Two X Chromosomes And Males essay example
These notions suggested, in turn, that when mutations induced in the X chromosome by ionizing radiation are incompatible with survival (are lethal), their expression would be manifested differently in the two genders and would depend, partly, upon whether the X chromosome was inherited from the mother or the father. More specifically, since a father was thought to transmit his X chromosome exclusively to his daughters, if a lethal mutation were present on the X chromosome in the father's sperm, it would find expression only in his daughters. Whereas, since mothers transmit their X chromosomes equally to their sons and daughters, a lethal mutation might find expression in either sex. If the mutation were dominant, i. e., expressed itself if only one copy was present, the two sexes would be affected equally often; however, if the mutation was recessive (normally requiring two copies for expression), since the male has only one X chromosome, it would invariably manifest itself in males, but in females, manifestation of the new mutant would occur only if the second X chromosome fortuitously carried a functionally similar gene. It follows that since the likelihood of a mutation would increase a dose increased, if the father were exposed, more female embryos would be lost, and at birth, the frequency of males would be greater than would be true if the father were not exposed. If, on the other hand, the mother were exposed, more male embryos would be lost, and at birth, the frequency of males would decrease.
If both parents were exposed, the resulting proportion of male births, would be related to the individual parental doses and the frequency of dominant versus recessive lethal mutations. As can be seen, this theory of sex determination made fairly specific predictions that could be compared with the actual observations that were accumulating. When the data from the initial study were examined, it appeared that the frequency of male births was, in fact, declining with dose when the mother was exposed, and increasing, albeit modestly, with increasing paternal dose. The rate of change with dose was not, however, statistically significant, although in the direction predicted by theory. It was for this reason that when the clinical phase of the studies ended, data on the sex ratio continued to be collected on the supposition that the rate of change might become statistically significant with further information. To this end, observations on the frequency of male births were continued through 1966, when information was available on 140,542 births of which 73,994 were to parents one or both of whom were exposed.
However, when these additional observations were analyzed, the results did not support the earlier findings; indeed, the modest changes seen were opposite to those predicted by theory. Today, the earlier arguments regarding sex determination are known to have been overly simplistic. First, they did not take into account the occurrence of X chromosome aneuploids (errors in X-chromosome number), such as the Klinefelter and Turner syndromes, which might confuse the determination of the gender of an individual as revealed by a clinical examination shortly after birth. The first of these aneuploids 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 extra embryonic 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.