Heredity

1. What is Heredity?

Definition:

Heredity is the transfer of traits/characters from parents to offspring through genes.

Examples:

• Eye colour

• Height

• Blood group

• Shape/colour of seeds in plants

Why heredity is important?

• It allows continuity of life.

• Offspring resemble parents.

• Gives variation in population → basis of evolution.

2. Variations

Definition:

Differences among individuals of the same species are called variations.

Types of Variations:

(A) Inherited Variation

• Passed from parents to offspring.

• Present in genes.

• Example: Blood group, eye colour.

(B) Acquired Variation

• Not inherited.

• Develop due to environment, habits.

• Example: Learning music, bodybuilding, tanning of skin.

3. Genes, Chromosomes & DNA

DNA (Deoxyribonucleic Acid):

• Genetic material present in nucleus.

• Carries instructions for traits.

• Has shape of double helix (spiral ladder).

Gene:

• A small segment of DNA controlling one trait.

• Example: Gene for tallness.

Chromosomes:

• Thread-like structures made of DNA.

• Present in pairs in nucleus.

• Humans have 46 chromosomes = 23 pairs.

4. Rules of Inheritance — Mendel’s Work

Gregor Johann Mendel – Father of Genetics

Worked on pea plants and discovered the laws of heredity.

A. Mendel’s Experiments

Traits Studied by Mendel

Trait	Dominant	Recessive
Height	Tall (T)	Dwarf (t)
Seed colour	Yellow (Y)	Green (y)
Seed shape	Round (R)	Wrinkled (r)

B. Important Terms

Dominant Trait

• Expressed even if one allele is present.

• Example: Tall (T)

Recessive Trait

• Expressed only when both alleles are recessive.

• Example: Dwarf (tt)

Phenotype

• Physical appearance (Tall, short)

Genotype

• Genetic makeup (TT, Tt, tt)

Homozygous

• Same alleles (TT or tt)

Heterozygous

• Different alleles (Tt)

5. Mendel’s Laws

Law 1: Law of Dominance

When two factors (T and t) are present, the dominant one expresses.

Example:
T (tall) + t (short) → offspring = Tall.

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Law 2: Law of Segregation

• Two factors separate during gamete formation.

• Gametes carry only one factor (T or t).

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Law of Independent Assortment

• Different traits pass independently.

• Example: Height and seed colour do not depend on each other.

6. Monohybrid Cross (Cross involving one trait)

Example: Tall (TT) × Dwarf (tt)**

F1 Generation:

All offspring = Tall (Tt)

F2 Generation:

Selfing: Tt × Tt
Punnett square gives:

	T	t
T	TT	Tt
t	Tt	tt

Genotype Ratio: 1 TT : 2 Tt : 1 tt

Phenotype Ratio: 3 Tall : 1 Dwarf

7. Dihybrid Cross (Two traits)

Example:

• Round Yellow (RRYY) × Wrinkled Green (rryy)

F2 Phenotype Ratio:

9 : 3 : 3 : 1
(9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green)

8. Sex Determination in Humans

Females: XX

Males: XY

Gametes:

• Female produces only X gametes.

• Male produces X and Y gametes.

Result:

Egg	Sperm	Offspring
X	X	Girl (XX)
X	Y	Boy (XY)

Conclusion:
👉 Father determines the sex of the child, not the mother.

9. Evolution & Heredity Link

• Evolution occurs due to accumulation of variations over generations.

• Natural selection favours beneficial variations.

10. Important Definitions

• Speciation: Formation of new species.

• Mutation: Sudden change in DNA.

• Cloning: Producing genetically identical copies.

• Fossils: Remains of ancient organisms.

IMPORTANT QUESTION–ANSWERS 

Q1. What is heredity? Explain with examples.

Answer:
Heredity is the transmission of traits from parents to offspring through genes.
For example, eye colour, height, blood group.

Q2. Define gene and chromosome.

Answer:

• Gene: Functional unit of DNA controlling specific traits.

• Chromosome: Thread-like structure made of DNA present in nucleus.

Q3. What is the difference between dominant and recessive traits?

Dominant	Recessive
Expressed when one allele is present	Expressed only when both alleles are recessive
Shown in heterozygous condition	Shown only in homozygous condition
Example: Tall (T)	Example: Dwarf (t)

Q4. Explain sex determination in humans.

Answer:

• Females: XX

• Males: XY
  Female gametes have only X, male gametes have X and Y.
  If sperm contributes X, baby = girl.
  If sperm contributes Y, baby = boy.
  Hence, father determines the sex of the child.

Q5. What is a monohybrid cross? Give an example.

Answer:
A cross in which only one trait is considered.
Example:
Tall (TT) × Dwarf (tt) → F1 = Tall (Tt)
F2 shows 3:1 ratio (Tall : Dwarf).

Q6. What is variation? Mention its types.

Variation = differences in characters among individuals.

Types:

1. Inherited variations – from parents.

2. Acquired variations – not inherited, due to environment.

Q7. What are acquired characters? Give examples.

Not inherited; developed during life.
Examples: Learning music, bodybuilding, pierced ears.

Q8. State Mendel’s laws of inheritance.

1. Law of Dominance

2. Law of Segregation

3. Law of Independent Assortment

Q9. Why are traits of father not always visible in children?

Because offspring inherits two sets of genes (one from each parent).
Recessive traits may remain hidden in heterozygous condition.

Q10. What is DNA? Describe its structure.

DNA is the genetic material found in cells.
Its structure is a double helix, resembling a twisted ladder.