“Heredity” is one of the important chapters in SSLC Science. This chapter explains how traits are inherited from parents to offspring, Mendel’s laws of inheritance, dominant and recessive traits, and sex determination in humans. This article is prepared based on the uploaded English LBA document.
Learning Points
This chapter mainly includes:
- Combination of differences during reproduction
- Heredity – Inherited traits
- Traits and Laws of Inheritance – Mendel’s contributions
- Sex determination
Heredity
The process by which traits are transferred from parents to offspring is called Heredity.
Importance of heredity:
- Transfers parental traits to next generation
- Maintains continuity of species
- Produces variations
- Helps evolution and adaptation
Gregor Mendel – Father of Modern Genetics
Gregor Mendel is called the Father of Modern Genetics.
He conducted experiments using pea plants and explained the laws of inheritance.
Mendel selected pea plants because:
- They have contrasting traits
- Self-pollination and cross-pollination are possible
- Easy to grow and maintain
Traits
Characteristics that are inherited from parents to offspring are called Traits.
Examples:
- Tallness and dwarfness
- Flower colour
- Seed shape
- Eye colour
Dominant and Recessive Traits
Dominant Trait
A trait that appears in the offspring generation is called a Dominant Trait.
Example:
Tall plant (T)
Recessive Trait
A trait hidden in presence of dominant trait is called a Recessive Trait.
Example:
Dwarf plant (t)
Mendel found that tallness is dominant over dwarfness.
Monohybrid Cross
A cross involving one pair of contrasting characters is called a Monohybrid Cross.
Example:
Tall plant × Dwarf plant
Parent Generation:
TT × tt
F₁ Generation:
All plants are tall (Tt)
F₂ generation phenotypic ratio:
3 : 1
Genotypic ratio:
1 : 2 : 1
Dihybrid Cross
A cross involving two pairs of contrasting traits is called a Dihybrid Cross.
Example:
Round yellow seeds × Wrinkled green seeds
F₂ generation ratio:
9 : 3 : 3 : 1
This ratio was obtained in Mendel’s dihybrid experiment.
Mendel’s Laws of Inheritance
Law of Dominance
Dominant traits express themselves in F₁ generation while recessive traits remain hidden.
Law of Segregation
Traits separate during gamete formation.
Law of Independent Assortment
Different traits assort independently during inheritance.
These laws explain inheritance of traits.
Gene and Chromosome
Gene
A gene is the basic unit of heredity.
Genes carry information for protein production and inheritance of traits.
Chromosome
Chromosomes are thread-like structures carrying genes.
They are present inside the nucleus.
Sex Determination in Humans
In humans, sex is determined by sex chromosomes.
Female:
XX
Male:
XY
XX + XY
Mother always contributes to the X chromosome.
Father contributes either X or Y chromosome:
- X + X → Female child (XX)
- X + Y → Male child (XY)
Therefore, the father determines the sex of the child.
Importance of Variation
Variation means differences among individuals of the same species.
Importance:
- Helps adaptation to environment
- Improves survival chances
- Supports evolution process
Sexual reproduction produces more variations than asexual reproduction.
Difference Between Dominant and Recessive Traits
| Dominant Trait | Recessive Trait |
| Expressed in offspring | Hidden in presence of dominant trait |
| Represented by capital letter | Represented by small letter |
| Appears in F₁ generation | Appears in F₂ generation |
Important Exam Questions
One Mark Questions
- Who is called the Father of Modern Genetics?
- Which plant was used by Mendel?
- What is heredity?
- Define genes.
Two Mark Questions
- Differentiate dominant and recessive traits.
- Write differences between male and female chromosomes.
- Explain monohybrid cross.
Three and Four Mark Questions
- Explain Mendel’s monohybrid experiment.
- Explain dihybrid cross.
- Explain sex determination in humans.
- How do Mendel’s experiments show dominant and recessive traits?
Preparation Tips for Students
- Learn Mendel’s experiments carefully
- Practice monohybrid and dihybrid ratios
- Revise sex determination diagram
- Learn dominant and recessive traits clearly
- Study genes and chromosomes regularly
Conclusion
“Heredity” is an important scoring chapter in SSLC Science. Understanding Mendel’s experiments, inheritance laws, genes and sex determination helps students score better marks in examinations.
