In some parts of Africa, the frequency of heterozygosity for the sickle-cell anemia allele is unusually high, presumably because this reduces the frequency of malaria. Which of the following statements is correct?

A) Homozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous
B) Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous
C) The malarial parasite changes the allele
D) Mendel's law of independent assortment
E) Mendel's law of segregation

B) Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous.

Explanation:

Sickle cell anemia is a recessive condition in which the red blood cells (RBCs) become sickle shaped. Due to the abnormal shape they are not able to transport oxygen efficiently which proves lethal for the person. Both, homozygous dominants and heterozygotes survive.

However in African countries with high rate of malaria, heterozygotes are selected over the dominant homozygotes. Dominant homozygotes have all normal RBCs which make them susceptible to malarial parasite since RBC is the host cell for it.

Heterozygotes produce some normal RBCs and some sickle shaped. Malarial parasite is not able to attack sickle shaped RBCs because of its unusual shape. So heterozygotes are less susceptible to malaria. Since they also have some normal RBCs, they are not affected much by sickle cell anemia too. Hence, heterozygotes evade both sickle cell anemia and malaria and heterozygous  is the most advantageous genotype in this case.

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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