Lung is the answer, its a breathing structure.
It because they give us oxygen and without oxygen we would all die from oxygen deprivation.
the process that gives us oxygen is photosynthesis
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Explanation:
In the majority of cases, the extra copy of chromosome 21 comes from the mother in the egg.
In a small percentage (less than 5%) of cases, the extra copy of chromosome 21 comes from the father through the sperm.
In the remaining cases, the error occurs after fertilization, as the embryo grows.