamete formation.
      • Genes for different traits assort independently if they are located on different chromosomes or are far apart on the same chromosome.
      • Dihybrid Ratio (F2): Phenotypic ratio = 9 (Round Yellow) : 3 (Round Green) : 3 (Wrinkled Yellow) : 1 (Wrinkled Green).

3. Deviations from Mendelism (Post-Mendelian Discoveries):

  • Incomplete Dominance:
    • The F1 hybrid phenotype does not resemble either of the two parents and is intermediate between the two.
    • Example: Flower colour in Snapdragon (Antirrhinum sp.) or Four o'clock plant (Mirabilis jalapa). Red (RR) x White (rr) -> Pink (Rr) in F1.
    • F2 Ratio: Phenotypic and Genotypic ratios are the same = 1 (Red) : 2 (Pink) : 1 (White) or 1(RR):2(Rr):1(rr).
  • Co-dominance:
    • Both alleles of a gene express themselves fully in the F1 hybrid. The F1 generation resembles both parents.
    • Example: Human ABO blood groups. Alleles IA and IB are co-dominant (produce A and B antigens respectively) and both are dominant over allele i (produces no antigen). A person with genotype IAIB has AB blood group.
    • Example: Roan coat colour in cattle (patches of red and white hair).
  • Multiple Alleles:
    • More than two alleles govern the same character. Found only in a population, an individual can only have two alleles.
    • Example: Human ABO blood groups (alleles IA, IB, i).
  • Pleiotropy:
    • A single gene exhibits multiple phenotypic effects.
    • Example: Phenylketonuria (PKU) in humans. Mutation in the gene coding for enzyme phenylalanine hydroxylase causes mental retardation, reduction in hair, and skin pigmentation.
    • Example: Sickle-cell anaemia gene (HbS) causes sickle-shaped RBCs and also confers resistance to malaria in heterozygotes.
  • Polygenic Inheritance:
    • Traits controlled by three or more genes. The phenotype reflects the contribution of each allele (additive effect).
    • Results in continuous variation in phenotypes.
    • Example: Human skin colour, human height.

4. Chromosome Theory of Inheritance:

  • Proposed by Walter Sutton and Theodor Boveri (1902).
  • They noted parallels between the behaviour of Mendel's factors (genes) and the behaviour of chromosomes during meiosis and fertilization.
    • Both genes and chromosomes occur in pairs in diploid cells.
    • Both segregate during gamete formation such that a gamete receives only one of each pair.
    • Pairs segregate independently of other pairs (independent assortment).
  • Conclusion: Genes are located on chromosomes.
  • Experimental Verification: By Thomas Hunt Morgan using fruit flies (Drosophila melanogaster).
    • Why Drosophila? Short generation time (~2 weeks), produce many progeny, clear differentiation of sexes, many hereditary variations observable under low power microscope, can be grown on simple synthetic medium.

5. Linkage and Recombination:

  • Linkage: Physical association of genes located on the same chromosome. These genes tend to be inherited together. The strength of linkage depends on the distance between the genes.
  • Recombination: Generation of non-parental gene combinations during meiosis due to crossing over between non-sister chromatids of homologous chromosomes.
  • Morgan's Experiments:
    • Crossed yellow-bodied, white-eyed females (yyww) with brown-bodied, red-eyed males (y+y+w+w+ - wild type).
    • Observed that the F2 ratio deviated significantly from the Mendelian 9:3:3:1 ratio.
    • Concluded that the genes for body colour and eye colour were linked on the X-chromosome.
    • Found that even linked genes showed some recombination (non-parental combinations).
    • Tightly linked genes: Show low recombination frequency (e.g., yellow body & white eye in Drosophila - 1.3% recombination).
    • Loosely linked genes: Show high recombination frequency (e.g., white eye & miniature wing in Drosophila - 37.2% recombination).
  • Genetic Maps: Alfred Sturtevant (Morgan's student) used recombination frequencies between gene pairs on the same chromosome to measure the distance between genes and map their position on the chromosome. 1 map unit (m.u.) or 1 centiMorgan (cM) = 1% recombination frequency.

6. Sex Determination:

  • The mechanism by which the sex of an individual is established. Often involves sex chromosomes (allosomes) which differ from autosomes.
  • Mechanisms:
    • XX-XY Type: (Humans, Drosophila) Females have XX, Males have XY. Males are heterogametic (produce X and Y sperm). Sex determined by the sperm fertilizing the egg.
    • XX-XO Type: (Grasshopper, some insects) Females have XX, Males have XO (only one X chromosome). Males are heterogametic (produce X and O sperm).
    • ZZ-ZW Type: (Birds, some fishes, reptiles) Females have ZW, Males have ZZ. Females are heterogametic (produce Z and W eggs). Sex determined by the egg.
    • Haplo-diploidy: (Honeybees, ants, wasps) Females develop from fertilized eggs (diploid, 2n), Males (drones) develop from unfertilized eggs (haploid, n) via parthenogenesis. Males produce sperm by mitosis.
  • Sex Determination in Humans: Determined by the Y chromosome. Presence of the SRY (Sex-determining Region Y) gene on the Y chromosome initiates male development.

7. Mutation:

  • A sudden, stable, heritable change in the genetic material (DNA sequence).
  • Source of new alleles and hence variation.
  • Mutagens: Agents that cause mutations (e.g., UV radiation, X-rays, certain chemicals).
  • Types:
    • Gene Mutations (Point Mutations): Change in a single base pair of DNA.
      • Substitution: One base replaced by another (can be transition or transversion).
      • Effect: May result in changed amino acid (missense), stop codon (nonsense), or no change (silent).
      • Example: Sickle-cell anaemia (GAG -> GUG in beta-globin gene, Glutamic acid -> Valine).
      • Frameshift Mutations: Insertion or deletion of one or more base pairs, changing the reading frame from the point of mutation onwards. Often results in non-functional proteins.
    • Chromosomal Mutations (Aberrations): Change in chromosome structure or number.
      • Structural Changes: Deletion (loss of segment), Duplication (segment repeated), Inversion (segment reversed), Translocation (segment attached to non-homologous chromosome).
      • Numerical Changes:
        • Aneuploidy: Gain or loss of one or more chromosomes. Caused by non-disjunction of chromatids during meiosis.
          • Monosomy: 2n - 1 (e.g., Turner's Syndrome: 44+XO).
          • Trisomy: 2n + 1 (e.g., Down's Syndrome: Trisomy 21; Klinefelter's Syndrome: 44+XXY).
        • Polyploidy: Increase in a whole set of chromosomes (e.g., 3n - triploidy, 4n - tetraploidy). Common in plants, rare in animals. Often leads to larger size.

8. Pedigree Analysis:

  • Analysis of inheritance of a particular trait over several generations in a human family.
  • Uses standard symbols to represent individuals and relationships.
  • Helps determine the mode of inheritance (autosomal/sex-linked, dominant/recessive).
  • Useful in genetic counselling to predict the risk of genetic disorders in future offspring.
  • Common Symbols: Square (male), Circle (female), Shaded symbol (affected individual), Horizontal line (mating), Vertical line (offspring), Diamond (sex unspecified), Double line (consanguineous mating).

9. Genetic Disorders:

  • Mendelian Disorders: Caused by alteration or mutation in a single gene. Follow Mendelian inheritance patterns. Can be traced using pedigree analysis.
    • Autosomal Recessive: (Manifests only when homozygous recessive)
      • Sickle-cell Anaemia: Defect in beta-globin gene (point mutation). HbS allele. Heterozygotes (carriers) resistant to malaria.
      • Phenylketonuria (PKU): Inborn error of metabolism. Lack of enzyme phenylalanine hydroxylase. Accumulation of phenylalanine causes mental retardation.
      • Thalassemia: Defect in synthesis of globin chains (alpha or beta) of haemoglobin. Quantitative problem. Causes anaemia. Alpha-thalassemia (controlled by genes HBA1, HBA2 on Chr 16), Beta-thalassemia (controlled by gene HBB on Chr 11).
      • Cystic Fibrosis: Defect in CFTR gene. Affects mucus production.
    • Autosomal Dominant: (Manifests even in heterozygous condition)
      • Huntington's Disease: Neurodegenerative disorder.
      • Myotonic Dystrophy: Muscle wasting disorder.
    • Sex-linked Recessive: (Usually affects males more; transmitted from carrier female to male progeny)
      • Haemophilia: Defect in blood clotting factors. Sex-linked recessive (gene on X chromosome).
      • Colour Blindness: Defect in red or green cone cells in retina. Sex-linked recessive (gene on X chromosome).
    • Sex-linked Dominant: (Less common, affects females more often).
  • Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of one or more chromosomes (Aneuploidy or Aberrations).
    • Down's Syndrome: Trisomy of chromosome 21 (47 chromosomes, XX or XY, +21). Symptoms: Short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic palm crease (simian crease), physical, psychomotor, and mental retardation. Increased risk with higher maternal age.
    • Klinefelter's Syndrome: Trisomy of sex chromosomes (47, XXY). Individuals are male but have overall masculine development with some feminine characteristics (gynaecomastia - development of breasts), sterile, mentally retarded sometimes.
    • Turner's Syndrome: Monosomy of sex chromosome (45, XO). Individuals are female, sterile (rudimentary ovaries), short stature, webbed neck, lack of secondary sexual characters.

Multiple Choice Questions (MCQs):

  1. Which of the following principles was NOT proposed directly by Mendel based on his pea plant experiments?
    a) Law of Dominance
    b) Law of Segregation
    c) Law of Independent Assortment
    d) Linkage

  2. In Snapdragon (Antirrhinum), a cross between true-breeding red-flowered (RR) and true-breeding white-flowered (rr) plants resulted in F1 progeny with pink flowers (Rr). If these F1 plants are self-pollinated, what will be the phenotypic ratio in the F2 generation?
    a) 3 Red : 1 White
    b) 1 Red : 2 Pink : 1 White
    c) 1 Red : 1 Pink : 1 White
    d) All Pink

  3. A man with blood group 'A' marries a woman with blood group 'B'. What are all the possible blood groups of their offspring?
    a) A and B only
    b) A, B and AB only
    c) AB only
    d) A, B, AB and O

  4. The phenomenon where a single gene influences multiple phenotypic traits is called:
    a) Polygenic inheritance
    b) Multiple allelism
    c) Pleiotropy
    d) Co-dominance

  5. The experimental verification of the Chromosomal Theory of Inheritance was done by:
    a) Gregor Mendel
    b) Sutton and Boveri
    c) Thomas Hunt Morgan
    d) Hugo de Vries

  6. In honeybees, females are diploid and males are haploid. This system of sex determination is called:
    a) XX-XY system
    b) ZZ-ZW system
    c) Haplo-diploidy
    d) Genic balance system

  7. Sickle-cell anaemia is an example of:
    a) Point mutation
    b) Frameshift mutation
    c) Chromosomal aberration
    d) Aneuploidy

  8. Which of the following genetic disorders is caused by trisomy of chromosome 21?
    a) Klinefelter's Syndrome
    b) Turner's Syndrome
    c) Down's Syndrome
    d) Haemophilia

  9. A cross between two tall pea plants resulted in offspring having few dwarf plants. What would be the genotypes of both the parents?
    a) TT and Tt
    b) Tt and Tt
    c) TT and TT
    d) Tt and tt

  10. The frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes was explained by:
    a) Gregor Mendel
    b) Alfred Sturtevant
    c) Walter Sutton
    d) Theodor Boveri

Answer Key for MCQs:

  1. d) Linkage (Linkage is a deviation/extension discovered later, primarily by Morgan)
  2. b) 1 Red : 2 Pink : 1 White (Classic incomplete dominance ratio)
  3. d) A, B, AB and O (Man could be IAIA or IAi; Woman could be IBIB or IBi. If both are heterozygous (IAi x IBi), all four blood groups are possible)
  4. c) Pleiotropy
  5. c) Thomas Hunt Morgan (using Drosophila)
  6. c) Haplo-diploidy
  7. a) Point mutation (Specifically, a substitution: GAG -> GUG)
  8. c) Down's Syndrome
  9. b) Tt and Tt (Only heterozygous parents can produce recessive offspring)
  10. b) Alfred Sturtevant (Morgan's student, used recombination frequency for mapping)

Remember to correlate these concepts with diagrams like Punnett squares and pedigree charts from your NCERT book for better visualization and understanding. Keep revising these fundamental principles! Good luck with your preparation.

Read more