Twins

Twins
Definition
The two children which develop and born together is called Twins.
Types of Twins
There are two types of Twins
1. Identical Twins
2. Fraternal Twins
1. Identical Twins

• Identical twins are exactly alike and of the some sex because both of them develop from just one zygote (monozygotic).
• One zygote divides mitotically into two separate blastomeres, each of which develops into a new individual.
• Sine both of those develop from one zygote they share the same genes and arc exactly identical in features.

2. Fraternal Twins

• In fraternal twins, the children are develops from two independent eggs.
• As each egg is fertilized by a separate sperm they are dizygotic.
• The genotype of each zygote is different hence they are not identical and are said to be the product of sexual reproduction.

Significance of Sexual Reproduction
Sexual reproduction has much significance which is as follows
1. Sexual reproduction is important to avoid genetic monotony, which is the result of asexual reproduction where generation after generation exactly identical progeny develops.
2. Sexual reproduction maintains the chromosome number in a species.
3. Through sexual reproduction variety of organisms are produced.
4. The diversity of characteristics increases the chances of survival of a species are far brighter in an unfavourable environment or during the out break of disease.
5. The genetic variation thus produced by sexual reproduction is the foundation of evolutionary changes.
Necessities of Sexual Reproduction
Sexual reproduction is a complicated process and it occurs in following steps
1. Gametogenesis
2. Mating
3. Fertilization
1. Gametogenesis
Definition
The process of formation of sex-cell (gametes) is called Gametogenesis.
Types of Gametogenesis
There are two types of gametogenesis
i. Spermetogenesis
ii. Oogenesis
i. Spermetogenesis
Definition
The process of gametogenesis which involves the formation of male gamete i.e. sperm is called spermatogenesis.
Process of Spermatogenesis
i. Male Gonad Testes
The spermatogenesis takes place in male gonad called Testes. Which contain spermatocytes producing mother called germ cells.
ii. Spermatogonia
In the testes germ cells take part in the spermatogenesis. The germ cells produce spermatogonia.
iii. Primary Spermatocytes
The spermatogonia are changed into primary spermatocytes.
iv. Secondary Spermatocytes
Each primary spermatocyte divides into two cells by meiosis called secondary spermatocytes.
v. Spermatids
Secondary spermatocytes divide further into four haploid spermatids.
vi. Sperm
Spermatids change into sperms
ii. Oogenesis
Definition

The process of gametogenesis which involves the formation of female gamete i.e. ova or egg cell is called Oogenesis.
Process of Oogenesis
i. Female Gonad Ovary
The oogenesis takes place in female gonad called ovary which contain ova producing mother cells called germ cells.
ii. Oogania
In the ovary, the germ cells change into special cells called oogania.
iii. Primary Oocytes
The oogania convert into primary oocytes.
iv. Secondary Oocytes
Primary oocytes divide into two cells by meiosis called Secondary oocytes.
v. Ovum
Secondary oocytes divide further into four cells, one ovum and three polar bodies which degenerates while ovum takes part in fertilization.
Development
Internal fertilization show different types of development.
1. Oviparity
2. Viviparity
1. Oviparity
Definition
The animals which lay eggs in the environment are called oviparous animals and the process is called Oviparity.
Properties of Oviparous Egg

• Eggs of oviparous animals are usually large in number.
• In terrestrial animals, eggs are always protested by tough water proof shells.
• The egg of aquatic oviparous animals covered over by gelatinous membrane.
• The egg of oviparous animals are large size.
• Oviparous egg contain enough store nutrients and yolk for embryo.
• Oviparous animals completes its development inside the egg before hatching.

Examples
Birds and Reptiles
2. Viviparity
Definition
The animals which given birth to young ones are called viviparous animals and the process is called Viviparity.
Properties of Viviparous Egg

• Eggs of viviparous animals are usually small in number.
• Viviparous animal eggs required no shells because they are well protected inside the females body.
• Eggs of viviparous animals are small in size.
• Viviparous egg contain very small amount of yolk because developing embryo in viviparous animals is taken by its mother with the help of its placenta.

Example
Mammals (Pouch mammals and placental mammals)