neuploids was discovered in 1959 (in unrelated studies), and soon, many others were identified. As a result of these discoveries, it is known that it is possible for some females to have only one X chromosome (or even as many as 5), and for some males to have two or more. Moreover, these individuals with abnormal numbers of X chromosomes are more frequent in most populations than new sex-linked lethal mutations would be expected following exposure to ionizing radiation at the dose of the average survivor. Second, it is now known that in females, only one of the two X chromosomes within a cell is functionally active. This inactivation of one of the X chromosomes makes the prediction of the behavior of a potentially lethal gene on the X chromosome more difficult, particularly if the inactivation is not always random (preferential inactivation of paternally derived X chromosome in extraembryonic tissue while random inactivation in embryonic tissues). Given these developments, most human geneticists no longer accept the simple, early arguments, and contend that prediction of the effects of lethal mutations on the frequency of male births is not possible. ...
