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b. gene expression varies by cell [type]
Every somatic (body) cell in your body has the same genome, however, not all of the cells express the same genes. For example, the cells in your retinas do not secrete insulin, and the cells in your pancreas do not enable you to see. This is due to the specific genes that are expressed in the different cell types.
The human genome has billions of base pairs that allows different combinations of genes to be chosen. Most cells do not use all of the information recorded in the genome. Parts of the genome is expressed in cell differentiation muscles cells use different parts of the genome than skin cells do.
C. gene expression varies by cell.
The human genome has a vast amount of information recorded in the DNA code. A cell will differentiate by using only a small part of the information in the DNA. A muscle cell will use different parts of the information in the DNA than a skin cell.