Nondisjunction occurs when O there is a change in a gene's DNA sequence which alters an amino acid sequence.
O homologous chromosomes fail to separate.
O the chromosomes separate, but genes are lost.
O genes are expressed, but the DNA sequence is damaged.

homologous chromosomes fail to separate.

Explanation:

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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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