One true-breeding line of mice is obese and dark, and the other true-breeding line is lean and light. Dark is dominant to light, but obese and lean exhibit incomplete dominance. The two genes involved are on separate chromosomes. What proportion of F2 offspring resulting from a cross of two F1 progeny from a cross of the two lines described above should be dark and intermediate between obese and lean?

Choose the incorrect statement (a) the motor end plate membrane is only capable of generating graded potentials, but it is contiguous with the skeletal muscle sarcolemma, which can generate action potentials. (b) propagation of an action potential through the t-tubules into the sarcoplasmic reticulum triggers the release of ca++ into the cytoplasm of a skeletal muscle fiber. (c) the generation of suprathreshold endplate potentials in multiple muscle fibers could be due to the increased frequency of action potential generation at the axon hillock of a single motor neuron. (d) a greater amount of ach would be found in the presynaptic vesicles of a motor unit that fired several action potentials after exposure to botulinum toxin, than in a motor unit that fired several action potentials after exposure to the venom of the krait snake (which contains α-bungarotoxin).