After hundreds or even thousands of generations both alleles for sickle cell anemia are still common in those of African ancestry. This is supported by two mechanisms: heterozygote advantage and gene flow.
Heterozygote advantage
When an allele is said to be heterozygous, it is carrying one copy of a gene. For example if one carries a copy of sickle cell allele, it will stay in the population even after many generations — natural selection will not be able to remove it and it will be passed on through generations. So, having one copy of that allele is advantageous but not deleterious.
Gene flow
With humans migrating throughout the different parts of the world, sickle cell allele is passed from one person to another or in different populations. It is actually advantageous to have one copy of this allele especially if there’s malaria in the area, which the allele is resistant.
Six dears migrated, this is the answer because immigration and emigration are both acts of migration.
The answer is A: The rough ER has visible ribosomes.
Jeremy is sitting quietly when the muscles in his left leg begin to “twitch.” This activation of movement in his voluntary muscles is most likely due to the release of the neurotransmitter Acetylcholine .
The primary function of acetylcholine is to carry signals from motor neurons to the body's skeletal muscles.
