The gotra exogamy, as a tradition among Hindus, prohibits marriage between members of the same gotra. The Hindu society traditionally expects people to marry someone outside their gotra. But The Hindu Marriage Act of 1955 legalizes marriage between members of the same gotra.
Sagotra exogamy, i.e., marrying outside one’s own “gotra” is very much prevalent even today among the upper castes such as the Brahmins, Kshatriyas and Vaishyas.
Gotra exogamy applies in its fullness to all the “twice-born” castes. These castes have a tradition of descent from certain sages who are believed to have lived in the remote past and two persons claiming descent from the same “gotra” were forbidden from marrying each other.
The gotra of a family is named after the Rishi or sage who founded the family in the immemorial past. There were only a few gotras (say, eight, according to a source) in the beginning, but in course of time, their numbers increased enormously.
There were no restrictions on gotra marriage up to 600 B.C.
The earliest mention of Gotra system in its technical sense is found in the Chandogya Upanishad, where the guru asks his pupil Satyakama Jabali his gotra. By the time of Gautama Buddha, that is, 6th century B.C., the gotra system had become an established institution. It is said that Smritikara Manu imposed restrictions on marriage in one’s own gotra.
It may be added that several ambitious non-Brahmanical castes have either claimed descent from traditional gotra-risis or have invented new gotras. The Lingayats of Mysore have gotras which are quite different from the Brahmanical gotras. Gujars, Ahirs, Jats and other castes in villages near Delhi have gotras but these are also different from Brahmanical gotras. Often, a non-Brahmin caste which was divided into exogamous clans with each clan claiming descent from a plant or animal changed the totem names of gotra-rsis.
Pravara is closely connected with “gotra” or family. Just as the sagotra marriages, sapravara marriages are also forbidden. People belonging to the same pravara cannot intermarry.