Gregor Johann Mendel (1822-1884) was an Austrian monk who conducted groundbreaking experiments on pea plants (Pisum sativum) in the monastery garden.

Why Mendel chose Pea Plant?

• ✅ Short life span (grows quickly)
• ✅ Easy to cultivate
• ✅ Large number of offspring
• ✅ Distinct contrasting characters (7 pairs)
• ✅ Bisexual flowers (self and cross-pollination possible)
• ✅ True-breeding varieties available

Parents (P):

Pure Tall Plant (TT) × Pure Dwarf Plant (tt)

Gametes:

T (from tall) and t (from dwarf)

F₁ Generation:

All plants were Tall (Tt)

Result: 100% Tall (dwarf trait disappeared)

F₂ Generation (Self-pollination of F₁):

3 Tall : 1 Dwarf

Genotypic Ratio: 1 TT : 2 Tt : 1 tt

Phenotypic Ratio: 3 Tall : 1 Dwarf

Example: Flower Color in Snapdragon (Antirrhinum)

Parents:

Red Flower (RR) × White Flower (rr)

F₁ Generation:

All Pink flowers (Rr)

Neither red nor white is dominant; blending occurs

F₂ Generation (Pink × Pink):

Ratio: 1 Red : 2 Pink : 1 White

Phenotypic Ratio = Genotypic Ratio = 1:2:1

Example: ABO Blood Group System in Humans

Alleles:

• I^A - produces antigen A
• I^B - produces antigen B
• i - produces no antigen (recessive)

Blood Groups and Genotypes:

Parents (P):

Round Yellow (RRYY) × Wrinkled Green (rryy)

F₁ Generation:

All plants: Round Yellow (RrYy)

Both dominant traits expressed

F₂ Generation (F₁ × F₁):

Phenotypic Ratio: 9:3:3:1

• 9 Round Yellow (R_Y_)
• 3 Round Green (R_yy)
• 3 Wrinkled Yellow (rrY_)
• 1 Wrinkled Green (rryy)

Proposed by: Walter Sutton and Theodor Boveri (1902)

Statement: Chromosomes are the physical carriers of genes. Genes are located on chromosomes in a linear order.

Sutton and Boveri's Observations:

• ✅ Chromosomes occur in pairs (diploid)
• ✅ Chromosomes segregate during meiosis (like Mendel's factors)
• ✅ Each gamete receives one chromosome from each pair
• ✅ Chromosomes maintain their individuality through cell divisions
• ✅ Behavior of chromosomes parallels behavior of genes

Types of Linkage

1. Complete Linkage:

• Genes are very close together
• No crossing over occurs
• Always inherited together
• Rare in nature; common in male Drosophila

2. Incomplete Linkage:

• Genes are far apart on same chromosome
• Crossing over can occur
• Some recombination possible
• Common in nature

Characteristics of Polygenic Traits:

• ✅ Controlled by multiple genes
• ✅ Show continuous variation (range of phenotypes)
• ✅ Cannot be classified into distinct categories
• ✅ Influenced by environment
• ✅ Follow normal distribution curve (bell-shaped)

Example: Human Skin Color

Let's assume 3 genes control skin color: A, B, C (simplified model)

Each dominant allele (A, B, C) adds pigment (melanin)

Result: Continuous range of skin colors from very light to very dark.

Example: Phenylketonuria (PKU)

A single gene defect causes multiple effects:

• ❌ Mental retardation
• ❌ Reduced hair and skin pigmentation
• ❌ Abnormal body posture
• ❌ Mousy odor in urine

Cause: Mutation in gene coding for enzyme phenylalanine hydroxylase

Human Sex Chromosomes

Humans have 46 chromosomes (23 pairs):

• 22 pairs of Autosomes (non-sex chromosomes)
• 1 pair of Sex Chromosomes

Sex Chromosome Types:

• Females: XX (homogametic)
• Males: XY (heterogametic)

Mechanism:

Parents:

Mother (XX) × Father (XY)

Gametes:

• Mother produces: All X eggs
• Father produces: 50% X sperm, 50% Y sperm

Offspring:

• X egg + X sperm = XX (Female child)
• X egg + Y sperm = XY (Male child)

Result: 50% chance of male, 50% chance of female

Opposite Mechanism to Humans

In birds, butterflies, and some fish, the sex determination mechanism is opposite to that of humans:

Sex Chromosomes in Birds:

• Males: ZZ (homogametic) - produce only one type of sperm (Z)
• Females: ZW (heterogametic) - produce two types of eggs (Z and W)

Mechanism:

Mother (ZW) × Father (ZZ)

Gametes:

• Mother produces: 50% Z eggs + 50% W eggs
• Father produces: All Z sperm (100%)

Offspring:

• Z (egg) + Z (sperm) = ZZ (Male chick - 50%)
• W (egg) + Z (sperm) = ZW (Female chick - 50%)

Result: Equal probability of male or female (1:1 ratio)

Unique System in Honey Bees

Honey bees use a special mechanism called Haplo-diploid system for sex determination.

Mechanism:

• Males (Drones): Develop from unfertilized eggs (Haploid - n)
• Females (Queen & Workers): Develop from fertilized eggs (Diploid - 2n)

1. Based on Cell Type:

A. Somatic Mutations:

• Occur in body (somatic) cells
• Not inherited (not passed to offspring)
• Affect only the individual
• Example: Most cancers

B. Germinal Mutations:

• Occur in germ cells (gametes)
• Inherited (passed to offspring)
• Affect future generations
• Example: Hemophilia, Color blindness

2. Based on Extent:

A. Gene Mutations (Point Mutations):

• Change in single or few nucleotides
• Types:
  • Substitution: One base replaced by another (e.g., Sickle cell: A→T)
  • Insertion: Addition of one or more nucleotides
  • Deletion: Removal of one or more nucleotides
• Insertion and Deletion cause Frame-shift mutations - shift in reading frame of codons
• Frame-shift mutations usually have severe effects as they change all amino acids after mutation point
• Examples: Sickle cell anaemia (substitution), Thalassemia (often deletion)

B. Chromosomal Mutations:

• Change in chromosome structure or number
• Examples: Deletion, duplication, inversion, translocation
• Can cause: Down syndrome, Turner syndrome

3. Based on Effect:

A. Beneficial Mutations:

• Increase survival and reproduction
• Source of evolution
• Example: Antibiotic resistance in bacteria

B. Harmful Mutations:

• Decrease survival
• Cause genetic disorders
• Example: Most human genetic diseases

C. Neutral Mutations:

• No effect on survival
• Silent mutations

Steps to Determine Inheritance Pattern:

1. Identify if trait is Dominant or Recessive:

• If trait skips generations → Recessive
• If trait appears in every generation → Dominant
• If two unaffected parents have affected child → Recessive

2. Identify if trait is Autosomal or Sex-linked:

• If affects males and females equally → Autosomal
• If affects mostly males → X-linked
• If affected males cannot pass to sons → X-linked

Definition:

Inability to distinguish between certain colors, most commonly red and green.

Inheritance Pattern:

• X-linked recessive disorder
• Gene located on X chromosome
• More common in males

Genotypes:

• Normal Male: X^CY
• Color blind Male: X^cY
• Normal Female: X^CX^C
• Carrier Female: X^CX^c
• Color blind Female: X^cX^c (very rare)

Why more common in males?

Males have only one X chromosome. If that X has mutant gene, they are color blind. Females need two mutant copies (rare).

Definition:

Hemophilia is a bleeding disorder where blood does not clot properly due to deficiency of clotting factors.

Inheritance Pattern:

• X-linked recessive disorder
• Affects mostly males
• Females are usually carriers
• Also called "Bleeder's disease"
• Famous in European royal families (Queen Victoria was carrier)

Types:

• Hemophilia A: Deficiency of Factor VIII (most common - 80%)
• Hemophilia B: Deficiency of Factor IX (less common)

Genotypes:

• Normal Male: X^HY
• Hemophilic Male: X^hY
• Normal Female: X^HX^H
• Carrier Female: X^HX^h
• Hemophilic Female: X^hX^h (extremely rare, usually lethal)

Definition:

A genetic blood disorder where red blood cells become sickle-shaped (crescent) instead of round, causing blockage in blood vessels.

Inheritance Pattern:

• Autosomal recessive disorder
• Both parents must be carriers
• Affects both males and females equally

Cause:

Point mutation in β-globin gene of hemoglobin:

• Substitution mutation: GAG → GTG
• 6th codon of β-globin gene
• Glutamic acid (Glu) replaced by Valine (Val)
• Results in abnormal hemoglobin (HbS)

Genotypes:

• Normal: HbA HbA (Homozygous dominant)
• Carrier: HbA HbS (Heterozygous - Sickle cell trait)
• Sickle cell anaemia: HbS HbS (Homozygous recessive)

Definition:

An inborn error of metabolism where body cannot break down amino acid phenylalanine, leading to its accumulation.

Inheritance Pattern:

• Autosomal recessive disorder
• Affects both males and females

Cause:

• Deficiency of enzyme phenylalanine hydroxylase
• Cannot convert phenylalanine → tyrosine
• Phenylalanine accumulates in blood and brain

Symptoms (Pleiotropy - one gene, multiple effects):

• Mental retardation (most serious)
• Reduced pigmentation (light skin, hair)
• Mousy odor in urine
• Abnormal body posture
• Seizures

Definition:

A group of inherited blood disorders characterized by reduced or absent synthesis of hemoglobin chains, leading to anaemia.

Inheritance Pattern:

• Autosomal recessive disorder
• More common in Mediterranean, Middle East, India, Southeast Asia

Types:

• α-Thalassemia: Defect in α-globin chain synthesis
• β-Thalassemia: Defect in β-globin chain synthesis (more common)

β-Thalassemia Forms:

• Thalassemia Minor: Heterozygous (one mutant allele) - Mild anaemia, usually asymptomatic
• Thalassemia Major (Cooley's Anaemia): Homozygous (two mutant alleles) - Severe anaemia, life-threatening

1. Numerical Abnormalities (Aneuploidy):

A. Monosomy: Loss of one chromosome (2n - 1)

• Example: Turner syndrome (45, XO)

B. Trisomy: Gain of one chromosome (2n + 1)

• Example: Down syndrome (47, +21)

C. Polyploidy: Gain of complete set(s) of chromosomes

• Triploidy (3n), Tetraploidy (4n) - Usually lethal in humans

2. Structural Abnormalities:

• Deletion: Loss of chromosome segment
• Duplication: Extra copy of chromosome segment
• Inversion: Reversal of chromosome segment
• Translocation: Transfer of segment to another chromosome

Cause:

• Extra copy of chromosome 21
• Karyotype: 47, +21 (total 47 chromosomes)
• Most common chromosomal disorder
• Risk increases with maternal age (>35 years)

Characteristics:

• Mental retardation (low IQ)
• Short stature
• Broad flat face
• Slanted eyes (Mongoloid features)
• Furrowed tongue
• Short neck
• Simian crease (single palmar crease)
• Congenital heart defects
• Poor muscle tone

Cause:

• Loss of one X chromosome in females
• Karyotype: 45, XO (total 45 chromosomes)
• Only viable monosomy in humans
• Affects only females

Characteristics:

• Short stature
• Webbed neck
• Broad chest with widely spaced nipples
• Underdeveloped ovaries (sterile)
• Lack of secondary sexual characteristics
• Usually normal intelligence
• Amenorrhea (no menstruation)

Cause:

• Extra X chromosome in males
• Karyotype: 47, XXY (total 47 chromosomes)
• Affects only males

Characteristics:

• Tall stature with long limbs
• Feminized body (gynaecomastia - breast development)
• Small testes (sterile)
• Reduced facial and body hair
• Lack of secondary sexual characteristics
• May have mild mental retardation