These are stems completed by a mudaf ilayh; do not read them as isolated words.
Noun Nominative Accusative Genitive
أَبٌ أَبُو أَبَا أَبِي
أَخٌ أَخُو أَخَا أَخِي
حَمٌ حَمُو حَمَا حَمِي
فَمٌ فُو فَا فِي
ذُو ذُو ذَا ذِي
The letters و، ا، ي show nominative, accusative, and genitive respectively.