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How does the study of genetic disorders such as pku biologists understand normal alleles?

Which of the following describes a negative feedback loop? when the heart rate is too high, the body sends hormones that continually increase the heart rate higher. when a pregnant woman is in labor, the body sends hormones that increase the intensity of contractions, which then increases the secretion of the same hormones. when blood sugar is too low, the body sends hormones that raise blood sugar until it reaches a typical level and hormone secretion slows. when a person is jogging, the body sends hormones that continually decrease the rate of oxygen supply to the legs.

How do you suppose living conditions differ between the country furthest along in the demographic transition compared to the country earliest in the transition? how would living conditions in these two countries affect both birth and death rates?