Mythology:
The name comes probably from Lat. gignere, because this deity was assigned to each human when he was conceived or he was taken under his protection (Varro), or he has created us himself or has been created together with us (Apuleius).

It's clear, that the Romans tried to integrate the Genius into their mythology. His parents should have been Jupiter and Gaea, who has born him, after Jupiter has created him when he was asleep. Others suggest that he was a son of the gods and the father of men. In any case all suggest that the Genii - there are many of them! - take a middle position between the gods and men. As soon as a human being was born one or two Genii were assigned to him, a white good one and a black bad one. The good one gives him all of his good thoughts, the bad one the contrary. Which ever is the stronger one he is the one who forms the character of the man. Genii always appear at males. Within women there were the Junones. The Genii stayed with their person until his death when they gave him to the gods. According to others each man has only one Genius. The Genius handed down his man to the court and blamed him if he was lying or praised him when he kept the truth. According to the Genius the judgement was given to him because the Genius knew all of his secret thoughts. Even families, cities and countries had these guardian spirits. The Genius of Rome had a golden statue in the VIII regio. 

Everyone gave honour to his own Genius, especially at his birthday when he gave offerings to him but only flowers, wine and incense, because it was not allowed to kill any animal on this day. A vow done by the Genius of the emperor was the most steadfast oath as if it was done by Jupiter himself. Some suggested that the Genius was identical with the Animus, others that he actually was the mind of men. But because also mountains, swamps, lakes, fountains, valleys and forests had their own Genius it could be concluded that he actually was a fictive entity invented only to put the humans in fear and to prevent them from vice (Hederich).

Background:
The Genius is the 'power' which is inherent in man, not only becoming manifest in his virility but signifying extensively his whole personality. The Genius is neither 'soul' nor 'life'. It's particular to each one and ceased with his death. It is a kind of active principle which could be found too in collectives like troop units, councils and so on. It is assigned too to localities like provinces or cities. Power and prestige of the pater familias explain that the domestics worshipped his Genius and swore by him. The oath by the Genius of the emperor became common in private and public fields. False oath was a crime against the emperor. The concept of the Genius Augusti was the possibility to assign divine attributs to the emperor without making him a god directly which was frowned especially in the western part of the Empire!

The need for protection resulted in the idea of the Genius as protection spirit, but it was never clear wether he was immanent to men or has his own existence. In later times these ideas were mixed with the conception of the soul which could be found in grave inscriptions. The conception of the Genius as sum of the personality expanded to the idea of the Genius of a god: Genius Iovi. This required the conception of a full personalized deity.   
 
Meaningful is the Genius Populi Romani which is not only the Roman interpretation of the Greek City Tyche. On October 9 the festival of the Genius Publicus was celebrated. The later snake shape was an amalgamation with the well-known incarnation and soul conception. The Genius indeed was linked to a person but not identical with him. Life arises 'by appearing of the Genius', who then obtained it continually. We can see that the ancient world had difficulties with the interpretation of the Genius. But worshipping of the Genius was alive until the beginning of Christianism.