Heredity

Class 10 CBSE Science • Chapter 8 • Easy, colourful notes for kids

Heredity Inherited vs Acquired Important terms Chromosomes Genes & Alleles Phenotype & Genotype Mendel Monohybrid Dihybrid Sex determination Revision

1. What is Heredity?

Heredity: the passing of genetic information and characters (traits) from one generation to the next.

Variation: the differences in characters among the individuals of a species.

Genetics: the branch of biology which deals with the study of heredity and variation.

Eye colour

brown, blue, green

Flower colour

pink, yellow, white, red

Earlobes

attached or free

Thumb

straight or bent

Family looks

child looks like parents

All these differences between individuals are examples of variation.

2. Inherited Traits vs Acquired Traits

Inherited traits	Acquired traits
These traits can be inherited as well as transmitted to the next generation.	These traits are neither inherited nor transmitted to the next generation.
Inherited from parents during reproduction.	Acquired after birth.
Examples: skin colour, eye colour, nose shape, blood group, etc.	Examples: acquiring weight, piercing of ear or nose, acquiring knowledge, etc.

Tip: Born with it → inherited. Got it later in life → acquired (and it is not passed to children).

3. Some Important Terms

These are the terms we will learn in this chapter. Tap a name to jump to its section.

DNA

Chromatin and Chromosome

Phenotype and Genotype

4. DNA, Chromosomes and Chromosome Number

DNA (Deoxyribo Nucleic Acid)

It is a double-stranded, thread-like structure present in the nucleus.
It carries all the information about the structure and function of an organism.
It is inherited from one generation to another.

DNA double helix — DNA is a double helix (double-stranded thread)

Chromatin and Chromosome

DNA is packed with proteins. In a normal (non-dividing) cell it looks like a tangled chromatin thread. When the cell divides, the chromatin condenses into short, thick chromosomes.

DNA → Chromatin (tangled thread) —(cell division)→ Chromosome (condensed, rod-like)

Haploid and Diploid

Haploid (n)

Single copy of each chromosome.
Chromosome number is denoted as ‘n’. Here n = 4.

Diploid (2n)

Two copies of each chromosome.
Chromosome number is denoted as 2n. Here 2n = 2 × 4 = 8.

Chromosome number in human body cells and gametes

All cells of the human body are diploid, except gametes.
Gametes (sperm cell and egg cell) are haploid cells.

Total chromosomes in a human cell = 46 (23 pairs)

Cell	Type	Chromosomes
Female body cell	Diploid	46
Male body cell	Diploid	46
Egg cell / ovum (made by meiosis)	Haploid	23
Sperm cell (made by meiosis)	Haploid	23
Zygote (after fertilisation)	Diploid	46 (23 pairs)

Egg (23) + Sperm (23) —(fertilisation)→ Zygote (46 = 23 pairs)

The chromosome that comes from the mother is the maternal chromosome.
The chromosome that comes from the father is the paternal chromosome.
A maternal and a paternal chromosome of the same kind form a pair called homologous chromosomes.

5. Genes and Alleles

Genes

Gene: a segment or part of DNA that carries the information for a particular character. Gene → Information → Protein.

One DNA thread has many genes, for example a gene for blood group, a gene for eye colour and a gene for hair shape.

Alleles

Allele: an alternative variant, version or form of a gene. Two or more possible forms of a gene are located at a specific location (place) on a specific chromosome.

Gene → Dominant allele or Recessive allele

Gene for	Allele 1	Allele 2
Plant height	T = Tall (dominant)	t = Dwarf (recessive)
Colour of flower	Violet	White
Seed shape	R = Round seed	r = Wrinkled seed

Dominant Allele vs Recessive Allele

Dominant allele	Recessive allele
The stronger one of the two alleles.	The weaker one of the two alleles.
Denoted by a capital letter (T).	Denoted by a small letter (t).
Expresses itself in both: homozygous condition (TT) and heterozygous condition (Tt).	Expresses itself only in the homozygous condition (tt).

6. Phenotype vs Genotype, Homozygous vs Heterozygous

Phenotype vs Genotype

Phenotype	Genotype
The appearance of an organism, or its observable physical trait. Example: tall or dwarf plant, round or wrinkled seed.	The combination of genes in an organism. Example: TT, Tt, tt.

Example (eye colour): Dad has brown eyes (dominant, B). Mom has blue eyes (recessive, b). The child has genotype Bb, so the child shows brown eyes (phenotype), and the blue colour stays hidden.

Dad BB (brown) + Mom bb (blue) → Child Bb (brown displayed, blue hidden)

Homozygous and Heterozygous conditions

Homozygous dominant
(pure condition)

Round seed (RR)
Tall (TT)

Heterozygous
(hybrid condition)

Round seed (Rr)
Tall (Tt)
one dominant + one recessive allele

Homozygous recessive
(pure condition)

Wrinkled seed (rr)
Dwarf (tt)

Tip: Homo = same, so both alleles are the same (TT or tt). Hetero = different, so the alleles are different (Tt).

7. Mendel and His Contributions

Gregor Mendel — Gregor Johann Mendel (1822–1884)

✔️ Gregor Johann Mendel (1822–1884).
✔️ Known as the “Father of Genetics”.
✔️ Performed his hybridisation experiments on pea plants from 1856 to 1863 and published the result in 1866.
✔️ His work explained how traits are inherited from one generation to the next and proposed the laws of inheritance.

Seven pairs of contrasting traits in the pea plant (Pisum sativum, the garden pea)

Trait	Shape of seeds	Colour of seeds	Colour of pods	Shape of pods	Plant height	Position of flowers	Flower colour
Dominant trait	Round	Yellow	Green	Full	Tall	At leaf junction	Purple
Recessive trait	Wrinkled	Green	Yellow	Flat, constricted	Short	At tips of branches	White

Hybridisation experiments

Mendel took pollen grains from the anther of one pea plant (parent P₁) and placed them on the stigma of another plant (parent P₂). This is cross-pollination.

After fertilisation:
1) Zygote → Embryo
2) Ovule → Seed
3) Ovary → Fruit

Mendel’s Laws of Inheritance

Based on a Monohybrid Cross:
(i) Law of Dominance: When an inherited pair of two alleles is heterozygous, the allele that is expressed is called dominant, while the other is called recessive.
(ii) Law of Segregation: During gamete formation, the copies of genes or alleles are divided (segregated) such that each gamete receives only one allele.

Based on a Dihybrid Cross:
(iii) Law of Independent Assortment: Alleles of two or more different genes get assorted into gametes independently of one another.

Breeding pea plants with violet flowers and white flowers

Parental generation (P): Violet flower plant × White flower plant
First generation (F₁): 100% plants have purple (violet) flowers

8. Monohybrid Cross (one character)

Monohybrid cross: a cross in which only one character is studied at a time.

In his first monohybrid cross, Mendel crossed a pure-breed tall plant and a pure-breed dwarf plant.

Tall: TT = Pure tall, Tt = Hybrid tall.
Dwarf: tt = Dwarf.

Steps of the cross (tall × dwarf)

Parent: Pure Tall TT × Pure Dwarf tt

↓

Gametes: T and T | t and t

↓ Fertilisation

F₁ generation: Tt
Genotype = Heterozygous
Phenotype = Hybrid Tall plant

↓ Selfing (F₁ × F₁: Tt × Tt)

Gametes: T, t | T, t

↓

F₂ generation: TT, Tt, Tt, tt

F₂ generation (Punnett square)

♀ ♂	T	t
T	TT Tall	Tt Tall
t	Tt Tall	tt Dwarf

Phenotypic ratio in F₂

Tall : Dwarf = 3 : 1

75% Tall, 25% Dwarf

Genotypic ratio in F₂

TT : Tt : tt = 1 : 2 : 1

Homozygous dominant TT 25%
Heterozygous Tt 50%
Homozygous recessive tt 25%

Another example: flower colour (Blue × White)

Parent: Blue flower BB × White flower bb

↓

Gametes: B and B | b and b

↓

F₁: Bb = Heterozygous blue flower

↓ Selfing (F₁ × F₁: Bb × Bb)

Gametes: B, b | B, b

♀ ♂	B	b
B	BB blue	Bb blue
b	Bb blue	bb white

Phenotypic ratio: Blue 75% : White 25% (3 : 1).
Genotypic ratio: BB : Bb : bb = 1 : 2 : 1.

2️⃣ 9. Dihybrid Cross (two characters)

Dihybrid cross: a cross in which the inheritance of two characters is studied at a time.

Mendel studied the inheritance of two characters:

Seed shape

Round (R) and Wrinkled (r)

Seed colour

Yellow (Y) and Green (y)

Steps of the cross (round green × wrinkled yellow)

Parent: Round, green RRyy × Wrinkled, yellow rrYY

↓

Gametes: Ry | rY

↓ Fertilisation

F₁ generation: RrYy = Round, yellow seeds

↓ Selfing (F₁ × F₁: RrYy × RrYy)

Gametes: RY, Ry, rY, ry | RY, Ry, rY, ry

F₂ generation (Punnett square, 16 boxes)

♀ ♂	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

Round, Yellow Round, Green Wrinkled, Yellow Wrinkled, Green

Phenotypic ratio in F₂

9 : 3 : 3 : 1

Round yellow : Round green : Wrinkled yellow : Wrinkled green

Genotypic ratio in F₂

1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy

Mendel’s actual result (556 seeds): 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green, which is close to 9 : 3 : 3 : 1. Notice that new combinations (round green and wrinkled yellow) appeared in F₂, which shows that the two characters are inherited independently.

⚧ 10. Sex Determination

Sex determination: the process by which the sex of a newborn individual is determined.

Factors affecting sex determination

🧬 Genetically

In humans, the chromosomes (genes) inherited from the parents decide the sex of the offspring.

🌡️ Non-genetically (environmental cues)

In turtles, alligators and crocodiles, the temperature at which the fertilised eggs are kept decides the sex. In snails, individuals can change sex.

Species	Temperature effect	Sex produced
Turtles	Low (26–28°C)	Male 🐢
Turtles	High (31–33°C)	Female 🐢
Crocodiles	Low or high	Female 🐊
Crocodiles	Medium	Male 🐊

Human chromosomes: Autosomes and Allosomes

Human male karyotype — Human karyotype (all 23 pairs of chromosomes arranged in order)

Autosomes	Allosomes (sex chromosomes)
Chromosomes that are not involved in determining sex.	Chromosomes involved in determining the sex of an individual.
Similar in males and females.	Male is XY and female is XX (not similar in males).
In humans there are usually a total of 44 or 22 pairs.	In humans there is usually 1 pair (2 chromosomes).

Sex chromosomes: Male = XY, Female = XX.

Cross between sex chromosomes

Parent: Male XY × Female XX
Gametes: X, Y | X, X

♂ sperm ♀ egg	X	Y
X	XX (Girl)	XY (Boy)
X	XX (Girl)	XY (Boy)

Boy : Girl = 1 : 1 and XY : XX = 1 : 1

Sex determination in human beings

Zygote formed after fusion of gametes. Solid lines = father’s gamete, dashed lines = mother’s gamete.

If a sperm with X chromosome fertilises the egg, the zygote will have XX in the 23rd pair. A zygote with XX will develop into a girl child.
If a sperm with Y chromosome fertilises the egg, the zygote will have XY in the 23rd pair. A zygote with XY will develop into a male (boy) child.
There is a 50% probability of a female child and a 50% probability of a male child.

Remember: The mother always gives an X. The father’s sperm (X or Y) decides whether the baby is a girl or a boy.

11. Quick Revision

Heredity

Passing of traits from one generation to the next

Variation

Differences among individuals of a species

Human chromosomes

46 (23 pairs); gametes 23

Dominant

Capital letter (T), shows in TT and Tt

Recessive

Small letter (t), shows only in tt

Monohybrid F₂

Phenotype 3 : 1, Genotype 1 : 2 : 1

Dihybrid F₂

Phenotype 9 : 3 : 3 : 1

Mendel

Father of Genetics, pea plant, 1856–1863, published 1866

Sex chromosomes

Male XY, Female XX

Boy : Girl

1 : 1 (50% each)

All the best for your exams!

class 10 Biology: Heredity : notes

Heredity

1. What is Heredity?

Eye colour

Flower colour

Earlobes

Thumb

Family looks

2. Inherited Traits vs Acquired Traits

3. Some Important Terms

4. DNA, Chromosomes and Chromosome Number

DNA (Deoxyribo Nucleic Acid)

Chromatin and Chromosome

Haploid and Diploid

Haploid (n)

Diploid (2n)

Chromosome number in human body cells and gametes

5. Genes and Alleles

Genes

Alleles

Dominant Allele vs Recessive Allele

6. Phenotype vs Genotype, Homozygous vs Heterozygous

Phenotype vs Genotype

Homozygous and Heterozygous conditions

Homozygous dominant(pure condition)

Heterozygous(hybrid condition)

Homozygous recessive(pure condition)

7. Mendel and His Contributions

Seven pairs of contrasting traits in the pea plant (Pisum sativum, the garden pea)

Hybridisation experiments

Mendel’s Laws of Inheritance

Breeding pea plants with violet flowers and white flowers

8. Monohybrid Cross (one character)

Steps of the cross (tall × dwarf)

F2 generation (Punnett square)

Phenotypic ratio in F2

Genotypic ratio in F2

Another example: flower colour (Blue × White)

2️⃣ 9. Dihybrid Cross (two characters)

Seed shape

Seed colour

Steps of the cross (round green × wrinkled yellow)

F2 generation (Punnett square, 16 boxes)

Phenotypic ratio in F2

Genotypic ratio in F2

⚧ 10. Sex Determination

Factors affecting sex determination

🧬 Genetically

🌡️ Non-genetically (environmental cues)

Human chromosomes: Autosomes and Allosomes

Cross between sex chromosomes

Sex determination in human beings

11. Quick Revision

Heredity

Variation

Human chromosomes

Dominant

Recessive

Monohybrid F2

Dihybrid F2

Mendel

Sex chromosomes

Boy : Girl

Homozygous dominant
(pure condition)

Heterozygous
(hybrid condition)

Homozygous recessive
(pure condition)

F₂ generation (Punnett square)

Phenotypic ratio in F₂

Genotypic ratio in F₂

F₂ generation (Punnett square, 16 boxes)

Phenotypic ratio in F₂

Genotypic ratio in F₂

Monohybrid F₂

Dihybrid F₂