Epigenetics: We often discuss genes as if their presence in our cells assures that they will be expressed. This is not true. The expression of genes is highly regulated. That is to say that our genes can be turned on or off, and this can be good (most of the time) or bad (sometimes). We understand the mechanisms in some cases, but not others. Section 14.3 in our text (genomic imprinting) is but one example. Sections 10.7-10.11 deal with the processes involved with differentiation of cells as we develop from a single, undifferentiated cell (zygote) to 100 trillion-cell being that you see in the mirror. The difference in your brain, muscle, liver, and skin cells is not the DNA/genes in their nuclei, but rather, these cells are different because of the genes that are being expressed in them. Genes are turned on an off in ways that we often don't understand. Sometimes this is a part of natural development and sometimes it is a result of external factors such as toxins, diet, stress, etc... Furthermore, some genes can be turned off (silenced) for several generations, and then turned on again. How? "Upstream" of all of our genes is a DNA sequence that we call the PROMOTER region. Chemical signals in this region - most commonly the addition of a methyl group (methylation) - can essentially instruct the RNA polymerase that would otherwise transcribe a gene to skip the gene with the methylated promoter... the gene is turned off. Another mechanism of gene regulation involves those histone proteins that wrap DNA tightly around them. When DNA is tightly wrapped around histones, it is not accessible to RNA polymerase and the genes are turned off. Histones can be modified by external factors as mentioned above and this affects coiling and thus the regulation of genes. Another mechanism of silencing genes is found in inactive regions of one of the X chromosomes in females. Inactivity of one X chomosome results from vast regions of DNA being condensed around histones, and this can be seen in stained preparations. The condensed areas of one X chromosome are inaccessible for transcription/translation. These regions are called Barr Bodies. As evidenced by males of our species, we really only need one X chromosome. Why we don't see something like Barr Bodies in other chromosomes is a mystery. Since the human genome has been sequenced (10 years), scientists have been frantically trying to make sense of what they found. Among the findings presented in the video "Ghost in Your Genes" is the evidence that the same gene/allele can have a different phenotypic effect if inherited from the mother or the father. The take home message of all this (to me) is that we are accumulating data faster than we can process it, and we realize now that we don't understand many of the very complex processes occurring in our cells. Recent studies have shown that the regulation of gene expression is itself a heritable trait. As strange as this may seem, the evidence is undeniable. Even identical twins diverge genetically beginning with their intrauterine development. This is how identical twins end up with different fingerprints. We now talk about "DNA Fingerprinting" as the ultimate test of indentification. However, a genetic test would show identical twins to be the same person. Their literal fingerprints would show otherwise. If the patterns in the epidermis of fingertips is different, then why not other genetically-controlled traits?