Heredity class 10 Notes

 

Heredity class 10 Notes

Introduction

• Reproductive processes produce new individuals that are similar but show some variations, even in asexual reproduction.
• Sexual reproduction increases the chances of successful variations, which is why organisms like humans show more visible differences than plants like sugarcane.
• This chapter studies how variations are produced and how they are inherited from one generation to the next.

Accumulation of Variation During Reproduction

• Accumulation of variation means the gradual build-up of small differences (variations) in organisms over many generations during the process of reproduction. 
• These variations are passed from parents to offspring and get added generation after generation.

How does variation arise during reproduction?

1. Asexual Reproduction

• Only one parent is involved.

• Offspring are almost identical to the parent.

• Small variations occur due to:

  • Errors in DNA copying

  • Environmental effects

• Variations are few and slow to accumulate.

Example:
Sugarcane, bacteria

2. Sexual Reproduction

• Two parents are involved.

• Variation arises due to:

  • Mixing of genes from both parents

  • Formation of different gametes

  • Random fertilisation

• Variations are more, useful, and accumulate faster.

Example:
Humans, animals, flowering plants

Why is accumulation of variation important?

1. Helps in survival
   Some variations help organisms survive sudden environmental changes.

2. Leads to evolution
   Accumulated variations over many generations can lead to the formation of new species.

3. Improves adaptability
   Species with more variations adapt better to changing conditions.

Example to understand

• In bacteria, a small DNA change may give resistance to antibiotics.
• Over many generations, these variations accumulate, and resistant bacteria survive.

Heredity

• The main outcome of reproduction is the generation of similar individuals, and heredity rules govern how traits and characteristics are reliably passed to offspring.

Inherited Traits

• Inherited traits are the characteristics or features that are passed from parents to their offspring through genes. 
• These traits are encoded in DNA and are responsible for the similarities seen between parents and their children.

Example: Eye color, hair color, blood group, height, shape of nose, and dimples.

Types of Inherited Traits

1. Physical Traits (Morphological Traits)

   • Traits related to appearance or structure of the body.

   • Examples: Skin color, eye color, hair type, height.

2. Physiological Traits

   • Traits related to functioning of the body.

   • Examples: Blood group, lactose tolerance, ability to digest certain foods.

3. Behavioral Traits

• Traits related to actions or behavior.

• Examples: Nest-building in birds, hunting skills in animals (some are instinctive).

How are Inherited Traits Transmitted?

• Through genes, which are segments of DNA.

• Genes are located on chromosomes in the nucleus of cells.

• During sexual reproduction, offspring inherit half of the genes from each parent, which determines their traits.

In short:Inherited traits are characteristics passed from parents to offspring through genes.

Rules for the Inheritance of Traits – Mendel’s Contributions

• Gregor Johann Mendel (1822–1884), known as the Father of Genetics, conducted experiments on pea plants and discovered the fundamental laws of inheritance. 
• His work explains how traits are passed from parents to offspring.

Mendel’s Experiments

• Mendel used pea plants because they have easily distinguishable traits (like tall/short, green/yellow seeds).

• He crossed plants with contrasting traits and observed the traits in offspring for several generations.

• He noticed patterns in inheritance, which led to the formulation of Mendel’s Laws.

Mendel’s Key Observations

1. Some traits dominate over others (e.g., tallness over shortness).

2. Traits are inherited as discrete units (now called genes).

3. Traits appear in predictable ratios in offspring.

Mendel’s Laws of Inheritance

1. Law of Dominance

• When two different forms of a trait (alleles) are present in an individual, one allele dominates and determines the appearance.

• The other allele is recessive and does not appear in the F1 generation.

Example:

• Tall (T) × Short (t) → All F1 plants are Tall (Tt).

2. Law of Segregation

• During gamete formation, the two alleles of a gene separate, so each gamete gets only one allele.

• During fertilization, alleles combine randomly to produce offspring.

Example:

• Tt × Tt → F2 generation shows 3 Tall : 1 Short ratio.

3. Law of Independent Assortment

• Different traits are inherited independently of each other.

• The inheritance of one trait does not affect the inheritance of another trait.

Example:

• Seed color (yellow/green) and seed shape (round/wrinkled) are inherited independently.

Significance of Mendel’s Work

1. Explained the mechanism of inheritance.

2. Introduced the concept of genes and alleles.

3. Formed the basis of modern genetics and study of variation.

4. Helps in predicting offspring traits through Punnett squares.

Key Terms

Term	Meaning
Gene	Unit of inheritance
Allele	Different forms of a gene
Dominant allele	Allele that masks another allele
Recessive allele	Allele masked by dominant allele
Homozygous	Two identical alleles for a trait (TT or tt)
Heterozygous	Two different alleles for a trait (Tt)

In short:Mendel discovered the basic rules of inheritance, showing that traits are passed from parents to offspring through discrete units called genes.

How do these Traits get Expressed?

• Traits are expressed when the information present in genes is used to produce visible characteristics in an organism. 
• Genes control traits by directing the production of proteins, which perform specific functions in the body.

Role of Genes in Trait Expression

• Genes are segments of DNA present on chromosomes.

• Each gene carries instructions to make a specific protein.

• These proteins control:

  • Physical features (like height, color)

  • Body functions (like enzymes, hormones)

Thus, traits are expressed through proteins made by genes.

Gene → Protein → Trait

1) Gene (DNA) contains information.

2) DNA instructions are used to make proteins.

3) Proteins determine traits by affecting structure or function.

Example:

• A gene that produces a functional enzyme leads to normal trait expression.
• A changed gene may produce a non-functional protein, leading to a different trait.

Example: Height in Plants

• Gene for tallness produces growth hormone (protein) → plant grows tall.
• Gene for shortness produces less or non-functional hormone → plant remains short.

Dominant and Recessive Traits

• A dominant gene produces enough functional protein to express the trait.
• A recessive gene may produce less or non-functional protein and is expressed only when dominant gene is absent.

Example:

• Tall (T) is dominant over Short (t).
• Tt plant is tall because dominant gene expresses the trait.

Effect of Environment

• Environment can influence the expression of traits but cannot change the gene.
• Example: Height depends on genes but is also affected by nutrition.

Sex Determination

• Sex determination is the biological process by which the sex of an individual (male or female) is decided at the time of fertilisation, depending on the type of sex chromosomes present.

Sex Determination in Humans

• Humans have 23 pairs of chromosomes:
• 22 pairs are autosomes
• 1 pair is the sex chromosome

Gender	Sex Chromosomes
Male	XY
Female	XX

Role of Parents

• Mother always contributes X chromosome.
• Father contributes either X or Y chromosome.

Fertilisation	Child’s Sex
X (mother) + X (father)	Female (XX)
X (mother) + Y (father)	Male (XY)

Therefore, father is responsible for determining the sex of the child, not the mother.

Sex Determination Diagram Explanation (Conceptual)

• Half of the sperms carry X chromosome.
• Half carry Y chromosome.
• Egg carries only X chromosome.
• Equal probability of male or female child (50%).

Sex Determination in Other Organisms

1. Birds

• Female: ZW
• Male: ZZ
• Female determines the sex.

2. Insects (Grasshopper)

• Male: XO
• Female: XX

3. Reptiles

• Sex determined by environmental temperature.

Importance of Sex Determination

1) Maintains balance in population.

2) Ensures continuity of species.

3) Helps understand genetic inheritance.

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