Class 12 Notes

Class 12 Biology Notes Chapter 5 (Principles of inheritance and variation) – Biology Book

Philoid

Philoid

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Biology
Alright, let's focus on a very important chapter for your exams – Chapter 5: Principles of Inheritance and Variation. This chapter forms the bedrock of genetics and frequently features in competitive exams. We'll break it down 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.

2. Mendel's Laws of Inheritance

  • Gregor Mendel (1822-1884): Known as the "Father of Genetics". Conducted hybridization experiments on garden peas (Pisum sativum) for 7 years (1856-1863).
  • Reasons for selecting Pea Plant:
    • Easy to cultivate.
    • Short life cycle (allows study of several generations quickly).
    • Has many distinct, easily observable contrasting traits (e.g., tall/dwarf stem, round/wrinkled seed).
    • Flowers are bisexual and naturally self-pollinating, but cross-pollination can be easily performed artificially.
    • Produces a large number of offspring.
  • Seven Pairs of Contrasting Traits Studied by Mendel:
    1. Stem height: Tall/Dwarf
    2. Flower colour: Violet/White
    3. Flower position: Axial/Terminal
    4. Pod shape: Inflated/Constricted
    5. Pod colour: Green/Yellow
    6. Seed shape: Round/Wrinkled
    7. Seed colour: Yellow/Green

3. Key Genetic Terms

  • Character: A heritable feature that varies among individuals (e.g., flower colour).
  • Trait: Each variant for a character (e.g., purple or white colour for flowers).
  • Gene: Mendel used the term 'factor'. It is the unit of inheritance, a sequence of DNA coding for a functional product (usually a protein).
  • Alleles: Alternative forms of a gene present on the same locus (position) on homologous chromosomes (e.g., T and t are alleles for the gene controlling height).
  • Homozygous: An organism having two identical alleles for a particular gene (e.g., TT or tt). Produces only one type of gamete.
  • Heterozygous: An organism having two different alleles for a particular gene (e.g., Tt). Produces different types of gametes.
  • Genotype: The genetic makeup of an individual for one or more traits (e.g., TT, Tt, tt).
  • Phenotype: The observable physical or physiological trait of an individual, resulting from the interaction of genotype and environment (e.g., Tall, Dwarf).
  • Dominant Allele: An allele that expresses its phenotypic effect even in the heterozygous condition (e.g., 'T' for tallness). Represented by a capital letter.
  • Recessive Allele: An allele that expresses its phenotypic effect only in the homozygous condition (e.g., 't' for dwarfness). Represented by a small letter.
  • Punnett Square: A graphical representation devised by Reginald C. Punnett to calculate the probability of all possible genotypes of offspring in a genetic cross.
  • F1 Generation (First Filial): The hybrid offspring resulting from a cross between two parental organisms (P generation).
  • F2 Generation (Second Filial): The offspring resulting from self-pollination or interbreeding of the F1 generation.

4. Mendel's Experiments and Laws

  • Monohybrid Cross: A cross involving the inheritance of a single pair of contrasting characters.

    • Example: Cross between Tall (TT) and Dwarf (tt) pea plants.
    • P: TT x tt
    • Gametes: T t
    • F1: Tt (All Tall - Phenotype; Heterozygous - Genotype)
    • Selfing F1: Tt x Tt
    • Gametes: T, t T, t
    • F2 Generation (using Punnett Square):
      • Genotypes: TT : Tt : tt = 1 : 2 : 1
      • Phenotypes: Tall : Dwarf = 3 : 1
    • Conclusions from Monohybrid Cross:
      • Characters are controlled by discrete units called factors (genes).
      • Factors occur in pairs.
      • In a dissimilar pair of factors (heterozygous), one member dominates (dominant) the other (recessive). This led to the Law of Dominance.
      • Law of Dominance: Characters are controlled by factors. Factors occur in pairs. In a heterozygous condition, one factor (allele) expresses itself (dominant) and prevents the expression of the other (recessive).
      • Alleles do not blend, and both characters are recovered as such in the F2 generation. During gamete formation, the alleles of a pair segregate from each other such that a gamete receives only one of the two factors. This is the Law of Segregation (also called the Law of Purity of Gametes). This law is universally applicable.

  • Test Cross: A cross between an individual with a dominant phenotype (unknown genotype, e.g., TT or Tt) and a homozygous recessive individual (tt) to determine the genotype of the dominant individual.

    • If the dominant phenotype individual is homozygous (TT), all F1 progeny will show the dominant phenotype (Tt). (TT x tt -> All Tt)
    • If the dominant phenotype individual is heterozygous (Tt), the F1 progeny will show dominant and recessive phenotypes in a 1:1 ratio. (Tt x tt -> 1 Tt : 1 tt)

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

    • Example: Cross between pea plants with Round Yellow seeds (RRYY) and Wrinkled Green seeds (rryy).
    • P: RRYY x rryy
    • Gametes: RY ry
    • F1: RrYy (All Round Yellow - Phenotype; Dihybrid - Genotype)
    • Selfing F1: RrYy x RrYy
    • Gametes: RY, Ry, rY, ry (from both parents)
    • F2 Generation (using 16-box Punnett Square):
      • Phenotypes: Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green = 9 : 3 : 3 : 1
      • Genotypes: Complex ratio (1:2:1:2:4:2:1:2:1)
    • Conclusion from Dihybrid Cross:
      • Led to the Law of Independent Assortment.
      • Law of Independent Assortment: 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. (This law applies to genes located on different homologous chromosomes or those far apart on the same chromosome).

5. Deviations from Mendelism (Post-Mendelian Discoveries / Non-Mendelian Inheritance)

  • Incomplete Dominance: The phenomenon where the F1 hybrid phenotype is intermediate between the two parental phenotypes. Neither allele is completely dominant.
    • Example: Flower colour in Snapdragon (Antirrhinum majus) or Four o'clock plant (Mirabilis jalapa).
    • Red (RR) x White (rr) -> F1: Pink (Rr)
    • Selfing F1 (Rr x Rr) -> F2: Red (RR) : Pink (Rr) : White (rr) = 1 : 2 : 1 (Phenotypic and Genotypic ratios are the same).
  • Co-dominance: Both alleles of a gene pair express themselves fully in the F1 hybrid. They do not blend but are expressed simultaneously.
    • Example: ABO blood groups in humans. Alleles I^A and I^B are co-dominant to each other and dominant over allele i.
      • Genotype I^A I^B results in AB blood group (both A and B sugars are expressed).
    • Example: Coat colour in cattle (Roan coat).
  • Multiple Alleles: When more than two alternative alleles exist for a particular gene in a population (though an individual diploid organism can only have two).
    • Example: ABO blood group system in humans. Three alleles: I^A, I^B, i. (Leads to 6 possible genotypes and 4 possible phenotypes: A, B, AB, O).
  • Pleiotropy: A single gene affects multiple phenotypic traits.
    • Example: Phenylketonuria (PKU) in humans. Mutation in the gene coding for the enzyme phenylalanine hydroxylase causes mental retardation, reduced hair pigmentation, and skin pigmentation.
    • Example: Starch synthesis gene in pea seeds. It controls both seed shape (Round/Wrinkled) and starch grain size (Large/Small). Starch synthesis shows incomplete dominance at the grain level.
  • Polygenic Inheritance: A trait controlled by three or more genes (multiple genes). The phenotype reflects the contribution of each allele (additive effect). Phenotypes often show a continuous range or gradient.
    • Example: Human skin colour, human height, kernel colour in wheat.

6. Chromosomal Theory of Inheritance

  • Proposed independently by Walter Sutton and Theodor Boveri (1902).
  • Noted parallels between the behaviour of Mendel's factors (genes) and the behaviour of chromosomes during meiosis and fertilization:
    • Both occur in pairs in diploid organisms.
    • Both segregate during gamete formation such that a gamete receives only one of each pair.
    • Independent pairs segregate independently of each other (parallels Law of Independent Assortment).
  • Conclusion: Genes are located on chromosomes. The segregation and independent assortment of chromosomes during meiosis explain Mendel's laws.
  • Experimental Verification: By Thomas Hunt Morgan (early 20th century) using fruit flies (Drosophila melanogaster).
    • Why Drosophila?: Short life cycle (~2 weeks), produce many offspring, easy to handle, clear differentiation of sexes, many hereditary variations observable under low power microscope, few chromosomes (2n=8).
    • Morgan discovered sex-linkage (genes located on sex chromosomes, e.g., eye colour in Drosophila).
    • Morgan also demonstrated linkage and recombination.

7. Linkage and Recombination

  • Linkage: The tendency of genes located close together on the same chromosome to be inherited together. They do not assort independently.
  • Recombination: The generation of non-parental gene combinations during meiosis due to crossing over between non-sister chromatids of homologous chromosomes.
  • Morgan found that the frequency of recombination between gene pairs on the same chromosome is a measure of the distance between the genes. Tightly linked genes show low recombination frequency, while loosely linked genes (farther apart) show higher recombination frequency.
  • Alfred Sturtevant (Morgan's student) used recombination frequencies to construct the first genetic maps (linkage maps) showing the relative positions of genes on a chromosome. Map unit = 1% recombination frequency (also called centimorgan, cM).

8. Sex Determination

  • The mechanism by which the sex of an individual is established. Usually genetically determined.

  • Sex Chromosomes: Chromosomes involved in sex determination (e.g., X and Y in humans, Z and W in birds).

  • Autosomes: Chromosomes other than sex chromosomes.

  • Mechanisms:

    • XX-XY Type: Females have homologous pair XX, Males have heteromorphic pair XY. Found in humans, Drosophila. Males are heterogametic (produce X and Y sperm).
    • XX-XO Type: Females have XX, Males have only one X (XO). Found in some insects (e.g., grasshoppers). Males are heterogametic (produce X and O sperm).
    • ZW-ZZ Type: Females have heteromorphic pair ZW, Males have homologous pair ZZ. Found in birds, reptiles, some fishes. Females are heterogametic (produce Z and W eggs).
    • Haplodiploidy: Sex determined by the number of sets of chromosomes. Females develop from fertilized eggs (diploid, 2n), Males develop from unfertilized eggs (haploid, n). Found in honeybees, ants, wasps.

  • Sex Determination in Humans: XX (Female), XY (Male). The Y chromosome carries the SRY (Sex-determining Region Y) gene, crucial for male development. The male determines the sex of the child.

9. Mutation

  • A sudden, stable, heritable change in the genetic material (DNA). It is the ultimate source of all genetic variation.
  • Mutagens: Agents that cause mutations (e.g., UV radiation, X-rays, certain chemicals).
  • Types:
    • Gene Mutations (Point Mutations): Changes in a single base pair of DNA.
      • Substitution: One base replaced by another (can be transition or transversion). Example: Sickle-cell anaemia (GAG -> GUG in beta-globin gene, Glutamic acid -> Valine).
      • Frameshift Mutations: Insertion or deletion of one or more base pairs, causing a shift in the reading frame of the genetic code from the point of mutation onwards. Usually results in a non-functional protein.
    • Chromosomal Mutations (Aberrations): Changes in the structure or number of chromosomes. Often observed in cancer cells.
      • Structural Changes: Deletion, Duplication, Inversion, Translocation.
      • Numerical Changes (Ploidy Changes):
        • Aneuploidy: Gain or loss of one or more chromosomes (not a whole set). Caused by non-disjunction of chromatids during cell division.
          • Trisomy: 2n + 1 (e.g., Down's Syndrome - Trisomy 21).
          • Monosomy: 2n - 1 (e.g., Turner's Syndrome - XO).
        • Polyploidy: Presence of more than two complete sets of chromosomes (e.g., 3n, 4n). Common in plants, rare in animals.

10. Pedigree Analysis

  • Analysis of the inheritance pattern of a particular trait in several generations of a human family. Presented as a family tree (pedigree chart).
  • Purpose: To determine the mode of inheritance (autosomal/sex-linked, dominant/recessive) of a genetic trait or disorder, and predict the risk for future generations. Control crosses are not possible in humans.
  • Standard Symbols: Square (male), Circle (female), Shaded symbol (affected individual), Horizontal line (mating), Vertical line (offspring), Diamond (sex unspecified), etc.

11. Genetic Disorders

  • Mendelian Disorders: Caused by alteration or mutation in a single gene. Follow Mendelian inheritance patterns (can be traced using pedigree analysis).

    • Autosomal Dominant: Trait appears in every generation. Affected individuals transmit to ~50% offspring. E.g., Myotonic dystrophy, Huntington's disease.

    • Autosomal Recessive: Trait appears only in homozygous state. Appears to skip generations. Affected individuals usually born to unaffected carrier parents. E.g., Sickle-cell anaemia, Phenylketonuria (PKU), Cystic fibrosis, Thalassemia.

    • Sex-linked Dominant: Affected males pass the trait to all daughters, no sons. Affected heterozygous females pass to 50% offspring of both sexes. Rare.

    • Sex-linked Recessive: More common in males. Affected males transmit the gene to all daughters (carriers), not sons. Carrier females transmit to 50% sons (affected) and 50% daughters (carriers). E.g., Haemophilia, Colour blindness.

    • Examples Explained:

      • Haemophilia: Sex-linked recessive. Defect in blood clotting factor.
      • Sickle-cell Anaemia: Autosomal recessive. Mutation in beta-globin gene (point mutation GAG -> GUG). Causes RBCs to become sickle-shaped under low oxygen tension. Heterozygotes (AS) show resistance to malaria.
      • Phenylketonuria (PKU): Autosomal recessive. Inborn error of metabolism. Lack of enzyme phenylalanine hydroxylase. Accumulation of phenylalanine leads to mental retardation.
      • Thalassemia: Autosomal recessive. Defect in synthesis of globin chains (alpha or beta) of haemoglobin. Causes anaemia. Quantitative problem (unlike sickle-cell which is qualitative).
      • Colour Blindness: Sex-linked recessive. Defect in red or green cone cells of the eye. Difficulty distinguishing red and green colours.
      • Cystic Fibrosis: Autosomal recessive. Defect in chloride ion transport. Affects lungs, pancreas, sweat glands.

  • Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of one or more chromosomes (Aneuploidy or Polyploidy).

    • 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 development retarded. Risk increases with 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, tall stature.
    • 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. In a dihybrid cross between RRYY (Round Yellow) and rryy (Wrinkled Green) pea plants, what is the phenotypic ratio of F2 generation?
    a) 1:2:1
    b) 3:1
    c) 9:3:3:1
    d) 1:1:1:1

  2. Which of the following is an example of co-dominance?
    a) Flower colour in Snapdragon
    b) ABO blood groups in humans
    c) Skin colour in humans
    d) Phenylketonuria

  3. The chromosomal theory of inheritance was experimentally verified by:
    a) Gregor Mendel
    b) Sutton and Boveri
    c) Thomas Hunt Morgan
    d) Alfred Sturtevant

  4. A cross between an F1 hybrid (dominant phenotype) and its homozygous recessive parent is called:
    a) Back cross
    b) Test cross
    c) Monohybrid cross
    d) Dihybrid cross

  5. Which genetic disorder is caused by Trisomy of chromosome 21?
    a) Klinefelter's Syndrome
    b) Turner's Syndrome
    c) Down's Syndrome
    d) Sickle-cell Anaemia

  6. Haemophilia is a:
    a) Autosomal dominant disorder
    b) Autosomal recessive disorder
    c) Sex-linked dominant disorder
    d) Sex-linked recessive disorder

  7. The phenomenon where a single gene influences multiple phenotypic expressions is known as:
    a) Polygenic inheritance
    b) Pleiotropy
    c) Multiple allelism
    d) Incomplete dominance

  8. In honeybees, males (drones) develop from unfertilized eggs. This type of sex determination is called:
    a) XX-XY type
    b) ZW-ZZ type
    c) Haplodiploidy
    d) XX-XO type

  9. A point mutation involving the change of GAG to GUG codon in the beta-globin gene results in:
    a) Thalassemia
    b) Phenylketonuria
    c) Haemophilia
    d) Sickle-cell Anaemia

  10. If a colour-blind man marries a woman who is homozygous for normal vision, the probability of their son being colour-blind is:
    a) 0%
    b) 25%
    c) 50%
    d) 100%

Answer Key for MCQs:

  1. c) 9:3:3:1
  2. b) ABO blood groups in humans
  3. c) Thomas Hunt Morgan
  4. b) Test cross (Note: A test cross is a type of back cross, but specifically with the recessive parent)
  5. c) Down's Syndrome
  6. d) Sex-linked recessive disorder
  7. b) Pleiotropy
  8. c) Haplodiploidy
  9. d) Sickle-cell Anaemia
  10. a) 0% (The son inherits the Y chromosome from the father and one X from the mother. Since the mother is homozygous normal (XX), she will pass a normal X chromosome to her son.)

Make sure you understand the concepts thoroughly, especially the laws, the exceptions, and the genetic disorders. These notes cover the core aspects needed for your exam preparation from this chapter. Good luck!

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