Being human is the only similarity that we share. Every human being is unique and is different from the other in some respects. In spite of the differences in physical characteristics and psychic behaviors human beings can be grouped according to certain characteristics.
Ayurvedic scholars have postulated a classification of human beings based on the characteristics expressed due to predominance of one or other somatic humor. These difference in physical characteristics and psychic behaviors are stated to be natural among individuals. These natural differences among human beings is termed as Prakruti.
This term Prakruti has been interpreted in Ayurveda as svabhavam which has different meanings viz, inherent property, temperament, constitution etc. Charaka Samhitha the oldest known Ayurveda text has stated that from the moment of conception, some men are equilibrated as regards the three humors, Vata Pitha and Kapha; some have a predominance of Vata, some of Pitha and some of Kapha. This statement clearly indicates that the human beings are divided in to four Prakrutis and that the Prakruti is decided at the time of the fertilization of the ovum with spermatozoon only.

The Prakrutis are as follows:
1. Sama Prakruti: a person with equilibrated humors.
2. Vata Prakruti:  a person with the predominance of Vata.
3. Pitha Prakruti: a person with the predominance of Pitha.
4. Kapha Prakruti: a person with the predominance of Kapha.

Vagbhata, a later luminary, has indicated the possibility of dvandvaja Prakruti, in whom two humors are predominant. But these varieties are stated to be the least variety of person that is that they always suffer from diseases.

Prakruti is not only dependent on the predominant humor at the time of fertilization of the ovum by the sperm but also the nature of the ovum and sperm which take part in the fertilization, nature of the mother's diet and behavior and the nature of the Panchabhutha combinations.
Among these factors, which ever humor or humors are predominant, will be observed to influence the nature of the foetus; therefore men are spoken of as of such and such Prakruti beginning from their foetal life.
Because of their subtlety, the nature of the Panchabhutha combinations at the time of fertilization is not possible to estimate. Similarly the time of the fertilization and its nature also cannot be decided because of the lack of requisite knowledge.

The possibility of a particular humor being predominant can be speculated with the correct knowledge of the mother’s diet and behavior. But sometimes it may not be sufficient to decide that Prakruti of the offspring.
The subject of Prakruti which is based on the fertilization of the ovum by spermatozoon, naturally brings forth the question of heredity also. Susruta Samhitha another important Ayurveda text has described the Adibala pravritha diseases which are attributed to the defects inherent in either the spermatozoon or ovum.

Discussing certain aspects of the development the embryo, Charaka has emphasized that the defects in the limbs or organs of the foetus are due to the defects in the representing the respective limbs in the fertilized ovum.
Chakrapani clarified the terms bijabhaga bijabhagavayava by stating that human bija (fertilized ovum) represents a community of bijabhaga representing the pratyangas of the body.  Therefore the nature of the ovum and spermatozoon which unite to develop in to an offspring, decides the development of different organs of the child, including the sex, genetic faults etc., in addition to the Prakruti.  
Prakruti is being studied as an indicator for genetic predisposition to various diseases by researchers. It can be used as an effective tool for prognosis of diseases.