Một số phần của điều này là những trang web nơi mà các phân tử được gọi là yếu tố phiên mã liên kết với DNA để ra hiệu cho một gen cho dù đó nên được kích hoạt hay không. Trong thời hạn gen cũng có phần của DNA hoạt động (exonic) và không hoạt động transcribable (intronic) cung cấp phức tạp hơn nữa | 39 Maternal-fetal conflicts most genes on the X chromosome have evolved to be optimal in a monoallelic state rather than affected by the potential for heterosis or heterozygosity . In mice one X chromosome is silenced to days after fertilisation the choice of chromosome usually being random. In humans X inactivation may start by the eight-cell stage. Similar considerations of gene silencing on one chromosome apply to individual genes on other chromosomes. As each allele comes from a different parent one can envisage situations where evolution found it advantageous to determine which allele is silenced so that only a maternal or paternal effect can be exerted. This parent-specific silencing of an allele is called The most common mechanisms for gene silencing involve chemical modification by methylation of cytosine nucleotides within special DNA sequence regions called CpG islands . This alters gene structure and function drastically and can stop the gene from being transcribed into RNA and thus into protein. DNA methylation is used to determine which of the alleles maternally- or paternally-derived of a particular gene will take precedence under given conditions. About 50 to 100 genes show maternal paternal imprinting and many of them are involved in the regulation of fetal growth and development. The best-described example of genomic imprinting involves a hormone termed insulin-like growth factor-2 usually abbreviated to IGF-2 . The hormone IGF-2 is made in a range of embryonic tissues and is a very important promotor of embryonic growth. It acts on receptors called IGF type 1 receptors in the target tissues to promote cell division differentiation and hence growth. It turns out that IGF-2 expression is imprinted such that only the paternal allele is expressed. Thus the production of IGF-2 and hence potentially embryonic growth are driven more by the paternal than the maternal genome. But we have just suggested that the mother must hold her .
