Dual inheritance.

Genetic inheritance in higher organisms normally refers to the transmission of information from one generation to the next. Nevertheless, there is also inheritance in somatic cells, characterised by the phenotypic stability of differentiated cells that divide (such as fibroblasts and lymphocytes), and also mitosis of stem line cells, which gives rise to another stem line daughter cell, and one that will differentiate. Thus, there is a dual inheritance systems in these organisms, one of which is genetic and the other epigenetic. In the latter, heritable information is superimposed on DNA sequences, and one well-known mechanism is heritable methylation of cytosine. Much information will come from the human epigenome project that will reveal the patterns of DNA methylation in distinct differentiated cells. There have also been innumerable studies on the abnormal de novo methylation and silencing of tumour suppressor genes in cancer cells.