Preheat oven to 350'F. Coat inside of 9x5 inch loaf pan with nonstick vegetable-oil cooking spray. Stir together whole-wheat flour, white flour, wheat germ, baking soda and salt in small bowl until well mixed.
Beat together margarine and sugar in medium-size bowl until light and fluffy. Beat in egg. Stir together banana and yogurt in another bowl until well blended. Mix the banana mixture alternately with the flour mixture, in batches, into the margarine mixture; stir just until combined. Spread the batter in the prepared pan.
Bake in preheated 350'F. oven 60-65 minutes or until wooden pick inserted in center comes out clean.
Cool in pan on wire rack 10 minutes. Remove bread from pan to wire rack to cool to room temperature.