CHAPTER 4: HEREDITY

1. Heredity

Definition:

Heredity is the natural process by which parents transmit their traits to their offspring through genes.

Traits passed from parents to offspring are called hereditary characters.

Genetics:

Genetics is the branch of biology that studies genes, heredity, and variation.

2. Chromosomes, DNA and Genes

Chromosomes

• Thread-like structures present in the nucleus.
• Made of DNA and proteins.
• Carry hereditary information.

Humans:

• Somatic cells: 46 chromosomes (2n)
• Gametes: 23 chromosomes (n)
• 22 pairs = Autosomes
• 1 pair = Sex chromosomes

Male: XY
Female: XX

Gene

A gene is a small segment of DNA that controls a specific trait.

Allele

Alternative forms of a gene.
Example: T (tall) and t (dwarf).

Genotype

The genetic makeup of an organism (TT, Tt, tt).

Phenotype

The visible expression of a trait (Tall or Dwarf).

Homozygous

Having identical alleles (TT or tt).

Heterozygous

Having different alleles (Tt).

3. DNA and RNA

DNA (Deoxyribonucleic Acid)

• Double-stranded structure
• Found in nucleus (eukaryotes)
• Made of nucleotides:

1. Sugar (deoxyribose)
2. Phosphate
3. Nitrogen bases: A, T, G, C

• Function: Stores and transmits hereditary information

RNA (Ribonucleic Acid)

• Single-stranded
• Sugar: Ribose
• Bases: A, U, G, C
• Types:

1. mRNA
2. tRNA
3. rRNA

• Function: Helps in protein synthesis

4. Cell Division

A. Mitosis

• Occurs in somatic cells
• 1 diploid cell → 2 identical diploid cells
• Maintains chromosome number (2n → 2n)
• Functions:

1. Growth
2. Repair
3. Asexual reproduction
4. Genetic stability

B. Meiosis

• Occurs in reproductive cells
• 1 diploid cell → 4 haploid cells
• Chromosome number reduced (2n → n)
• Crossing over occurs
• Produces variation

Function:

• Sexual reproduction
• Genetic variation
• Basis of evolution

Difference Between Mitosis and Meiosis

5. Fertilization and Sex Determination

Fertilization

Fusion of male gamete (sperm) and female gamete (ovum) to form a zygote.

Sperm: 22 + X or 22 + Y
Ovum: 22 + X

Zygote:
44 autosomes + XX (Female)
44 autosomes + XY (Male)

Chance of male or female child = 50%

Sex is determined by the father.

6. Mendelism

Gregor Johann Mendel is known as the Father of Genetics.

He conducted experiments on pea plants.

Reasons for choosing pea plants:

• Short life cycle
• Large number of seeds
• Many contrasting traits
• Easy to cultivate
• Self-pollinating

7. Monohybrid Cross

Cross between parents differing in one pair of contrasting traits.

Example:
TT (Tall) × tt (Dwarf)

F1 Generation:
All Tt (Tall)

F2 Generation:
Phenotypic ratio = 3 Tall : 1 Dwarf
Genotypic ratio = 1 TT : 2 Tt : 1 tt

Punnett square is used to show genetic crosses.

8. Dihybrid Cross

Cross involving two pairs of contrasting traits.

Phenotypic ratio in F2:
9 : 3 : 3 : 1

9. Mendel’s Laws

1. Law of Dominance

In F1 generation, only the dominant trait appears.

2. Law of Segregation

Alleles separate during gamete formation.

3. Law of Independent Assortment

Different traits assort independently during gamete formation.

10. Seven Pea Traits

11. Genetic Technology

Definition:
Manipulation of genes to develop desired traits.

Techniques:

• Genetic engineering
• Gene transfer
• Molecular markers

Applications:

• High-yield crops
• Disease treatment
• DNA fingerprinting

12. Selective Breeding

Definition:
Breeding organisms with desirable traits to produce improved offspring.

Types:

• Inbreeding
• Crossbreeding
• Line breeding
• Self-pollination

Advantages:

• Improved quality
• High yield
• Disease resistance

Disadvantages:

• Reduced genetic diversity
• Risk of sterility
• Loss of natural traits

13. Artificial Insemination (AI)

Definition:
Introduction of semen into female reproductive tract without mating.

Advantages:

• Cost effective
• Disease control
• Long-distance fertilization

Disadvantages:

• Requires skill
• Risk of infection

14. In Vitro Fertilization (IVF)

Definition:
Fertilization outside the body in laboratory.

Steps:

1. Egg collection
2. Fertilization in lab
3. Embryo transfer

Advantages:

• Helps infertile couples
• Reduces genetic disorder risk

Disadvantages:

1. Expensive
2. Emotional stress
3. Risk of multiple births

15. Important Concepts

Variation

Differences between individuals of the same species.

Evolution

Gradual change in organisms over time.

Genetic variation produced during meiosis is the basis of evolution.

Summary

Heredity is the transmission of traits through genes located on chromosomes. DNA stores genetic information. Mendel explained inheritance through his laws of dominance, segregation, and independent assortment. Mitosis maintains chromosome number, while meiosis produces variation. Modern genetic technologies and reproductive techniques improve agriculture and medicine.