As the cells of an early female embryo divide they randomely inactivate one of the two X chromosomes. By chance, some cells end up with an active X from their mother, and others the X chromosome they received from their father. As the embryo develops, the descendants of the early dividing cells will inherit the same paternal or maternal active X chromosomes and the specific gene forms on those chromosomes. On average, half of the cells express their maternal X and half express their paternal X. The result of X inactivation is a mosaicism of expressed X-linked traits.
21. There is a tendency among Homo sapiens to cornucopian thinking and an exaggerated assessment of human culture. These inclinations became hardwired into our brains during our evolutionary past when the human population was small, technology was relatively unsophisticated and had no lasting impact on the environment. These inclinations are at the foundation of our culture but their validity has now been superseded by the artefacts and impacts of human civilization, multiplied by the sheer number of Homo sapiens . The cornucopian perspective is comforting, comfortable and easy - and therefore seductive - but it is anachronistic in a world where the ecological footprint score exceeds . Biophysical reality will ultimately and always trump human culture. Back to text