Why single nucleotide polymorphisms occur at only certain nucleotides in our genome?

Single nucleotide polymorphisms (SNPs) are polymorphisms that are caused by point mutations that give rise to different alleles containing alternative bases at a given position of nucleotide within a locus. Due to their high abundance in the genome, SNPs already serve as the predominant marker type.

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answer; /// i believe that the correct answer is; ///a combination of the birth and death

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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