Topics to be Learn : 

  • Chromosomes and Mechanism of inheritance
  • Genetic Terminology
  • Mendel's Laws of Inheritance
  • Back Cross and Test Cross
  • Deviations from Mendel's findings
  • Chromosomal Theory of Inheritance
  • Chromosomes
  • Linkage and Crossing Over
  • Autosomal Inheritance
  • Sex Linked Inheritance
  • Sex Determination
  • Genetic Disorders

Genetics and Inheritance:Heredity or inheritance is the transmission of genetic information from generation to generation.

Gregor Mendel:

  • Born in Moravia in 1822.
  • Provided the first accurate explanation for the mechanism of inheritance using the hybridization technique.
Carl Correns:
  • Contemporary German botanist.
  • Independently discovered principles of heredity and verified Mendel's work through experiments on other model organisms.
  • Did not propose fundamental laws of heredity, which were established by Mendel.

Mendel's Contributions: Coined the term factors, now known as genes.

Reasons for Mendel's Success:

  • Carefully planned experiments.
  • Conducted studies with a large sample size and kept meticulous recordings, yielding accurate ratios.
  • Selected contrasting characters that were easily recognizable.
  • Each character controlled by a single factor.
  • Discovered the concepts of dominance and recessiveness among pairs of characters.
  • Seven pairs of contrasting characters of pea plant studied by Mendel were 
  •  
  •  Genetic Terminology:
    1. Character: Specific feature an organism possesses.
    2. Trait: Inherited character with variant form, e.g., tall or dwarf.
    3. Factor: Unit of heredity responsible for inheritance and expression of a genetic character.
    4. Gene: Specific DNA segment responsible for inheritance and expression of a character.
    5. Alleles or Allelomorphs: Alternative forms of a gene present on homologous chromosomes.
    6. Dominant: Allele expressing its trait even in presence of an alternative allele.
    7. Recessive: Allele not expressed in presence of an alternative allele; expressed only in homozygous condition.
    8. Phenotype: External appearance of an organism for a trait, e.g., tallness or dwarfness.
    9. Phenotypic ratio: Ratio of phenotypes in offspring, e.g., 3 Tall : 1 Dwarf.
    10. Genotype: Genetic constitution of an organism, e.g., TT, Tt, tt.
    11. Genotypic ratio: Ratio of genotypes in offspring, e.g., 1 TT : 2 Tt : 1 tt.
    12. Homozygous (pure): Identical alleles for a trait, producing only one type of gametes.
    13. Heterozygous: Pairs of contrasting alleles for a trait, producing two types of gametes.
    14. Pure line: Individual or population homozygous for one or more traits.
    15. F1 generation (First filial generation): Progeny of pure parents with contrasting characters.
    16. F2 generation: Progeny of self-breeding F1 organisms.
    17. Punnett square/checker board: Probability table representing combinations of fertilization between gametes.
    18. Homologous Chromosomes: Identical chromosomes morphologically, genetically, and structurally similar.

  • Back cross: Cross of F1 progeny with one of the parents from which they were derived.

    Graphical representation of back cross : 

    F1 crossed back with its dominant parent :
    •  
      F2 offspring :
       
      Genotypic Ratio: 1:1 Tt to Tt 

  • ------------------------------------------------------------------------------------------------------------- 

    Test Cross: Cross of F1 hybrid with homozygous recessive parent.
    Purpose: Determine if an individual is homozygous (pure) or heterozygous (hybrid).
    Graphical representation of test cross :

     

    F1 crossed back with its recessive parent : 

    F2 Generation :
  • Phenotypic Ratio: 1:1 Tall (Tt) to Dwarf (tt)
  • Genotypic Ratio: 1:1 Tt to tt
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Monohybrid Cross: Cross considering one trait during inheritance.
  • Cross Type: Cross between parents differing in one heritable trait.
  • Parents (P1): TT x tt,                   Gametes of P1: T and t
  • F1 generation: Tt (Heterozygous)
    • On selfing F1: Tt x Tt
  • Gametes of F1: T and t

F2 generation:

 

  • Phenotypic Ratio: 3 Tall : 1 Dwarf
  • Genotypic Ratio: 1 TT : 2 Tt : 1 tt
--------------------------------------------------------------------------------------------------------------  
Dihybrid Cross: Cross considering two traits during inheritance.
    • Cross Type: Cross between parents differing in two heritable traits.
    • Parents (P1) : RRYY x rryy, Gametes of P1 : RY and ry
    • F1 generation : RrYy(Yellow round)
      • On selfing F1 : RrYy x RrYy
    • Gametes of F1 : RY, Ry, rY, ry
    • F2 generation :
      
     
    • Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.
    • Genotypic ratio : 1:2:1:2:4:2:1:2:1
    Mendel's Laws of Inheritance:
  1. Law of Dominance:
  2. Definition: When two homozygous individuals with contrasting characters are crossed, dominant alleles appear in F1, while recessive alleles do not.
  3. Law of Segregation (or Law of Purity of Gametes):
  4. Definition: Alleles segregate during gamete formation in F1 hybrids, resulting in pure gametes carrying only one allele each.
  5. Law of Independent Assortment:
  6. Definition: Alleles of different gene pairs segregate independently during gamete formation in hybrids possessing two or more pairs of contrasting factors.
Deviations from Mendel's Findings:
  • Mendel's Generalizations:
  • Single trait due to single gene with two alleles.
  • Two alleles interact, one being completely dominant.
  • Genes for different traits present on different chromosomes, showing independent assortment.
Deviation Types (Neo-Mendelism):
  • Phenomena of Co-dominance and Incomplete Dominance:Single trait with two alleles showing interactions.
  • Multiple Allelism:Single gene with more than two alleles.
  • Polygenic Inheritance:Single trait influenced by multiple genes, showing interactions (epistatic or additive effects).
  • Pleiotropy:Single gene influencing many traits.
Gene Interactions:Modification or influence of phenotypic expression of a gene by another gene.

  • Types:

    • Intragenic Interactions:
    • Interactions between alleles of the same gene.
    • Examples: Incomplete dominance, co-dominance, multiple allele series.
    • Intergenic (Non-allelic) Interactions:
    • Interactions between alleles of different genes on the same or different chromosomes.
    • Examples: Pleiotropy, polygenes, supplementary and complementary genes.
Incomplete Dominance:
  • Definition: Both alleles express partially; neither completely suppresses the other.
  • Characteristics: Intermediate expression in the F1 hybrid.
  • Example: Flower color of Mirabilis jalapa.
  • Cross: Red-flowered (RR) plant x White-flowered (rr) plant.
  • F1 Offspring: Pink (Rr) flowers.
  • Genotypic Ratio: 1 RR : 2 Rr : 1 rr.
  • Phenotypic Ratio: 1 Red : 2 Pink : 1 White.
Co-dominance:
  • Both alleles express equally in hybrids.
  • Example: Coat color in cattle.
Multiple Alleles:
  • Definition: More than two alleles of a gene occupying the same locus on a chromosome or its homologue.
  • Formation: Result from mutation of the wild and original gene, leading to a series of alleles.
  • Allele Types: Show dominance, co-dominance, or incomplete dominance; the most dominant is the wild type.
  • Examples:
    • Drosophila Wing Size: From normal wings to vestigial wings (vg) in homozygous condition.
    • Blood Groups in Humans: A, B, O blood groups showcase multiple alleles.
Pleiotropy:
  • Definition: Single gene controlling two or more different traits.
  • Phenomenon: Also known as pleiotropism.
  • Phenotypic Ratio: Deviates to 1:2 instead of 3:1 due to death of recessive homozygote.
  • Example: Sickle-cell anemia caused by gene Hbs.
  • Genetic Makeup:
  • Dominant gene: HbA (normal).
  • Heterozygotes: Carriers (HbA/Hbs), exhibit mild anemia.
  • Homozygotes: With recessive gene Hbs, suffer severe anemia, often fatal.
  • Impact: Lethal in homozygous condition, manifests sickle-cell trait in heterozygous carriers.
  • Offspring: Marriage between carriers results in normal carriers and sickle-cell anemic children in 1:2:1 ratio.

  • Chromosomal Theory of Inheritance:

    • Rediscovery of Mendel's Work: Hugo de Vries, Correns, and von Tschermak independently rediscovered Mendel's work in 1900.
    • Chromosomal Theory Pioneers: Walter Sutton and Theodore Boveri (1903) formulated the chromosomal theory of inheritance after studying meiotic chromosome behavior.
Chromosomes: 
  • Carriers of heredity composed of DNA, histone, and non-histone proteins.
  • Coined Term: 'Chromosome' coined by W. Waldeyer in 1888.
    Specificity: Chromosome number is species-specific.

Ploidy:

  • Definition: Degree of repetition of the primary basic number of chromosomes ('x') in a cell.
  • Euploidy: Condition where chromosome number is an exact multiple of the primary basic number. Types: Monoploid/haploid (n), diploids (2n), triploids (3n), tetraploid (4n), etc.
  • Aneuploidy: Condition where chromosome number shows addition or deletion by one or more.  

  • Structure of Chromosome:

    1. Chromatids:
    • Two chromatid arms joined at the centromere.
    • Chromatid arms bear long, slender, highly coiled DNA thread called chromonema.
    2. Centromere:
    • Primary constriction with kinetochore.
    • Site where spindle fibers attach during cell division.
    3. Secondary Constriction:
    • Secondary constriction I: Organizes nucleolus during interphase.
    • Satellite Body (SAT Body) attached at secondary constriction II.
    4. Telomeres: Ends of chromosomes.

      • Types of Chromosomes Based on Centromere Position:

      1. Metacentric: Structure: Centromere at the midpoint. 

      Pattern: Resembles the letter 'V'. Arm: Arms are equal in length.

      2. Sub-metacentric: Structure: Centromere near the midpoint.

      Pattern: Resembles the letter 'L'. Arm: One arm slightly shorter than the other.

      •  

      3. Acrocentric: Structure: Centromere near one end.

      Pattern: Resembles the letter 'J'. Arm: One arm much smaller than the other.

        •  

      4. Telocentric: Structure: Centromere at one end.

      Pattern: Resembles the letter 'I'. Arm: Consists of only one arm.

        •  

      Types of Chromosomes Based on Function:

    1. Homologous Chromosomes:Similar in shape and organization.
    2. Heterologous Chromosomes: Dissimilar chromosomal pairs.
    3. Sex Chromosomes (Allosomes): Determine the sex of sexually reproducing organisms.
    4. Somatic Chromosomes (Autosomes): Determine body characteristics other than sex.
    Linkage and Crossing Over:
    Linkage: Tendency of two or more genes to be inherited together.
    Discovery: Bateson and Punnett in plants; H. Morgan in animals.
  • Types of Linkage:
  • Complete Linkage:
  • Example: X chromosome in Drosophila males.
  • Incomplete Linkage:
  • Example: Color and shape of Zea mays grain.
  • Linkage Groups:
  • All linked genes in a chromosome.
  • Number equals species' haploid chromosome count.
  • Sex-Linkage:
  • Inheritance of X-linked and Y-linked genes.
  • Types: X-linked, Y-linked, XY-linked.
  • Types of Sex Linkage:
  • Complete Sex Linkage:
  • Genes on non-homologous X and Y chromosome regions.
  • Examples: Haemophilia, red-green color blindness.
  • Incomplete Sex Linkage:
  • Genes on homologous X and Y chromosome regions.
  • Examples: Total color blindness, nephritis.

  1. Crossing Over: Formation of gene re-combinations by exchanging segments of non-sister chromatids of homologous chromosomes.

    • Occurrence: Takes place during pachytene of prophase I of meiosis.Term Origin: Coined by Morgan.
    • Mechanism:
    1. Synapsis: Pairing of homologous chromosomes.
    2. Tetrad Formation: Formation of four-chromatid structure.
    3. Crossing Over: Exchange of genetic material between chromatids.
    4. Terminalization: Separation of homologous chromosomes.
    • Significance: Generates variations essential for natural selection and evolution.

  2.  

    Morgan's Experiments on Linkage and Crossing Over:

    Experimental Setup: Organism: Drosophila melanogaster.
    Advantages: Easily cultured in lab, Short lifespan (~two weeks), High reproduction rate.

Morgan's Findings:

  • Principle of Linkage:
  • Genes on the same chromosome are strongly linked.
  • Re-combinations among them are minimal, approximately 1.3%.
  • Sex Linkage and Crossing Over:
  • Genes located far apart on a chromosome are loosely linked.
  • Shows higher re-combinations, around 37.2%.

  1. Autosomal Inheritance:

  • Human somatic cells: 23 pairs of chromosomes (2n).
  • Autosomes: 22 pairs, determining bodily characteristics.
  • Sex chromosomes: 1 pair, determining sex.

Autosomal Inheritance Types:

  • Dominant Traits: Examples: Widow's peak, Huntington's disease.
  • Recessive Traits: Examples: Phenylketonuria (PKU), Cystic fibrosis, Sickle-cell anemia.
  • Phenylketonuria (PKU):
  • Definition: Autosomal recessive inborn error.
  • Cause: Deficiency of phenylalanine hydroxylase (PAH) enzyme.
  • Effects: Accumulation of phenylalanine leads to toxic compound production, causing mental retardation.
  • Gene Location: Chromosome 12 (PAH gene).
  • Treatment: Early detection prevents further abnormalities.
  • Widow’s Peak:
  • Description: Prominent 'V' shaped hairline on forehead.
    Mode of Inheritance: Autosomal dominant.
    Genotypes and Expression:
  • Homozygous dominant (WW) and heterozygous (Ww) individuals have widow's peak.
    Homozygous recessive (ww) individuals lack widow's peak.
  • Inheritance: Equal chance in both males and females.
  • Sex-Linked Inheritance: Genes present on non-homologous regions of sex chromosomes.
    Types of Sex-Linked Genes:
  • X-Linked Genes:
    • Located on non-homologous region of X chromosome.
    • Examples: Haemophilia, color blindness, night blindness, myopia, muscular dystrophy.
  • Y-Linked Genes (Holandric Genes):
    • Located on non-homologous region of Y chromosome.
    • Example: Hypertrichosis.
  • X-Y-Linked Genes:
    • Located on homologous region of X and Y chromosomes.
    • Also known as incompletely sex-linked genes.
    • Examples: Total color blindness, nephritis, retinitis pigmentosa.

Sex-Linked Inheritance (Color Blindness):

    • Red-Green Color Blindness:
    • Type: X-linked recessive disorder.
    • Characteristics: Inability to distinguish red and green colors.

    Genotypes of male and female individuals for colour blindness are as follows :

    Sex linked inheritance (colour blindness) :

     Haemophilia (Bleeder’s Disease):
      • Type: X-linked recessive disorder.
      • Cause: Lack of clotting factors (VIII or IX) in blood.
      • Effect: Impaired blood clotting process.
      • Inheritance: Passed down through the X chromosome.
      Genotypes of male and female individuals for haemophilia are as follows :
    •  

    Remember : XhXh combination is lethal, such females do not survive.

    Sex Determination: Mechanism establishing sexual phenotype.

      • Sex Types:
        • Bisexual or Hermaphrodite or Monoecious: Both sexes' organs exist in the same body.
        • Dioecious or Unisexual: Organism has either male or female reproductive organs.

      X-Body:

    • Discovery: German biologist Henking in 1891.
    • Study Subject: Spermatogenesis of squash bug (Anasa tristis).
    • Observation: 50% of sperms received unpaired chromosomes.
    • Naming: Termed structure as "X-body."
    • Clarification: Further research revealed X-body was a chromosome, leading to the name "X-Chromosome."
      • Sex Determination in Humans: Chromosomal Mechanism: XX-XY type.
      • Male:
      46 chromosomes: 44 autosomes + XY sex chromosomes.
      Heteromorphic due to two different sex chromosomes.
      • Female:
      46 chromosomes: 44 autosomes + XX sex chromosomes.
      Homomorphic due to similar sex chromosomes.
      • Gamete Formation:
      Diploid germ cells undergo meiosis to produce haploid gametes.
      Homologous chromosomes separate into different gametes.
      • Male Gametes:
      Produce two types of sperm: X and Y.
      Heterogametic: Produces different types of gametes.
      • Female Gametes:
      Produce only one type of egg: X.
      Homogametic: Produces identical gametes.
      • Fertilization:
      X-containing sperm + X egg = Female (XX).
      Y-containing sperm + X egg = Male (XY).
      • Sex Determination: Heterogametic parent (father) determines the child's sex.

      •  

    Sex Determination in Birds:
      • Mechanism: ZW-ZZ mechanism.
      • Males: Homogametic, Produce one type of sperm, each carrying a Z chromosome.
      • Females: Heterogametic, Produce two types of eggs: 50% Z-bearing eggs, 50% W-bearing eggs.
      • Sex Determination:
      • Offspring's sex depends on the type of egg fertilized by the sperm.
      • Fertilization of Z-bearing egg results in a male offspring.
      • Fertilization of W-bearing egg results in a female offspring.
        •   
    Sex Determination in Honey Bees:
    • Mechanism: Haplodiploid system.
    • Determination:
      • Sex determined by the number of chromosome sets received.
      • Fertilized egg becomes diploid, developing into a female.
      • Unfertilized egg develops via parthenogenesis, becoming a male.
    • Chromosome Count:
      • Queen and worker bees: 32 chromosomes.
      • Drones (males): 16 chromosomes.
    • Reproduction:
      • Sperms produced by mitosis.
      • Eggs produced by meiosis.

    Environmental Sex Determination in Bonellia viridis:

    • Mechanism: Environmental factors determine offspring sex.
    • Sexual Dimorphism: Extreme dimorphism: Females ≈ 10 cm, males tiny and parasitic.
    • Development:
      • Proximity to mature female determines larva's sex.
      • Larva near female becomes male, settles on proboscis, enters female's mouth, and resides in uterus.
      • Away from female or on sea bottom, larvae develop into females.
    • Outcome: Sex determination influenced by environment.

    Genetic Disorders:

    • Mendelian Disorders:
      • Result from gene mutations.
      • E.g., Thalassemia, Sickle-cell anemia, Color blindness, Hemophilia, Phenylketonuria. 
    • Thalassemia:
      • Autosomal recessive disorder.
      • Alpha chains by HBA1 and HBA2 on chromosome 16, beta chain by HBB on chromosome 11.
      • Defective gene leads to abnormal hemoglobin synthesis.
      • Symptoms: Pale skin, Anemia, Slow growth, Abnormal RBCs.
      • Treatment: Regular blood transfusions.
    Chromosomal Disorders:
      • Due to chromosome absence, excess, or abnormal arrangement.
      • E.g., Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome
    1. Down’s Syndrome:
      • Aneuploidy due to trisomy of chromosome 21.
      • Symptoms: Facial features, Mental retardation, Skeletal issues, Short stature.
    2. Turner’s Syndrome:
      • Monosomy of X chromosome.
      • Symptoms: Short stature, Webbed neck, Lack of secondary sexual characteristics.
    3. Klinefelter’s Syndrome:
      • Trisomy of X chromosome.
      • Symptoms: Tall stature, Sterility, Underdeveloped testes, Subnormal intelligence.