Principles of Inheritance and Variation -Class 12 Biology(Complete Notes)

🌿 Principles of Inheritance and Variation (Class 12 NCERT – Full Chapter Notes)

✦ With Designing, Figures, Diagrams, MCQs — Complete & Error-free

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⭐ 1. Introduction to Inheritance and Variation

Inheritance is the biological process by which traits are passed from parents to offspring.
Variation is the differences in characteristics among individuals of the same species.

Inheritance ensures continuity, and variation provides diversity which is essential for evolution.

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♛ 2. Mendel – Father of Genetics

Gregor Johann Mendel (1822–1884), an Austrian monk, performed experiments on garden pea (Pisum sativum) for 7 years.

Reasons for choosing pea plant:

• Short life cycle

• Clear contrasting traits

• Easy to grow

• Naturally self-pollinating

• Could be artificially cross-pollinated

• Large number of seeds produced

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♡ 3. Mendel’s Experimental Approach (Scientific Method)

Mendel followed a scientific method which included:

1. Selection of pure lines

2. Identification of contrasting traits

3. Controlled crosses

4. Collection and analysis of data

5. Mathematical interpretation

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🌱 4. Important Terms in Genetics

✔ Gene – A unit of inheritance controlling a trait.

✔ Alleles – Different forms of the same gene.

✔ Dominant allele – Expresses itself even in heterozygous condition.

✔ Recessive allele – Expresses only in homozygous condition.

✔ Genotype – Genetic constitution (e.g., TT, Tt, tt).

✔ Phenotype – Physical expression (e.g., Tall, Dwarf).

✔ Homozygous – Same alleles (TT, tt).

✔ Heterozygous – Different alleles (Tt).

✔ Monohybrid cross – Cross involving one trait.

✔ Dihybrid cross – Cross involving two traits.

✔ Back cross – F1 hybrid crossed with either parent.

✔ Test cross – F1 crossed with recessive parent.

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🌿 5. Mendel’s Laws of Inheritance

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♢ 5.1 Law of Dominance

When two contrasting alleles are together, only one is expressed (dominant) while the other is masked (recessive).

Example:

T = tall (dominant)
t = dwarf (recessive)

Tt → Tall plant

Figure: Monohybrid Cross

Parent Generation:
TT (Tall) × tt (Dwarf)

Gametes:
T   T     ×     t   t

F1 Generation:
All Tt (Tall)

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🜚 5.2 Law of Segregation (Law of Purity of Gametes)

During gamete formation, alleles do not mix.
Each gamete receives one allele from each gene pair.

Diagram:

Parent genotype: Tt

Gametes:   T     t
Alleles separate during meiosis

This explains why recessive traits reappear in the F2 generation.

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✦ 5.3 Law of Independent Assortment

Alleles of different genes assort independently during gamete formation.

This was discovered through dihybrid crosses.

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🌾 6. Monohybrid Cross (One Trait)

Example: Tall (T) vs Dwarf (t)

★ Cross:

TT × tt → F1 (all Tt)

F1 Selfing:

      T     t
    -------------
T |   TT   |  Tt  |
    -------------
t |   Tt   |  tt  |
    -------------

Phenotypic ratio (F2): 3 Tall : 1 Dwarf

Genotypic ratio: 1 TT : 2 Tt : 1 tt

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🌾 7. Dihybrid Cross (Two Traits)

Traits:

• Yellow (Y) > Green (y)

• Round (R) > Wrinkled (r)

Parents:

YYRR × yyrr
↓
F1: YyRr (All Yellow Round)

Gametes formed by F1:

YR, Yr, yR, yr

Dihybrid Square:

       YR     Yr     yR     yr
--------------------------------------
YR | YYRR | YYRr | YyRR | YyRr |
--------------------------------------
Yr | YYRr | YYrr | YyRr | Yyrr |
--------------------------------------
yR | YyRR | YyRr | yyRR | yyRr |
--------------------------------------
yr | YyRr | Yyrr | yyRr | yyrr |
--------------------------------------

Phenotypic ratio (F2):

9 : 3 : 3 : 1

• 9 Yellow Round

• 3 Yellow Wrinkled

• 3 Green Round

• 1 Green Wrinkled

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🌙 8. Deviations from Mendelian Inheritance

Sometimes Mendel’s ratios do not appear. These exceptions occur due to special gene interactions.

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♣ 8.1 Incomplete Dominance

Dominant allele does not fully mask recessive allele → intermediate phenotype.

Example: Snapdragon flower color

RR = Red
Rr = Pink
rr = White

Diagram:

RR × rr → F1 (all Rr = Pink)

F2 ratio = 1 Red : 2 Pink : 1 White

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✿ 8.2 Codominance

Both alleles express equally.

Example: Human blood group AB

IA IB → AB blood group (both A & B antigens expressed)

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🧬 8.3 Multiple Allelism

One gene has more than two allele forms, but a person can have only two.

Example:

Human ABO blood group alleles: IA, IB, i

Phenotypes = A, B, AB, O

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⭐ 8.4 Pleiotropy (Single gene → many traits)

Example:

• Sickle cell anaemia gene affects shape, oxygen capacity, and health.

• PKU → multiple symptoms.

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🌸 8.5 Polygenic Inheritance (Multiple genes → one trait)

Traits controlled by several genes.

Examples:

• Human skin color

• Height

• Eye color

Polygenic traits show continuous variation.

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🧫 9. Chromosomal Theory of Inheritance

Proposed by Sutton and Boveri (1902).

Key points:

• Genes are present on chromosomes.

• Pairing and segregation of chromosomes explain Mendel’s laws.

• Male and female gametes contribute equally to heredity.

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🧲 10. Linkage and Recombination

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📌 10.1 Linkage

Genes located close together on a chromosome are inherited together.

Discovered by Morgan in Drosophila.

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📌 10.2 Recombination

Formation of new allele combinations due to crossing over.

Recombination frequency helps construct linkage maps.

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🧬 11. Sex Determination Mechanisms

Different organisms use different mechanisms:

XY type (humans, mammals)

Male: XY
Female: XX
Male decides sex of child.

ZW type (birds)

Male: ZZ
Female: ZW (female decides sex)

XO type (insects)

Male: XO
Female: XX

Haplo-diploid system (honeybee)

• Male (drone): haploid

• Female (queen & workers): diploid

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⚠️ 12. Genetic Disorders

Genetic disorders are of two types:

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12.1 Mendelian (Single Gene) Disorders

Inherited like Mendelian traits.

Examples:

• Sickle cell anaemia

• Phenylketonuria (PKU)

• Cystic fibrosis

• Haemophilia

• Thalassemia

• Colour blindness

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12.2 Chromosomal Disorders

Due to change in chromosome number or structure.

Examples:

• Down syndrome (Trisomy 21)

• Turner syndrome (XO)

• Klinefelter syndrome (XXY)

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🩸 13. Human Blood Groups

ABO System:

• IAIA / IAi → A

• IBIB / IBi → B

• IAIB → AB

• ii → O

Rh System:

Rh+ (dominant)
Rh– (recessive)

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⚙️ 14. Pedigree Analysis

Used to trace inheritance of traits across generations.
Symbols:

• ♂ = Male

• ♀ = Female

• ■ = Affected male

• ● = Affected female

• Line between ♂–♀ = marriage

• Vertical line = children

Pedigree helps identify dominant, recessive, and sex-linked inheritance.

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🧬 15. DNA as Genetic Material – Support from Chromosomal Basis

Genes located on chromosomes are passed from parents to offspring. During meiosis, chromosomes segregate and assort independently → same as genes.

Thus, Mendel’s laws are explained by the behavior of chromosomes.

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🌻 16. Important Diagrams (Text-Based)

16.1 Monohybrid Cross

      T     t
    -------------
T |   TT   |  Tt  |
    -------------
t |   Tt   |  tt  |
    -------------

16.2 Dihybrid F2 Checkerboard

Traits: Y,y and R,r
Gametes: YR, Yr, yR, yr

Dihybrid ratio = 9:3:3:1

16.3 Incomplete Dominance Flower Color

RR (Red)
Rr (Pink)
rr (White)

16.4 ABO Blood Group Relationship

IA   IB   i   ← Three alleles

16.5 Sex Determination (XY System)

 Father:    X     Y
 Mother:    X     X
 Offspring: XX (Girl), XY (Boy)

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📝 17. NCERT-Based Important MCQs

1. Mendel conducted experiments on which plant?

A. Wheat
B. Maize
C. Pea plant
D. Rose
Answer: C

2. Which law explains purity of gametes?

A. Dominance
B. Segregation
C. Independent assortment
D. Co-dominance
Answer: B

3. Phenotypic ratio in monohybrid F2?

A. 1:1
B. 9:3:3:1
C. 3:1
D. 2:1
Answer: C

4. Example of codominance:

A. Height in plants
B. ABO blood groups
C. Skin colour
D. Sickle cell anaemia
Answer: B

5. Down syndrome is caused by:

A. Trisomy 13
B. Trisomy 21
C. Monosomy X
D. XXY
Answer: B

6. Human male is:

A. Homogametic
B. Heterogametic
C. Diploid
D. Haploid
Answer: B

7. Dihybrid ratio:

A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 2:1
Answer: Cknowledgeallinonehub.blogspot.com