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Genetics and Inheritance Patterns

DNA and Genetic Material​

1. DNA Structure (डीएनए संरचना)​

Double Helix:

  • Two strands twisted together
  • Complementary base pairing
  • Sugar-phosphate backbone
  • Base stacking in center

Nucleotides (न्यूक्लिओटाइड):

  • Three components:
    • Deoxyribose sugar
    • Phosphate group
    • Nitrogenous base
  • Units of DNA

Nitrogenous Bases:

  • Purines: Adenine (A), Guanine (G) - double ring
  • Pyrimidines: Thymine (T), Cytosine (C) - single ring

Base Pairing (क्षार युग्मन):

  • Adenine ↔ Thymine (2 hydrogen bonds)
  • Guanine ↔ Cytosine (3 hydrogen bonds)
  • Complementary pairing
  • A-T and G-C ratios constant

DNA Functions:

  • Store genetic information
  • Pass information to offspring
  • Control protein synthesis

2. Genes (जीन) and Alleles (एलील)​

Gene:

  • Section of DNA
  • Codes for specific protein
  • Determines trait
  • Located on chromosomes
  • Example: Gene for eye color

Allele:

  • Alternative form of gene
  • Different versions
  • Example: Blue or brown color alleles
  • Inherited from parents

Dominant and Recessive (प्रभावी और अप्रभावी):

  • Dominant: Expressed in heterozygote
    • Represented by capital letter (A)
  • Recessive: Only expressed in homozygote
    • Represented by lowercase letter (a)
  • Codominant: Both alleles expressed equally
    • Neither dominant over other

Protein Synthesis​

1. Transcription (प्रतिलेखन)​

Definition:

  • DNA → RNA (messenger RNA)
  • Occurs in nucleus
  • Using DNA as template

Process:

  1. RNA polymerase binds to promoter
  2. Separates DNA strands
  3. mRNA synthesized using DNA template (complementary)
  4. Adenine pairs with Uracil (in RNA)
  5. mRNA leaves nucleus (in eukaryotes)

mRNA Characteristics:

  • Single strand
  • Temporary copy of gene
  • Carries genetic message
  • Contains codons (3-base sequences)

2. Translation (अनुवाद)​

Definition:

  • mRNA → Protein
  • Occurs at ribosomes
  • tRNA transfers amino acids

Process:

  1. mRNA binds to ribosome
  2. tRNA brings amino acids
  3. Anticodon matches mRNA codon
  4. Peptide bond forms between amino acids
  5. Ribosome moves along mRNA
  6. Protein chain grows
  7. Stop codon signals end

Codon:

  • Three-base sequence
  • Specifies amino acid
  • 64 codons total
  • Some code for same amino acid (redundant)

Mutation Effects:

  • Silent Mutation: Different codon, same amino acid
  • Missense Mutation: Different amino acid (may affect protein)
  • Nonsense Mutation: Stop codon, incomplete protein

3. Gene Expression Control​

Regulation:

  • Not all genes always active
  • Cells different types control expression
  • Lac operon example in bacteria
  • Developmental genes activation/inactivation

Mitosis and Meiosis​

1. Mitosis (सूत्री विभाजन)​

Purpose:

  • Growth and repair
  • Produces identical daughter cells
  • Maintains chromosome number (diploid)

Stages:

Prophase:

  • Nuclear envelope breaks down
  • Centrioles move to poles
  • Spindle fibers form
  • Chromosomes condense (visible)

Metaphase:

  • Chromosomes align at metaphase plate (center)
  • Spindle fibers attached to centromeres
  • Chromosomes visible

Anaphase:

  • Centromeres split
  • Sister chromatids separate
  • Move to opposite poles
  • Spindle fibers shorten

Telophase:

  • Nuclear envelopes reform
  • Chromosomes decondense
  • Spindle fibers disappear
  • Cleavage furrow forms (animals)

Cytokinesis:

  • Cell division completes
  • Two daughter cells form
  • Animal: Cleavage furrow
  • Plant: Cell plate forms

Result: 2 diploid cells, identical to parent

2. Meiosis (अर्धसूत्री विभाजन)​

Purpose:

  • Produce gametes (sex cells)
  • Produces haploid cells (half chromosome number)
  • Variation through crossing over and independent assortment

Two Divisions:

Meiosis I:

  • Homologous chromosomes separate
  • Crossing over (exchange material) during prophase I
  • Independent assortment (random orientation)
  • Results in 2 haploid cells

Meiosis II:

  • Sister chromatids separate (like mitosis)
  • Results in 4 haploid gametes
  • Non-identical (due to crossing over and reassortment)

Crossing Over (विनिमय):

  • Homologous chromosomes exchange segments
  • Non-sister chromatids involved
  • Creates genetic variation
  • Occurs during prophase I

Independent Assortment:

  • Random orientation of chromosome pairs
  • Each gamete different combination
  • Creates variation

Results:

  • 4 haploid cells from 1 diploid
  • Variation (genetic diversity)
  • Foundation of sexual reproduction

3. Comparison​

FeatureMitosisMeiosis
PurposeGrowth, repairGamete formation
Divisions12
Parent cells11
Daughter cells2 identical4 non-identical
Chromosome numberDiploid → DiploidDiploid → Haploid
Crossing overNoYes
Independent assortmentNoYes
WhereSomatic cellsGerm cells

Inheritance Patterns​

1. Monohybrid Cross (एकल संकरण)​

Single Trait Inheritance:

  • One gene with two alleles
  • Dominant and recessive
  • Example: Flower color (red vs. white)

Punnett Square:

  • Shows possible offspring
  • Parent genotypes on axes
  • Offspring in squares

Ratios:

  • F₁ (all Aa): All show dominant phenotype
  • F₂ (1 AA : 2 Aa : 1 aa): 3:1 ratio (three dominant : one recessive)
  • Monohybrid test cross 1:1 ratio

2. Dihybrid Cross (द्विसंकरण)​

Two Trait Inheritance:

  • Two genes, each with two alleles
  • Traits assort independently
  • Example: Seed color and shape

9:3:3:1 Ratio (F₂):

  • 9 double dominant
  • 3 first dominant only- 3 second dominant only
  • 1 double recessive

3. Codominance and Multiple Alleles​

Codominance:

  • Both alleles expressed
  • Example: AB blood type (both A and B antigens)
  • Bl^A Bl^B = AB phenotype

Multiple Alleles:

  • More than two alleles per gene
  • Example: Blood type (A, B, AB, O)
  • But only two in diploid organism

4. Sex-Linked Inheritance​

X-Linked Traits:

  • Gene on X chromosome
  • Males (XY) show phenotype with single allele
  • Females (XX) need homozygous for recessive
  • Example: Color blindness in males

Sex-Linked Cross:

  • X^N X^N female (normal) × X^n Y male (color blind)
  • All females normal, all males normal
  • X^N X^n female (carrier) × X^n Y male (color blind)
  • Females 1:1 (normal and color blind)
  • Males 1:1 (normal and color blind)

Variation and Evolution​

1. Genetic and Environmental Variation​

Genetic Variation:

  • Differences in alleles
  • Inherited from parents
  • Creates population diversity
  • Mutation source
  • Sexual reproduction source

Environmental Variation:

  • Caused by environment
  • Not inherited
  • Example: Height affected by nutrition
  • Phenotype = Genotype + Environment

2. Polygenic Inheritance​

Definition:

  • Many genes control trait
  • Small additive effects
  • Continuous variation (bell curve)
  • Example: Human height, skin color, intelligence

Characteristics:

  • Quantitative traits
  • Normal distribution
  • Many phenotypes
  • Environment also affects

3. Natural Selection (प्राकृतिक चयन)​

Definition:

  • Differential reproductive success
  • Organisms with advantageous traits survive/reproduce more
  • Allele frequency changes over time

Requirements:

  • Variation exists
  • Traits heritable
  • More offspring than survival
  • Resources limited
  • Survival/reproduction varies with traits

Types:

Directional Selection:

  • One allele increases frequency
  • Shifts population toward one extreme
  • Example: Industrial melanism in peppered moths

Stabilizing Selection:

  • Intermediate phenotype favored
  • Extremes selected against
  • Reduces variation
  • Example: Birth weight (too small or large problematic)

Disruptive Selection:

  • Extreme phenotypes favored
  • Intermediate selected against
  • Increases variation
  • Example: Seed size in plants

4. Evidence for Evolution​

Fossil Record:

  • Shows gradual change
  • Intermediate forms
  • Extinction
  • Index fossils show age

Comparative Anatomy:

  • Homologous structures (same origin, different function)
    • Example: Bat wing, human arm, whale flipper
    • Shows common ancestor
  • Vestigial structures (non-functional remnants)
    • Example: Human appendix, snake pelvis

Molecular Biology:

  • DNA similarity (more similar = closer ancestor)
  • Humans 98% DNA similar to chimpanzees
  • All organisms use same genetic code

Biogeography:

  • Species distribution patterns
  • Similar species in same regions
  • Explains by continental drift and isolation

Summary​

Genetics and inheritance:

  • DNA: Stores genetic information
  • Genes: Control traits through proteins
  • Mitosis: Growth and repair
  • Meiosis: Sexual reproduction and variation
  • Inheritance: Traits passed from parents
  • Evolution: Population change over time
  • Natural Selection: Mechanism of evolution

Understanding genetics explains heredity, variation, and evolution of life.