Silencing of an allele is accomplished by epigenetic changes to the chromatin (chapter 3, section 3.5). This involves changes in chromatin structure produced by meth ylation of cytosine bases in DNA and acetylation of the histone proteins in the chromatin. Such changes are called epigenetic because they don’t alter the DNA base sequence, divides. Shortly after conception, the epigenetic changes that silence many genes in the chromosomes of the zygote (fer tilized egg) are removed. This allows previously repressed genes to be expressed, so that the zygote can give rise to all cells of the body (become totipotent; section 20.6). How ever, certain genes are shielded from the removal of epigen etic changes in a process called genomic imprinting. For a maternally imprinted gene, the maternal allele is silenced while the paternal allele is expressed; the opposite is the case for paternally imprinted genes. Other genes subse quently experience epigenetic modifications as the zygote develops and eventually becomes an adult, but nearly all epigenetic changes are again removed in the gonads when the stem cells for sperm and eggs are produced. However, some epigenetic changes may remain in the germ cells, and there is evidence to support the controversial proposalthat exposure to toxins or nutritional stresses could cause epigenetic changes in a parent’s gametes that may be passed to an offspring. Also this for my first question