Sexual reproduction is a complex process involving two individuals or parents of different sexes i.e. male and female. The formation of two different types of gametes by male and female takes place in their gonads i.e., testis in male and ovary in female. The sperms are produced in the testis and ova are produced by ovary. Therefore testes and ovaries are called primary sex organs. The spermatozoa are formed in the testis by the process of spermatogenesis and ova in the ovary by the process of oogenesis. The spermatogenesis and oogenesis are altogether known as gametogenesis. The formation of gametes by gonads is the primary sex character of male and female. In association with this, secondary sex characters also develop in sexually reproducing animals.

The gametes from the two sexes of a species are quite different in structure and behaviour. Sperms are very small, active and motile but ova are comparatively larger, passive, non-motile and receptive in nature. Both the gametes are haploid and fuse with each other in the process of fertilization.

Fertilisation

It is the common phenomenon seen in all sexually reproducing animals. Fertilisation may be of two types: external fertilisation and internal fertilisation. When fertilisation occurs outside the body of parents i.e., in water, this is called external fertilisation. This is mostly found in invertebrates and some lower aquatic vertebrates such as fishes and amphibians. When external fertilisation takes place, the sperms and ova are shed in large numbers, approximately during same time and place to facilitate fertilisation. But when fertilisation takes place inside the body of the parent i.e., female, it is called internal fertilisation. It is generally seen in higher vertebrates such as reptiles, birds and mammals including man. In these animals in addition to gonads, the accessory reproductive structures are also developed which facilitate internal fertilisation. The male possesses penis as copulatory organ and the female possesses vagina ti receive the penis during copulation. During copulation male ejaculates the seminal fluid containing sperms into the vagina of the female from which sperms travel through the oviduct and as a result, formation of zygote takes place. The zygote is diploid in nature.

When the gametes are similar in size and form and not differentiated into male and female, such gametes are called isogametes and their union is called isogamy. It occurs mainly in protozoa such as Monocystis. The gametes are produced by multiple fission and usually they are provided with flagella. In majority of cases, two fusing gametes differ in size, shape and behaviour. Such dissimilar gametes are called heterogametes and their fusion is known as heterogamy. The gametes differ from each other in their morphology and physiology. The small gametes are motile and active and generally provided with long tail. They function as male gametes. The larger gametes are passive, non-motile and termed as female gametes. So sexual reproduction depends on the union of a sperm with an ovum to form a zygote from which a new individual develops.

In the case of viviparous animals the development of zygote takes place inside the body of the mother. Complete embryonic development takes place in the uterus of mother. The period of carrying the embryo till complete development is called gestation period. It varies greatly in different mammals. For example, in human being 280 days. In birds and reptiles the egg is laid onto the land after fertilisation where development takes place and eggs are covered with hard protective porous shell.

Hermaphroditism

When an animal has both functional male and female reproductive systems, along with their gonads i.e., testis and ovary, it is called hermaphrodite. The dual sexuality is called monoecious condition. Although some hermaphrodites fertilise themselves but most of them mate with another member of the same species. When this occurs, each animal serves as both male and female i.e. donating and receiving sperms. Hermaphroditism is beneficial to sessile animals. Among vertebrates, hermaphroditic individuals occasionally occur but as an abnormality, reported in fishes and amphibians.

Parthenogenesis

When egg without fertilisation can develop into an organism, it is called Parthenogenesis. This is treated as a mode of modified sexual reproduction. It is a spontaneous activation of a mature egg, followed by normal egg division and subsequent embryonic development. This is seen in certain flat worms, rotifers, round worms, lobster and insects like bees, wasps and ants. In honey bee, a large number of males (Drones) are produced parthenogenetically whereas sterile females workers and reproductive female (Queen) are produced sexually from fertilized ova.

Basic Features of Sexual Reproduction

Sexual reproduction is common in vertebrates and in invertebrates but exclusively occurring in vertebrates. Various basic features of sexual reproduction are:

-         The gametes are reproductive units of sexual reproduction.

-         Two individuals or parents belonging to two different sexes.

-         The gametes are formed in the gonads. i.e. in testis and ovary through the process of meiotic division. As a result, haploid numbers of chromosomes are retained by gametes.

-         Fusion of two haploid gametes, the sperm and ovum takes place by the process of fertilisation. As a result, the zygote is formed which is diploid in nature. So fertilisation forms the basis of sexual reproduction.

-         The zygote develops into an embryo or new organism, which contains diploid number of chromosomes. Due to reduction division, the chromosomes number remains constant in the species from generation to generation.

-         New individuals are genetically different from parents and also different from each other showing great variations among themselves. These variations are important from evolutionary point of view.

-         This is a slower method of reproduction and complex one.

Merits of Sexual Reproduction

-         In sexual reproduction, each parent individual produces a variety of gametes. The gametes differ in the kinds of hereditary units or genes that they contain. When sperms and ova combine during fertilisation, many different variations arise. So, raw combination of characters can arise more rapidly because of genetic combination.

-         The great advantage of sexual reproduction is the variations in individuals that it produces. This variation is the raw material for organic evolution.

-         It ensures a variety of genetic type in the population on which evolution depends.

-         The resulting genetic diversity and variability increase the chance of the species surviving under sudden environmental changes.

-         Copies of deleterious and lethal mutations tend to be eliminated from sexually reproducing population.

Demerits of Sexual Reproduction

-         An animal which can not reproduce asexually can never handover willfully its own exact set of genetic material to its progeny.

-         Sexual reproduction hand over the responsibility to the progeny a reassortment of maternal and paternal chromosomes. Thus the same mixing process that created the adaptive gene combinations in the adult, work to dismantle it partially in the offspring.

-         In addition, many of the gametes that are released will not be fertilised, leading to a significant waste of metabolic efforts.