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CHAPTER 14 



expected is 1 : 1 as boy : girl. Even if the 

 four meiotic products of spermatogenesis are 

 usually X, X, Y, Y, the possibility exists that 

 during or after spermiogenesis (conversion of 

 the telophase II cell into a sperm) some X- 

 bearing sperm are lost. This is supported 

 by a report that human ejaculates contain 

 sperm heads of two sizes and shapes, the 

 smaller type being in sufficient excess to 

 explain an excess of males at fertilization, if 

 the smaller sperm contains the smaller Y 

 chromosome and the larger sperm carries 

 the larger X chromosome. It is likely, then, 

 that at conception males and females are 

 unequal in number. A study of the sex ratio 

 at birth has shown that the ratio of 1.067 : 1 

 is found only among young parents, and that 

 it decreases steadily until it is about 1.036 : 1 

 among the children of older parents. How 

 might this significant decrease be explained? 

 It might be due to a greater chance for male 

 babies to abort in older mothers. It might 

 be due, in part, to the increased likelihood, 

 among older mothers, for a chromosome to 

 be lost, in the earliest mitotic divisions of the 

 fertilized egg, -by failing to be included in 

 one of the daughter nuclei. For, if the chro- 

 mosome lost was an X, and the zygote was 

 XY, then the loss would be expected to be 

 lethal, and what would have been a boy 

 would be aborted, while if the zygote losing 

 an X was XX, a girl would still be born. 

 Moreover, if the chromosome lost in the XY 

 individual was a Y, a girl might be born 

 instead of a boy. Part of the effect must be 

 due to the increase in meiotic nondisjunction 

 with maternal age (zygotes of XXX type form 

 viable females, while zygotes of YO type are 

 expected to be lost before birth). 



We must not preclude the possibility that 

 the fathers are somewhat responsible for this 

 shift in sex ratio. There may be an increase, 

 with paternal age, in postmeiotic selection 

 against Y-carrying sperm. Perhaps, as 

 fathers become older, there is an increased 

 chance for certain abnormal events to take 



place during the meiotic divisions preceding 

 sperm formation. How could this be of sig- 

 nificance? Suppose the XY tetrad undergoes 

 nondisjunction in such a way that from a 

 given prophase I cell, the four meiotic prod- 

 ucts, each forming a sperm, contain, re- 

 spectively, X, X, YY, 0. The first two sperm 

 listed would produce normal daughters, the 

 last one an underdeveloped XO daughter, 

 while only the YY would be capable of pro- 

 ducing maleness. Moreover, this "male" 

 individual would be XYY and we do not 

 know if such a genotype is viable. While 

 still other genetic and nongenetic explanations 

 for the shift in sex ratio with age are possible, 

 the present discussion will suffice to demon- 

 strate how the basic facts of sex determina- 

 tion, chromosome loss, and nondisjunction 

 may be used to set up various explanatory 

 hypotheses whose validity may subsequently 

 be subject to test. 



The sex ratio has been found abnormal in 

 another respect. When pedigrees are ex- 

 amined for sex ratio, it occasionally is found 

 that a dozen consecutive births are of the 

 same sex. This could happen purely as a 

 matter of chance if enough pedigrees are 

 scored, just as it is possible (but unlikely) 

 that you could have a coin fall with the same 

 side up in a dozen consecutive tosses. But 

 one family is reported to have had only boys 

 in 47 births, and in another well substantiated 

 case a family has had 72 births, all girls. In 

 both these cases the result is so improbable 

 that it is not reasonable to consider it to be 

 merely due to chance! While we do not know 

 the basis for such results in man, we can ex- 

 amine two different cases in Drosophila in 

 which almost only female progeny are pro- 

 duced; these might suggest an explanation 

 for those human pedigrees in which only one 

 sex occurs in the progeny. 



In the first case in Drosophila, the males 

 were found to be responsible for the almost 

 exclusive production of daughters. These 

 males are XY but carry a gene called "sex 



