Class 12 Notes

Class 12 Biology Notes Chapter 5 (Principles of Inheritance and Variation) – Biology Book

Priyanka Rawat

— 11 min read

Biology

Read detailed notes for Chapter 5: Principles of Inheritance and Variation from the NCERT Class 12 Biology textbook, tailored for exam preparation.

Chapter 5: Principles of Inheritance and Variation

Introduction:
Genetics is the branch of biology dealing with inheritance and variation.

  • Inheritance: The process by which characters (traits) are passed from parent to progeny. 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. Humans knew from early times (8000-1000 B.C.) that causes of variation were hidden in sexual reproduction. They exploited variations in wild populations (plants and animals) for selective breeding and domestication (e.g., Sahiwal cows in Punjab).

1. Mendel's Laws of Inheritance:
Gregor Johann Mendel conducted hybridisation experiments on garden peas (Pisum sativum) for seven years (1856-1863) and proposed the laws of inheritance.

  • Why Pea Plants?
    • Easy to cultivate.
    • Short life cycle (annual).
    • Produces many seeds.
    • Has many distinct contrasting traits.
    • Flowers are bisexual and naturally self-pollinating, but cross-pollination can be easily performed (emasculation and bagging).
    • Hybrids were fertile.
  • Contrasting Traits Studied by Mendel (7 pairs):
    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
  • Steps in Mendel's Experiments:
    1. Selection of true-breeding parent plants (plants that consistently produce offspring with the same trait after self-pollination).
    2. Hybridisation (cross-pollination) between parents with contrasting traits to get the First Filial (F1) generation.
    3. Self-pollination of F1 plants to get the Second Filial (F2) generation.
  • Terminology:
    • Factors/Genes: Units of inheritance, passed from parents to offspring. Mendel called them 'factors'.
    • Alleles: Different forms of the same gene (e.g., T and t are alleles for the height gene). Usually represented by letters.
    • Homozygous: Having two identical alleles for a trait (e.g., TT or tt). True-breeding lines are homozygous.
    • Heterozygous: Having two different alleles for a trait (e.g., Tt).
    • Genotype: The genetic constitution of an individual for a trait (e.g., TT, Tt, tt).
    • Phenotype: The observable characteristic or trait of an individual (e.g., Tall, Dwarf). It is determined by the genotype and environmental factors.
    • Dominant Allele: The allele that expresses itself in the heterozygous condition (e.g., T in Tt). Represented by a capital letter.
    • Recessive Allele: The allele that fails to express itself in the heterozygous condition (e.g., t in Tt). It only expresses itself in the homozygous condition (tt). Represented by a small letter.
    • Punnett Square: A graphical representation developed by Reginald C. Punnett to calculate the probability of all possible genotypes of offspring in a genetic cross.

1.1 Inheritance of One Gene (Monohybrid Cross):

  • A cross involving parents differing in only one pair of contrasting characters.

  • Example: Cross between Tall (TT) and Dwarf (tt) pea plants.

    • Parents: TT (Tall) x tt (Dwarf)
    • Gametes: T t
    • F1 Generation: Tt (All Tall) - Genotype Tt, Phenotype Tall.
    • Selfing F1: Tt x Tt
    • Gametes from F1: T, t T, t
    • F2 Generation (using Punnett Square):
      T t
      T TT Tt
      t Tt tt
    • F2 Genotypic Ratio: 1 TT : 2 Tt : 1 tt (1:2:1)
    • F2 Phenotypic Ratio: 3 Tall : 1 Dwarf (3:1)

  • Mendel's Postulates based on Monohybrid Cross:

    1. Characters are controlled by discrete units called factors (genes).
    2. Factors occur in pairs.
    3. In a dissimilar pair of factors (heterozygous), one member of the pair dominates (dominant) the other (recessive). (This is the basis of the Law of Dominance).

  • Law of Dominance:

    • Characters are controlled by factors.
    • Factors occur in pairs.
    • In a heterozygous individual, only one allele (dominant) expresses itself, while the other (recessive) remains unexpressed.

  • Law of Segregation (Purity of Gametes):

    • Alleles do not blend.
    • During gamete formation (meiosis), the two alleles of a pair segregate (separate) from each other such that each gamete receives only one of the two factors.
    • Homozygous parents produce similar gametes; heterozygous parents produce two kinds of gametes, each having one allele with equal proportion. This law is universally applicable.

  • Test Cross (Monohybrid):

    • A cross between an individual with a dominant phenotype (unknown genotype, e.g., TT or Tt) and its homozygous recessive parent (tt).
    • Purpose: To determine the genotype of the individual with the dominant phenotype.
    • If the unknown is homozygous dominant (TT): TT x tt -> All offspring Tt (Tall). Phenotypic ratio 100% dominant.
    • If the unknown is heterozygous (Tt): Tt x tt -> 1 Tt (Tall) : 1 tt (Dwarf). Phenotypic ratio 1:1.

1.2 Inheritance of Two Genes (Dihybrid Cross):

  • A cross involving parents differing in two pairs of contrasting characters.

  • Example: Cross between pea plants with Round Yellow seeds (RRYY) and Wrinkled Green seeds (rryy).

    • Parents: RRYY (Round Yellow) x rryy (Wrinkled Green)
    • Gametes: RY ry
    • F1 Generation: RrYy (All Round Yellow)
    • Selfing F1: RrYy x RrYy
    • Gametes from F1 (by independent assortment): RY, Ry, rY, ry (each with 1/4 probability)
    • F2 Generation (using Punnett Square - 16 squares): Results in four phenotypes in the ratio 9:3:3:1.
      • 9 Round Yellow (R_Y_)
      • 3 Round Green (R_yy)
      • 3 Wrinkled Yellow (rrY_)
      • 1 Wrinkled Green (rryy)
    • The F2 genotypic ratio is complex (1:2:1:2:4:2:1:2:1).

  • Law of Independent Assortment:

    • Based on the dihybrid cross.
    • States that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters (alleles) is independent of the other pair of characters (alleles) during gamete formation.
    • The alleles for different traits located on different chromosomes (or far apart on the same chromosome) sort independently during meiosis.

  • Test Cross (Dihybrid):

    • Cross F1 hybrid (RrYy) with double recessive parent (rryy).
    • RrYy x rryy -> Offspring: 1 RrYy : 1 Rryy : 1 rrYy : 1 rryy (Phenotypic and Genotypic ratio 1:1:1:1).

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

  • Incomplete Dominance:

    • The F1 hybrid phenotype does not resemble either parent but is intermediate between the two.
    • 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: 1 Red (RR) : 2 Pink (Rr) : 1 White (rr)
    • Here, both phenotypic and genotypic ratios in F2 are 1:2:1.
    • The R allele is not completely dominant over r; the Rr heterozygote produces less pigment than RR, resulting in a pink phenotype.

  • Co-dominance:

    • Both alleles of a pair express themselves fully in the F1 hybrid. The F1 generation resembles both parents.
    • Example: ABO blood groups in humans.
      • Gene 'I' controls ABO blood groups. It has three alleles: IA, IB, i.
      • IA and IB produce slightly different forms of sugar antigen (A and B) on the RBC surface; allele 'i' produces no sugar.
      • IA and IB are completely dominant over i.
      • When IA and IB are present together (genotype IAIB), both express their own types of sugars. This is co-dominance, resulting in AB blood group.
    • Example: Coat colour in cattle (Roan coat). Red (RR) x White (WW) -> Roan (RW - patches of red and white hair).

  • Multiple Alleles:

    • When more than two alleles exist for a gene in a population (although an individual diploid organism can have only two alleles).
    • Example: ABO blood groups (alleles IA, IB, i).
    • Possible Genotypes and Phenotypes:
      Genotype Blood Group (Phenotype)
      IAIA or IAi A
      IBIB or IBi B
      IAIB AB
      ii O

  • Pleiotropy:

    • A single gene affects multiple phenotypic expressions (traits).
    • Example: Phenylketonuria (PKU) in humans.
      • Caused by mutation in the gene coding for the enzyme phenylalanine hydroxylase.
      • Leads to accumulation of phenylalanine.
      • Results in multiple phenotypes: mental retardation, reduction in hair, skin pigmentation.
    • Example: Starch synthesis in pea seeds.
      • Gene B controls starch synthesis and seed shape.
      • BB: Round seeds, large starch grains.
      • bb: Wrinkled seeds, small starch grains.
      • Bb: Round seeds, intermediate-sized starch grains. (Shows incomplete dominance for starch grain size).

  • Polygenic Inheritance:

    • Traits controlled by three or more genes (multiple genes).
    • The phenotype reflects the contribution of each allele (additive effect).
    • Shows continuous variation in the population, often influenced by the environment.
    • Examples: Human skin colour, human height.
    • Human skin colour: Assumed to be controlled by 3 genes (A, B, C). Dominant alleles (A, B, C) determine dark skin; recessive alleles (a, b, c) determine light skin. The phenotype depends on the total number of dominant alleles present. The distribution forms a bell-shaped curve.

3. Chromosome Theory of Inheritance:

  • Proposed independently by Walter Sutton and Theodor Boveri in 1902.
  • Noted parallels between the behaviour of Mendel's factors (genes) and the behaviour of chromosomes during meiosis and fertilisation.
  • Key Points:
    • Both genes and chromosomes occur in pairs in diploid cells.
    • Both segregate during gamete formation (meiosis) such that a gamete receives only one of each pair.
    • Pairs of genes and pairs of homologous chromosomes separate independently of other pairs (Independent Assortment applies to genes on different chromosomes).
    • Fertilisation restores the diploid condition for both chromosomes and genes.
  • Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the Chromosomal Theory of Inheritance.
  • Experimental Verification: By Thomas Hunt Morgan using fruit flies (Drosophila melanogaster).
    • Why Drosophila?
      • Short life cycle (about 2 weeks).
      • Can be grown on simple synthetic medium.
      • Single mating produces a large number of progeny.
      • Clear differentiation of sexes (male and female).
      • Many hereditary variations can be seen with low power microscopes.
      • Has only 4 pairs of chromosomes (easily studied).

4. Linkage and Recombination:

  • Morgan's experiments with Drosophila showed deviations from Mendel's Law of Independent Assortment.
  • Linkage: The physical association of genes located on the same chromosome. Linked genes tend to be inherited together.
    • Morgan found that genes for body colour (Yellow/Brown) and eye colour (White/Red) in Drosophila were sex-linked (on the X chromosome) and were often inherited together because they were located on the same chromosome.
    • The strength of linkage depends on the distance between the genes: Tightly linked genes show very low recombination; loosely linked genes show higher recombination.
  • Recombination: The generation of non-parental gene combinations during meiosis due to crossing over between homologous chromosomes.
    • Morgan observed that even linked genes showed some recombination (non-parental combinations).
    • Frequency of recombination between gene pairs on the same chromosome is a measure of the distance between the genes.
  • Alfred Sturtevant (Morgan's student) used recombination frequencies to map the positions of genes on chromosomes (genetic mapping). Units of distance are map units (m.u.) or centimorgans (cM). 1% recombination = 1 cM.

5. Sex Determination:

  • The mechanism by which the sex of an individual is established. Henking (1891) observed a specific nuclear structure ('X body') in sperm of some insects; later identified as the X chromosome.

  • Chromosomal Basis: Involves specific sex chromosomes (allosomes) which differ from autosomes.

  • Mechanisms:

    • XX-XY Type:
      • Females have homologous pair XX; Males have heteromorphic pair XY.
      • Females produce only one type of egg (with X). Males produce two types of sperm (50% with X, 50% with Y).
      • Sex of offspring determined by the sperm fertilizing the egg.
      • Found in: Humans, many insects (like Drosophila).
    • XX-XO Type:
      • Females have XX; Males have only one X chromosome (XO). O denotes absence of a sex chromosome.
      • Females produce eggs with X. Males produce two types of sperm (50% with X, 50% without any sex chromosome).
      • Found in: Many insects like grasshoppers, cockroaches.
    • ZW-ZZ Type:
      • Females are heterogametic (ZW); Males are homogametic (ZZ). (Opposite of XX-XY).
      • Females produce two types of eggs (with Z or W). Males produce only one type of sperm (with Z).
      • Sex determined by the type of egg fertilized.
      • Found in: Birds, some reptiles, some fishes.
    • Haplodiploidy:
      • Based on the number of sets of chromosomes an individual receives.
      • Females develop from fertilized eggs (diploid, 2n).
      • Males develop from unfertilized eggs (haploid, n) by parthenogenesis.
      • Found in: Honeybees, ants, wasps. Males (drones) have no father and cannot have sons, but have a grandfather and can have grandsons.

  • Sex Determination in Humans:

    • XX-XY system. 22 pairs of autosomes + XX (female) or XY (male).
    • The Y chromosome carries the SRY (Sex-determining Region Y) gene, crucial for male development.
    • The male is responsible for determining the sex of the child. There is a 50% probability of having a male or female child.

6. Mutation:

  • A sudden, stable, heritable change in the genetic material (DNA sequence).
  • Leads to variation in DNA.
  • Types:
    • Gene Mutations (Point Mutations): Change in a single base pair of DNA.
      • Example: Sickle-cell anemia. Caused by substitution of Glutamic acid by Valine at the sixth position of the beta-globin chain due to a single base change (GAG -> GUG) in the gene.
      • Can also be insertions or deletions of base pairs.
    • Frameshift Mutations: Insertion or deletion of one or two base pairs changes the reading frame of the genetic code from the point of mutation onwards, leading to a completely different protein. Insertion/deletion of three bases (or multiples of three) adds/removes codons without shifting the frame.
    • Chromosomal Aberrations (Chromosomal Mutations): Changes in the structure or number of chromosomes. Often observed in cancer cells.
      • Changes in structure:
        • Deletion: Loss of a segment of a chromosome.
        • Duplication: A segment of a chromosome is repeated.
        • Inversion: A segment breaks off, reverses orientation, and reattaches.
        • Translocation: A segment breaks off one chromosome and attaches to a non-homologous chromosome.
      • Changes in number (covered under Genetic Disorders - Aneuploidy/Polyploidy).
  • Mutagens: Agents that cause mutations.
    • Physical Mutagens: High energy radiation like UV rays, X-rays, gamma rays.
    • Chemical Mutagens: Mustard gas, acridines, nitrous acid, etc.

7. Genetic Disorders:

  • Disorders caused by alterations in genes or chromosomes.

  • Pedigree Analysis:

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

  • Mendelian Disorders:

    • Caused by alteration or mutation in a single gene.
    • Follow Mendelian inheritance patterns (can be traced using pedigree analysis).
    • Examples:
      • Autosomal Recessive: Require both alleles to be defective (homozygous recessive) for disease expression. Affects males and females equally. Skips generations.
        • Sickle-cell Anemia: Point mutation in beta-globin gene (HbS allele). RBCs become sickle-shaped under low oxygen tension, leading to anaemia, clotting. Heterozygotes (HbAHbS) are carriers, generally unaffected, but show resistance to malaria.
        • Phenylketonuria (PKU): Inborn error of metabolism. Lack of enzyme phenylalanine hydroxylase. Phenylalanine accumulates, converted to phenylpyruvic acid. Causes mental retardation.
        • Thalassemia: Quantitative defect in globin chain synthesis (alpha or beta). Causes anaemia. Alpha-thalassemia (controlled by genes HBA1, HBA2 on chr 16), Beta-thalassemia (controlled by gene HBB on chr 11). Autosomal recessive.
        • Cystic Fibrosis: Defect in CFTR gene leading to faulty chloride ion transport. Affects lungs, pancreas, sweat glands. Autosomal recessive.
      • Autosomal Dominant: Only one defective allele is sufficient to cause the disorder. Appears in every generation. Affects males and females equally.
        • Myotonic Dystrophy: Progressive muscle weakness and wasting.
        • Huntington's Disease: Neurodegenerative disorder.
      • Sex-linked Recessive (X-linked Recessive): Gene located on X chromosome. More common in males (as they have only one X). Females are usually carriers. Transmission from carrier female to son, or affected father to carrier daughter. Never father to son.
        • Haemophilia (Bleeder's disease): Defect in blood clotting factors. A minor cut can lead to non-stop bleeding. Queen Victoria was a carrier.
        • Colour Blindness: Defect in red or green cone cells in the retina. Difficulty distinguishing red and green colours. Gene on X chromosome. More common in males (~8%) than females (~0.4%).
      • Sex-linked Dominant (X-linked Dominant): Rare. Affected father transmits to all daughters, no sons. Affected heterozygous mother transmits to 50% offspring (both sexes).

  • Chromosomal Disorders:

    • Caused by absence, excess, or abnormal arrangement of one or more chromosomes.
    • Aneuploidy: Gain or loss of one or more chromosomes. Caused by failure of segregation of chromatids during cell division (non-disjunction).
      • Trisomy: Gain of one chromosome (2n+1). E.g., Down's Syndrome, Klinefelter's Syndrome.
      • Monosomy: Loss of one chromosome (2n-1). E.g., Turner's Syndrome.
    • Polyploidy: Gain of a whole set of chromosomes (e.g., 3n, 4n). More common in plants. Caused by failure of cytokinesis after telophase.
    • Examples of Human Chromosomal Disorders:
      • Down's Syndrome (Trisomy 21): Extra copy of chromosome 21 (Total 47 chromosomes). Described by Langdon Down (1866).
        • Symptoms: Short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic palm crease (simian crease), physical, psychomotor and mental retardation, congenital heart disease. Increased risk with higher maternal age.
      • Klinefelter's Syndrome (XXY): Extra X chromosome in males (Total 47 chromosomes: 44 + XXY).
        • Symptoms: Overall masculine development, but feminine characteristics develop (gynaecomastia - development of breasts), poor beard growth, often sterile, tall stature.
      • Turner's Syndrome (XO): Absence of one X chromosome in females (Total 45 chromosomes: 44 + XO).
        • Symptoms: Females are sterile as ovaries are rudimentary, lack of other secondary sexual characters, short stature, webbed neck.

This covers the core concepts of Chapter 5 as per the NCERT Class 12 Biology textbook. For government exams, focus on understanding the principles, definitions, specific examples (especially diseases, their causes, and inheritance patterns), and the scientists associated with key discoveries. Remember to practice Punnett squares and pedigree analysis problems.

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