Sexual Reproduction in Animals

• It is the process which involves the production of offspring by the formation and fusion of gametes. It is also called amphimixis. In animals, gamete formation primarily involves meiosis.
• Sexually reproducing animals shows two main phases in their life time.
• Fusion of gametes is called fertilization which results in variation.

Why the sexual mode of reproduction:-

• Sexual reproduction provides variation in DNA thereby making the offspring adapted for better survival. Sexual reproduction ensures the mixing of gene pool of the species. Due to genetic recombination, variations occur in the process of sexual reproduction.
• Reproduction incorporates such a process of combining DNA from two different individuals during reproduction.
• But this creates a major difficulty. If each new generation is to be the combination of the DNA copies from two pre-existing individuals, then each new generation will end up having twice the amount of DNA that the previous generation had.
• One solution that many multi-cellular organisms have found for the problem mentioned above is to have special lineages of cells in specialised organs in which only half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells. This is achieved by a process of cell division called meiosis.
• When such germ cell form two individuals unite during sexual reproduction the normal chromosome number and DNA content are restored.
• In multicellular organism body design become more complex, the germ cells also specialize. One germ cell is large and contains the food stored while the other is smaller and likely to be motile. The motile germ cell is called the male gamete and the germ cell containing the stored food is called the female gamete.

Sexually reproducing animals shows two main phases in their life time. 

1. Juvenile phase - The earlier juvenile phase mainly represents physical growth phase starting from birth. The animals cannot reproduce sexually in this phase. 

2. Reproductive maturity phase- The later Reproductive maturity phase is attained usually after physical growth is almost over. It involves growth and activity of the sex organs. Animals can reproduce sexually in this phase. 

After attainment of maturity, all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarities, even though the structures associated with sexual reproduction are indeed very different. The events of sexual reproduction though elaborate and complex, follow a regular sequence. Sexual reproduction is characterised by the fusion (or fertilisation) of the male and female gametes, the formation of zygotes and embryogenesis. For convenience, these sequential events may be grouped into three distinct stages namely,

• Pre-fertilisation events
• Fertilisation events
• Post-fertilisation events

• Sexual reproduction involves the formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex.
• These gametes fuse to form the zygote which develops to form the new organism.
• It is an elaborate, complex and slow process as compared to asexual reproduction.
• Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves.
• Even with different external morphology, anatomy, and physiology, sexual reproduction in the patterns of plants, animals, and fungi is similar.
• All organisms have to reach a certain stage of growth and maturity in their life before they can reproduce sexually.
• Sexual reproduction can be grouped into three distinct stages pre-fertilization, fertilization and post-fertilization events.

• The involvement of two parents results in the intermingling of genes resulting in the production of a new offspring. 
• Gamete formation and fertilization take place.
• The whole process is slow and lengthy.
• Genetically identical offspring are produced.
• Offspring are different from parents, genetically and physically.
• Variations in species increase the chances of survival hence the evolutionary advancements.