Sexual Reproduction in Animals
Sexual reproduction is the production of a new organism from two parents by making use of their sex cells or gametes. The humans, fish, frogs, cats and dogs, all reproduce by the method of sexual reproduction.
It is important to know the meaning of some important terms like male sex, female sex, gametes, sperms, ova or eggs, fertilization, zygote and embryo which are involved in sexual reproduction.
- An animal having male sex cells called ‘sperms’ in its body is called male and an animal having female sex cells called ‘ova’ or ‘eggs’ in its body is called female.
- Gametes: The cells involved in sexual reproduction or we can say that they are sexual reproductive cells. They are of two types: male gametes and female gametes. The male gamete in animal is called ‘sperm’ and the female gamete in animals is called ‘ovum’ or ‘egg’. Also female gamete or female sex cell is known by two names: ovum and egg both are same. The plural of ovum is ova. The ovum or egg contains water and stored food. Nucleus is the important part of ovum. Sperm cells are hundreds or thousands time smaller than ovum or egg and have a long tail. The sperm are motile which can move independently with the help of their tails.
- Fertilization: The fusion of a male gamete with a female gamete to form a zygote during the sexual reproduction i.e. the fusion of a sperm with an ovum or egg to form zygote is called fertilization. The zygote is also known as ‘fertilized egg’ or ‘fertilized ovum’. This zygote grows and develops to form a new baby. The stage of development between the zygote or fertilized egg and the newly formed baby is called embryo.
- Internal and external fertilization: The fertilization which occurs inside the female body is called internal fertilization i.e. it takes place in mammals including human beings, birds and reptiles. The fertilization which occurs outside the female body is called external fertilization i.e. it takes place in amphibians like frogs, toads and fishes.
The method in which a zygote grows and develops in to a full organism also varies in different animals. As in humans zygote grows and develops in to a baby inside female body and gives birth to a young ones, in animals like cats, dogs etc also gives birth to young ones but in birds it is entirely different which lay eggs. For example: Hen sits on its egg to give warmth so that zygote grows and develops to form a complete chick. This chick then comes out from egg by breaking its shell. So, all the organisms do not give birth to individuals like humans do.
- It is important to understand that why the amount of DNA does not get doubled during sexual reproduction?
Gametes are also known as reproductive cells which contains only half the amount of DNA or half number of chromosomes as compared to the normal body cells of an organism. So, when male gamete combines with female gamete during sexual reproduction, then the new cell ‘zygote’ will have the normal amount of DNA. Human sperm has 23 chromosomes and human egg has also 23 chromosomes, after fusion 23 + 23 = 46 chromosomes, which is the normal number of chromosomes.
How Sexual reproduction in Animals takes place?
It occurs in following steps:
- Sperms or male gametes are produced by male parent and sperm contains long tail i.e. flagellum for movement.
- Ova, eggs or female gametes are produced by female parent which is a bigger cell than the sperm having a lot of cytoplasm.
- The sperm enters in to the ovum or egg and fuses to form a new cell called ‘zygote’. This process is known as fertilisation. So, the zygote is fertilized ovum.
- The zygote then divides again and again to form a large number of cells, ultimately grows and develops to form a new baby.
Fertilization of an ovum or egg by a sperm to form a zygote.
Advantages of Sexual reproduction:
The Sexual reproduction has many advantages over asexual reproduction. In asexual reproduction, the offspring produced is almost identical to their parent because of same genes. So, much genetic variation is not possible. This is a disadvantage as it inhibits the further evolution of the organism.
In Sexual reproduction the offsprings although similar to their parents, are not identical to them or to one another. Because the offsprings receive some genes from the mother and some from the father. So, mixing of genes makes various different combinations and so all the offsprings have genetic variations. In this way sexual reproduction leads to a greater variety in population i.e. species can adapt more quickly to changes in its surroundings or environment.
We can say that sexual reproduction promotes diversity of characters in the offsprings by providing genetic variation. Sexual reproduction plays an important role in the origin of new species having different characteristics. This genetic variation continuous leads to the evolution of species to form a better and still better organism which is not possible in asexual reproduction.
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