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Word Count: 816
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 persons 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 fat her was thought to transmit his X chromosome exclusively to his daughters if a lethal mutation were present on the X chromosome in the fathers 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 ie 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 embryos would be lost and at birth the frequency of males
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