Which level of biological classification is the lowest level that butterflies (invertebrates) and dogs (vertebrates)
have in common

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.

i would say it is