Modes of Reproduction - Sexual Reproduction

A mode of reproduction involving the fusion of male and female gametes (sperm and egg) to form a zygote that develops into a new organism.

The period of growth and development before an organism becomes sexually mature is called the juvenile phase.

Sexual reproduction is the mode of reproduction in which male and female gametes fuse to form a zygote, which develops into a new organism.

• It usually involves two parents.
• The process includes the formation and fusion of gametes.
• It produces genetic variation in the offspring.
• Genetic variation supports adaptation and evolution over generations.

A. Juvenile Phase

• This is the pre-reproductive phase of life.
• During this stage, the organism grows but does not reproduce.
• In plants, this phase is often called the vegetative phase.

B. Reproductive Phase

• During this phase, the organism attains maturity and becomes capable of reproduction.
• In animals, this stage begins after puberty.
• In flowering plants, this stage is marked by flowering and seed formation.

C. Senescent Phase

• This is the post-reproductive stage associated with ageing.
• Physiological functions decline gradually during this phase.

1. Formation of male and female gametes.
2. Transfer or meeting of gametes.
3. Fusion of gametes during fertilisation.
4. Formation of zygote.
5. Development of embryo and then a new individual.

• Pre-fertilisation events: Formation and transfer of gametes.
• Fertilisation: Fusion of male and female gametes.
• Post-fertilisation events: Zygote formation, embryo development, and growth into a new individual.

In Plants

• Flowering marks the reproductive phase in flowering plants.
• Some plants flower only once in their lifetime and are called monocarpic plants.
• Some plants flower repeatedly in different seasons or years and are called polycarpic plants.

In Animals

• Some animals reproduce only in a particular breeding season and are called seasonal breeders.
• Some animals reproduce throughout the year and are called continuous breeders.

• Sexual reproduction involves the formation and fusion of male and female gametes (amphimixis) through meiotic division to create a diploid zygote.
• Life cycles progress from an initial juvenile phase focused purely on vegetative growth and development to a mature reproductive phase in which sex organs become functional.
• The reproductive process is strictly regulated by hormones and sequence-driven, consisting of three sequential stages: pre-fertilisation, fertilisation, and post-fertilisation.
• Pre-fertilisation focuses on gamete production and transfer, while post-fertilisation events track zygote development and the formation of embryonic structures.
• Meiotic cell division and cross-fertilisation blend parental genetic material, generating the vital variations needed to drive species adaptation and organic evolution.