6.2.2 Fertilization

What it is: Fertilization is the union of a male gamete (sperm) and a female gamete (egg) to form a zygote.

What happens:

• It restores the diploid number of chromosomes (46 in humans).
• Each parent contributes half of the genetic material (23 chromosomes each).

Simple explanation: Fertilization is when a sperm and an egg join together. This creates a zygote with a full set of chromosomes — half from the mother and half from the father.

Types of Fertilization

Internal fertilization:

• In human, the fertilization occurs inside the body of female within oviduct.
• Such fertilization is called internal fertilization.

External fertilization:

• In most animals living in aquatic environment such as fish, amphibians etc., the female animal lays its eggs in water at suitable place.
• The male animal then releases its sperms over the eggs.
• This fertilization is called external fertilization.

For Your Information

Development is the process that involves a series of progressive changes, transforming an organism from a simpler stage (zygote) to a more complex one (newborn).

In aquatic organisms that perform external fertilization, development occurs entirely outside the body.

In land organisms that perform internal fertilization, development can occur:

• Completely inside the body
• Completely outside
• Partially inside and partially outside

Types of development:

Difference Between Internal and External Fertilization

Intermediate Level

Gametogenesis and fertilization are the foundation of sexual reproduction.

Gametogenesis:

Fertilization:

Key points:

• Meiosis reduces chromosome number by half.
• Fertilization restores diploid number.
• Internal fertilization occurs in land animals.
• External fertilization occurs in aquatic animals.
• Viviparous = live young.
• Oviparous = shelled eggs.
• Ovoviviparous = eggs hatch inside body.

Important Points

• - Sexual reproduction involves the steps: gametogenesis, fertilization, and development of zygote.
• - The process of gametogenesis and fertilization are closely related and form the foundation of sexual reproduction.
• - Together, they ensure the combination of genetic material from two parents, resulting in offspring with genetic diversity.
• - Gametogenesis is the first step in sexual reproduction in which the specialized sex cells, called gametes, are produced in the reproductive organs.
• - This process involves meiosis, a type of cell division that reduces the chromosome number by half.
• - In males, this process is called spermatogenesis, where sperm cells are produced in the testes.
• - Spermatogenesis is a continuous event that starts from puberty and remains continue throughout the life simultaneously in both testes.
• - In oogenesis, some cells in the ovary form vesicular structures called follicles.
• - Oogenesis is a cyclic event that generally occurs only once in the period of one reproductive cycle.
• - It starts before birth of an individual but arrests in prophase I.
• - At puberty, it restarts and remain continue alternatively in both ovaries till specific age in the life.
• - Fertilization is the union of a male gamete (sperm) and a female gamete (egg) to form a zygote.
• - It restores the diploid number of chromosomes (46 in humans), with each parent contributing half of the genetic material (23 chromosomes each).
• - In human the fertilization occurs inside the body of female within oviduct. Such fertilization is called internal fertilization.
• - In most animals living in aquatic environment such as fish, amphibians etc., the female animal lays its eggs in water at suitable place.
• - The male animal then releases its sperms over the eggs.
• - This fertilization is called external fertilization.

Remember

• - Gametogenesis = production of gametes
• - Spermatogenesis = sperm production in testes
• - Oogenesis = egg production in ovaries
• - Meiosis = reduces chromosome number by half
• - Fertilization = union of sperm and egg
• - Zygote = fertilized egg (diploid)
• - Internal fertilization = inside female body
• - External fertilization = outside body (water)
• - Viviparous = live young
• - Oviparous = shelled eggs
• - Ovoviviparous = eggs hatch inside body

Common Mistakes

• 1. Confusing spermatogenesis and oogenesis.
• - Spermatogenesis = sperm production (4 sperm cells).
• - Oogenesis = egg production (1 egg cell + polar bodies).
• 2. Mixing up internal and external fertilization.
• - Internal = inside female body (land animals).
• - External = outside body in water (aquatic animals).
• 3. Forgetting the role of meiosis.
• - Meiosis reduces chromosome number by half.
• 4. Thinking fertilization restores haploid number.
• - Fertilization restores diploid number.
• 5. Confusing viviparous, oviparous, and ovoviviparous.
• - Viviparous = live young.
• - Oviparous = shelled eggs.
• - Ovoviviparous = eggs hatch inside body.
• 6. Believing oogenesis produces four functional eggs.
• - Oogenesis produces one functional egg and polar bodies.
• 7. Thinking spermatogenesis occurs only in adults.
• - Spermatogenesis starts at puberty and continues throughout life.

Quick Revision

60-Second Revision

• Gametogenesis = production of gametes
• Spermatogenesis = sperm production in testes (4 sperm cells)
• Oogenesis = egg production in ovaries (1 egg cell + polar bodies)
• Meiosis = reduces chromosome number by half
• Fertilization = union of sperm and egg → zygote (diploid)
• Internal fertilization = inside female body (land animals)
• External fertilization = outside body in water (aquatic animals)
• Viviparous = live young
• Oviparous = shelled eggs
• Ovoviviparous = eggs hatch inside body
• Warning: Do not confuse spermatogenesis (4 sperm) with oogenesis (1 egg + polar bodies). Internal fertilization occurs inside the body; external fertilization occurs outside.