Single cell fertilized egg 1 cell type, single DNA code Embryo hundreds of cell types, same DNA code Each cell type is defined by the genes that are expressed in that cell ?Gene expression? Single cell fertilized egg 1 cell type, single DNA code Neuron Hemoglobin ? Dopamine + Myoglobin - Muscle cells Hemoglobin ? Dopamine + Myoglobin - Red blood cells Hemoglobin ? Dopamine + Myoglobin - Each cell type only expressed a restricted subset of genes Single cell fertilized egg 1 cell type, single DNA code Neuron Hemoglobin ? Dopamine + Myoglobin - Muscle cells Hemoglobin ? Dopamine + Myoglobin - Red blood cells Hemoglobin ? Dopamine + Myoglobin - Each cell type only expressed a restricted subset of genes How??? Epigenetic marks applied to DNA