Professor Rusty Lansford explains that all genes are not expressed in the same levels in different cells; there is a lot of differential regulation.

From what I understand at this point in time is that there are 25-30 thousand genes that are described by the genomic biologists in my genome. Obviously in my heart, my heart DNA is turned on and is expressing transcripts that will be processed in a myriad of different ways. There is probably anywhere from as few as 5,000 (of that 25,000 or so) being expressed to maybe 12,000 and all the rest are being repressed or turned off. Some of them are housekeeping genes, doing the basic RNA polymerase, DNA polymerase, actin, some of the essential factors. Then some of the others, a smaller subset, are just the ones that are going to be specific for brain. These are saying 'do brain or neuron functions'. Other ones will be saying ‘those neural functions will be turned off, those neural genes will be turned off, and glial genes will be turned on’. And that’s often through the process of transcription and transcription regulation, but more we find out that some of that’s due to alternative splicing or micro RNAs or regulation at the level of translation. So it is again another loaded question that has a whole lot of different answers. The quick answer is no. In fact, all genes are not expressed in the same levels in different cells; there is a lot of differential regulation in that too.

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