Set in the city of what one critic calls "psychic dark corners," there are three major relationships in which power and control are exerted in The Merchant of Venice as the play is constructed around contrasting value systems and relationships.
Clearly there is great animosity between the money-lenders, one of whom does not charge interest and the other who does exact interest on all his loans. This difference is at the heart of Shylock's hatred for Antonio
I hate him for he is a Christian;
But more for that in low simplicity
He lends out money gratis, and brings down
The rate of usance here with us in Venice. (1.3.)
Certainly, there are controls exerted upon Shylock in Venice as the Jews were confined to a certain area of the city; in fact, this area called ghetto is the derivative of the modern word. It is derived from the Italian word "gheto" or "ghet", which means slag or waste, for the slag was on the same part of the island as the Jews were forced to live. So, the relationship between Antonio and Shylock is not only religious, but it is also one of class.
The value systems of each theology/social class is also one of marked contrast. During the climactic trial of Act IV, Portia speaks of the Christian quality of mercy,
It is an attribute of God himself
And earthly power doth then show likest God's
When mercy seasons justice. (4.1.195-197)
In contrast, Shylock believes in the Old Testament's concept of revenge. For, the Duke of Venice characterizes Shylock as
A stony adversary, an inhuman wretch,
Uncapable of pity, void and empty
From any dram or mercy (4.1.4-6)
Of course, when Antonio defaults upon his loan, Shylock is eager for his pound of flesh, although he knows cutting this from Antonio will kill him. Further, it is Bassiano who wins Portia's hand with his demonstration of Christian values and true worth as does Portia in her famous speech.
At the end of the trial, Shylock not only loses against Antonio, but he is forced in retributive oppression to convert to Christianity a religion that is anathema to him.
The dominant position of male/female in Venetian society is best exemplified in the paternal relationship of Portia with her father, who controls her even after his death--"so is the will of a living daughter curbed by the will of a dead father" (1.2.24-25). For, she must marry the man who chooses the correc one from the three caskets that the father leaves, or she will lose her inheritance. Yet, Portia does not surrender to this male dominance as she acts on her own and disguises herself as a lawyer in order to defend Antonio against Shylock's revenge.
Shylock's daughter Jessica also disguises herself as a man, but she does so for ignoble reasons as she wishes to avoid notice after she steals her father's money and jewels when she elopes with Lorenzo. Also, in defiance of her culture as well as her father, Jessica converts to Christianity.