What is the K-T Boundary?

Considering that we are all made up of the same 4 nucleotides in our DNA, the same 4 nucleotides in our RNA, and the same 20 amino acids in our proteins, why are we so different from each other? For example, why do some people have sickle cell  anemia and others don't?

What is Homoplasmy

57. A 65-year-old man is brought to the physician by his wife. She says, "His appearance has been so strange lately. He has been neglecting his grooming, and I have to put his left arm into the sleeve of his shirt." The left side of his face is unshaven. When asked to fill in the numbers on a clock face, he writes the numbers as shown in the diagram. The most likely explanation for these findings is a lesion in which of the following nondominant areas of the brain? A) Frontal lobe B) Occipital lobe C) Optic nerve D) Parietal lobe E) Temporal lobe