CLASS 10 SCIENCE NOTES: CHAPTER - HEREDITY

HEREDITY

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1. VARIATION

Variation means differences among individuals of the same species.

Examples:

• In a sugarcane field (asexual reproduction) → very little variation

• In humans (sexual reproduction) → large variation

Key Points:

• Variations are less in asexual reproduction

• Variations are more in sexual reproduction

• Variations help organisms survive in changing environments

Important:

Variations produced during reproduction may be inherited and help in survival of species.

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2. HEREDITY

Heredity is the transfer of characters from parents to offspring.

Inheritance:

The process by which characters pass from one generation to another is called inheritance.

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3. TRAITS

Traits are characteristic features of an organism that can be:

• Physical → height, eye colour

• Physiological → blood group

Acquired Traits	Inherited Traits
These traits develop during a person’s lifetime.	These traits are present in a person since birth.
They occur due to changes in body parts, not in DNA.	They occur due to changes in DNA.
These traits are not passed to children. Example: pierced ears, big muscles due to exercise.	These traits are passed to children. Example: eye colour, skin colour, hair type.

Example:

• Tall plant

• Dwarf plant

• Round seeds

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4. GENES

• Genes are units of heredity

• Genes are the units of heredity which transfer characteristics from parents to their offsprings during reproduction.

• They are present on chromosomes

• Genes are functional units of DNA

• Due to the differences in genetic makeup, human population show a great deal of variations.

• Each gene controls one trait

Example:

• Gene controlling height

• Gene controlling seed colour

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5. ALLELES

Alleles are different forms of the same gene, present at the same position on a chromosome.

Example:

• T → Allele responsible for tallness

• t → Allele responsible for dwarfness

Difference between gene and allele.

Gene	Allele
A gene is a part of DNA that controls a trait.	An allele is a specific type of a gene.
Genes decide which trait will appear.	Alleles decide the different forms of a trait.
Genes do not always work in pairs.	Alleles always occur in pairs.
Examples: eye color, hair color, height.	Examples: blue eyes, brown eyes, tall, short.

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6. IMPORTANT TERMS

Chromosome

Thread-like structures present in the nucleus that carry genes.

Phenotype

The observable physical appearance of an organism
➡ Example: Tall plant, Dwarf plant, Round seeds

Genotype

It refers to genetic code responsible for any given phenotype.
➡ Example: 

• TT is the genotype responsible for tallness of plant
• Tt is the genotype responsible for tallness of plant
• tt is the genotype responsible for dwarfness of plant

Genotype	Phenotype
Genotype means the genes an organism has.	Phenotype means the visible traits of an organism.
It is the genetic makeup inside the body.	It is the physical appearance or expressed character.
It cannot be seen directly.	It can be seen easily.
Example: TT, Tt, tt.	Example: Tall plant, short plant.
Genotype decides what trait will appear.	Phenotype is the result of genotype and environment.

Homozygous

It refers to condition when SAME TYPE allele is present for a given phenotype. 

e.g. (TT)-> homozygous tall

Heterozygous

It refers to condition when DIFFERENT TYPE allele is present for a given phenotype. 

e.g. (Tt)-> homozygous tall

Haploid (n)

Cells with one set of chromosomes
➡ Sperm, Ovum

Diploid (2n)

Cells with two sets of chromosomes
➡ All body cells

F1 Generation

Generation produced as a result of cross fertilization.

F2 Generation 

Generation produced as a result of selfing or self-fertilization of F1 generation.

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7. FATHER OF GENETICS

Gregor Johann Mendel (1822–1884) is known as the Father of Genetics.

Why Mendel chose Pea Plants?

1. Easily visible traits (tall/short)

2. Short life cycle

3. Self-pollination possible

4. Cross-pollination possible

5. Produces many seeds

8. MENDEL’S EXPERIMENTS

What did Mendel do?

• Mendel used several contrasting visible characters in garden peas.

• Examples of these characters were:

  • Round seeds / Wrinkled seeds

  • Tall plants / Short plants

  • White flowers / Violet flowers

• He selected pea plants with different characteristics for his experiments.

• For example, he crossed a tall plant with a short plant.

• He then studied the progeny (offspring) produced from the cross.

• Finally, he calculated the percentage of tall and short plants in the progeny.

9. MONOHYBRID CROSS

A cross involving one pair of contrasting traits

Example: Tall (TT) × Dwarf (tt)

Results:

• F1 Generation: All Tall (Tt)

• F2 Generation:

  • Phenotypic ratio → 3 Tall : 1 Dwarf

  • Genotypic ratio → 1 TT : 2 Tt : 1 tt

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10. DIHYBRID CROSS

• A cross in which inheritance of two pairs of contrasting characters is simultaneously studied.

Example:

• Seed shape → Round / Wrinkled

• Seed colour → Yellow / Green

F1 Phenotypic Ratio:

9 : 3 : 3 : 1

Called Law of Independent Assortment

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11. MENDEL’S LAWS OF INHERITANCE

(i) Law of Dominance

• Every plant gets two genes for height — one from each parent.
• The gene T stands for tall, and t stands for short.
• If a plant has TT or Tt, it will be tall.
• This is because T (tall) is a strong (dominant) gene.
• Only when a plant has tt, it becomes short.
• So, just one T is enough to make the plant tall, but both genes must be t for the plant to be short.

(ii) Law of Segregation

• Alleles separate during gamete formation

• Each gamete carries only one allele

Also called Law of Purity of Gametes

(iii) Law of Independent Assortment

• Each pair of traits is passed on independently from other traits.
• This means one trait does not affect the inheritance of another trait.
• During the formation of gametes (reproductive cells), gene pairs separate independently.
• Because of this, every pair of traits gets an equal chance to appear in the offspring.
• The gene (allele) received for one character does not influence the gene received for another character.

12. HOW DO TRAITS GET EXPRESSED?

• DNA in a cell carries the information needed to make proteins.
• A gene is a small part of DNA that gives instructions to make one specific protein.
• For example, the height of a plant depends on a growth hormone controlled by a gene.
• If the gene works properly and makes more growth hormone, the plant grows tall.
• If the gene is changed and makes less growth hormone, the plant remains short.
• During sexual reproduction, both parents give equal DNA to the next generation.
• Gametes contain one set of chromosomes
• Zygote restores diploid condition
• This ensures stability of species
• Each parent passes one copy of a gene to the offspring.
• For example, when a tall plant is crossed with a short plant, the gametes carry one gene for tallness or one gene for shortness.
• The F1 generation receives one gene for tallness and one gene for shortness from the parents.

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13. SEX DETERMINATION

What is Sex Determination?

The process by which sex of an individual is decided based on genetic material.

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Sex Determination in Humans

Important Points:

• Humans have 23 pairs of chromosomes

• 22 pairs → Autosomes

• 1 pair → Sex chromosomes

Autosomes	Sex Chromosomes
Do not decide the sex of a person.	Decide the sex of a person.
Males and females have the same autosomes.	Males have XY, females have XX.
Humans have 22 pairs of autosomes (44 chromosomes).	Humans have 1 pair of sex chromosomes.
Control body traits like height, skin color, etc.	Control sex and sex-related traits.

Mother always gives X
Father decides sex of child

• X from father → Girl (XX)

• Y from father → Boy (XY)