Class 12 Biology Chapter 5 Biology Principles of Inheritance and Variation Notes
5.1 Mendel’s Laws of Inheritance
Why Mendel Selected Pea Plant (Pisum sativum)
Gregor Mendel selected garden pea for his experiments due to the following advantages:
Short life cycle (results in ~3 months)
Easy to grow and maintain
Naturally self-pollinated but allows controlled cross-pollination
True-breeding varieties available
Seven pairs of contrasting characters (easily observable)
F₁ hybrids are fertile
Important Genetic Terms
Dominant vs Recessive
Dominant allele expresses itself in heterozygous condition
Recessive allele expresses only in homozygous condition
Homozygous vs Heterozygous
Homozygous: Same alleles (TT or tt)
Heterozygous: Different alleles (Tt)
Monohybrid vs Dihybrid Cross
Monohybrid cross: One character (e.g., Tall × Dwarf)
Dihybrid cross: Two characters (e.g., Seed colour + seed shape)
Number of Gametes Formula
If an organism is heterozygous for n loci:
Number of gametes = 2ⁿ
👉 For 4 heterozygous loci:
2⁴ = 16 types of gametes
Law of Dominance (Monohybrid Cross)
In a heterozygote, only one allele expresses (dominant)
Example:
TT (Tall) × tt (Dwarf)
F₁: All Tt (Tall)
Shows Tall (T) is dominant over Dwarf (t)
Test Cross
A test cross is crossing an individual with a homozygous recessive
Purpose:
To determine genotype of dominant phenotype
Helps identify homozygous vs heterozygous condition
Single Locus Cross (Homozygous × Heterozygous)
TT × Tt
Genotype ratio: TT : Tt = 1 : 1
Phenotype: All Tall
5.3 Inheritance of Two Genes
Linked Genes
Genes located on the same chromosome
Do not assort independently
Produce parental combinations more frequently
Contribution of T. H. Morgan
Thomas Hunt Morgan worked on Drosophila melanogaster and established:
Linkage and crossing over
Sex-linked inheritance
Chromosomal theory expansion
Gene mapping (1% recombination = 1 centiMorgan)
Criss-cross inheritance
Pedigree Analysis
Pedigree analysis studies inheritance of traits across generations.
Uses:
Identify mode of inheritance (dominant/recessive)
Genetic counselling
Trace origin of defective genes
Predict risk of genetic disorders
5.4 Sex Determination in Humans
Female: XX
Male: XY
Ovum always carries X
Sperm carries X or Y
👉 Father determines the sex of the child
ABO Blood Group Inheritance
Child with blood group O → genotype ii
Father (A): Iᴬi
Mother (B): Iᴮi
Possible offspring genotypes:
Iᴬi (A)
Iᴮi (B)
IᴬIᴮ (AB)
ii (O)
5.5 Non-Mendelian Inheritance
Co-dominance
Both alleles express equally
Example: AB blood group (IᴬIᴮ)
Incomplete Dominance
Heterozygote shows intermediate phenotype
Example: Red × White → Pink (Snapdragon / Mirabilis jalapa)
5.6 Mutation
Point Mutation
Change in a single base pair of DNA
Example:
Sickle cell anaemia
Haemophilia
Chromosomal Theory of Inheritance
Proposed independently by:
Walter Sutton and Theodor Boveri (1902)
Key Idea:
Genes are located on chromosomes
Chromosomes segregate and assort during meiosis
Expanded by Morgan and his team
5.8 Genetic Disorders (Autosomal)
Sickle Cell Anaemia
Abnormal haemoglobin
Sickle-shaped RBCs
Reduced oxygen transport
Phenylketonuria
Accumulation of phenylalanine
Mental retardation
Hypopigmentation of skin and hair
5.9 Summary (Exam-Focused)
Mendel laid foundation of genetics
Dominance, segregation & independent assortment explain inheritance
Linked genes show deviation from Mendel
Humans show XY type sex determination
Mutations cause genetic disorders
Pedigree analysis aids diagnosis and counselling
✍️ Top 10 Short Question–Answers
Q1. Why did Mendel select pea plant for his experiments?
Answer: Because it has short life cycle, clear contrasting traits, true-breeding varieties and allows controlled pollination.
Q2. What is a dominant allele?
Answer: An allele that expresses itself in heterozygous condition.
Q3. What is meant by heterozygous condition?
Answer: Presence of two different alleles of a gene (e.g., Tt).
Q4. Define monohybrid cross.
Answer: A genetic cross involving inheritance of one character.
Q5. What is the law of dominance?
Answer: In a heterozygote, only the dominant allele is expressed.
Q6. What is a test cross?
Answer: Crossing a dominant individual with a homozygous recessive to determine genotype.
Q7. What are linked genes?
Answer: Genes located on the same chromosome that do not assort independently.
Q8. Who proposed the chromosomal theory of inheritance?
Answer: Walter Sutton and Theodor Boveri.
Q9. What is co-dominance?
Answer: Condition in which both alleles express equally in heterozygote.
Q10. Who determines the sex of a child in humans?
Answer: Father determines the sex of the child.
📝 Long Answer Questions
Q1. Explain why Mendel selected pea plant for his genetic experiments.
Answer:
Mendel selected pea plant (Pisum sativum) because:
Short life cycle (~3 months)
Easy to grow and maintain
Naturally self-pollinated but allows cross-pollination
True-breeding varieties available
Seven easily observable contrasting characters
Fertile F₁ hybrids
These features made it ideal for genetic studies.
Q2. Describe Mendel’s Law of Dominance with example.
Answer:
The law of dominance states that in a heterozygous condition, only one allele expresses itself.
Example:
TT (Tall) × tt (Dwarf)
F₁ generation: All Tt (Tall)
This shows that Tall (T) is dominant over Dwarf (t).
Q3. Explain non-Mendelian inheritance with examples.
Answer:
Co-dominance:
Both alleles express equally
Example: AB blood group (IᴬIᴮ)
Incomplete dominance:
Heterozygote shows intermediate phenotype
Example: Red × White → Pink (Snapdragon / Mirabilis jalapa)
📝 PYQs (Previous Year Questions)
🔹 1️⃣ Very Short Answer Questions (1 Mark)
Name the plant selected by Mendel for his experiments.
What is meant by dominant allele?
Define heterozygous condition.
What is a monohybrid cross?
Write the formula to calculate number of gametes.
What is a test cross?
Name the scientist who worked on Drosophila melanogaster.
What type of sex determination is seen in humans?
Give one example of co-dominance.
Name a genetic disorder caused by point mutation.
🔹 2️⃣ Short Answer Questions (2–3 Marks)
Why did Mendel select pea plant for inheritance studies?
Differentiate between dominant and recessive alleles.
What is the law of dominance?
Define test cross and state its significance.
What are linked genes?
Explain pedigree analysis and its importance.
Explain inheritance of ABO blood group system.
What is incomplete dominance? Give one example.
Write any two contributions of T. H. Morgan.
What is point mutation? Give one example.
🔹 3️⃣ Long Answer Questions (3–5 Marks)
Explain Mendel’s monohybrid cross with suitable example.
Describe dihybrid cross and law of independent assortment.
Explain linkage and crossing over.
Describe chromosomal theory of inheritance.
Explain sex determination in humans.
Describe non-Mendelian inheritance patterns.
Explain inheritance of ABO blood groups.
Describe sickle cell anaemia as a genetic disorder.
🔹 4️⃣ Difference-Based PYQs (Very Important)
Dominant vs Recessive
Homozygous vs Heterozygous
Monohybrid cross vs Dihybrid cross
Linked genes vs Unlinked genes
Co-dominance vs Incomplete dominance
🔹 5️⃣ Assertion–Reason / Concept-Based PYQs
Linked genes do not assort independently. Explain.
Father determines the sex of the child in humans. Justify.
Mendel’s laws are universally applicable. Comment.
Pedigree analysis is useful in genetic counselling. Explain.
Mutations can lead to genetic disorders. Explain.
🔹 6️⃣ Numerical / Diagram-Based PYQs
Calculate number of gametes produced by an organism heterozygous for four loci.
Draw a Punnett square for monohybrid cross.
Draw a pedigree showing inheritance of a genetic disorder.
❓ FAQs (5)
FAQ 1. What is the formula to calculate number of gametes?
Number of gametes = 2ⁿ, where n is number of heterozygous loci.
FAQ 2. Why do linked genes not follow Mendel’s law of independent assortment?
Because they are located on the same chromosome.
FAQ 3. What is pedigree analysis used for?
To study inheritance patterns and provide genetic counselling.
FAQ 4. Why is AB blood group an example of co-dominance?
Because both Iᴬ and Iᴮ alleles express equally.
FAQ 5. What causes sickle cell anaemia?
A point mutation resulting in abnormal haemoglobin.
| Chapter No. | Chapter Name | Visit |
|---|---|---|
| 1 | Reproduction in Organisms | Visit |
| 2 | Sexual Reproduction in Flowering Plants | Visit |
| 3 | Human Reproduction | Visit |
| 4 | Reproductive Health | Visit |
| 5 | Principles of Inheritance and Variation | Visit |
| 6 | Molecular Basis of Inheritance | Visit |
| 7 | Evolution | Visit |
| 8 | Human Health and Disease | Visit |
| 9 | Strategies for Enhancement in Food Production | Visit |
| 10 | Microbes in Human Welfare | Visit |
| 11 | Biotechnology: Principles and Processes | Visit |
| 12 | Biotechnology and its Applications | Visit |
| 13 | Organisms and Populations | Visit |
| 14 | Ecosystem | Visit |
| 15 | Biodiversity and Conservation | Visit |
| 16 | Environmental Issues | Visit |
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