The genitive case is used to show possession, more often in writing than in speech. When speaking, most people use von (of) plus the dative case to show possession. For proper nouns, German only adds an -s to the noun, whereas English would add an apostrophe and an -s. Femine and Plural nouns do not change in the Genitive case. Masculine and Neuter nouns add an -s if the word is more than one syllable, or an -es if the word is one syllable.  Except the weak masculine nouns that added -(e)n in the accusative and dative; they also also add -(e)n in the genitive.  There are some irregular nouns that add -s after -en in the genitive case as well, for example der Name becomes des Namens and das Herz becomes des Herzens.

die Farbe des Vogels - the color of the bird
die Grösse des Hauses - the size of the house
die Tasche meiner Mutter - my mother's purse
der Bleistift des Studenten - the student's pencil