Were more boys or girls born to atomic-bomb survivors? Normally, somewhat more pregnancies terminate in boys than girls in all populations, and this normal preponderance of male births has not been demonstrated to be significantly altered when the parents (one or both) were exposed to atomic radiation. However, when the genetic studies began, it was believed that a person's gender was simply determined. Individuals inheriting an X chromosome from their father and one from their mother were destined to be females; whereas those individuals who inherited a Y chromosome from their father and an X from their mother would be males. Thus, females would have two X chromosomes and males only one. 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 as dose increased, if the father were exposed, more female embry...
