10 



CHAPTER 8 



babies to abort, or tor chromosome loss in 

 the earliest mitotic divisions o( the fertilized 

 egg. If the chromosome lost is an X and 

 the zygote is \Y. the loss is expected to be 

 lethal, so that a potential boy is aborted. 

 If the zygote losing an X is XX. a girl can 

 still be born. Moreover, if the chromosome 

 lost in the XY individual is a Y, a girl can 

 be born instead of a boy. Part of the effect 

 must be due to the increase in meiotic non- 

 disjunction with maternal age (zygotes of 

 XXX type form viable females, whereas 

 zygotes of Y0 type are expected to abort). 



We must include the possibility that the 

 fathers may also contribute to this shift in 

 sex ratio. Postmciotic selection against Y- 

 carrying sperm may increase with paternal 

 age. Or. as fathers become older, the XY 

 tetrad may be more likely to undergo non- 

 disjunction to produce sperm containing re- 

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

 produce normal daughters; the last one can 

 produce an underdeveloped X0 daughter; 

 and only the YY is capable of producing 

 males. Even though the XYY individual 

 is male, it may frequently abort. Other 

 genetic and nongenetic explanations for the 

 shift in sex ratio with age are also possible. 

 This discussion merely demonstrates how 

 the basic facts of sex determination, chro- 

 mosome loss, and nondisjunction may be 

 used to formulate various hypotheses whose 

 validity is subject to test. 



When many pedigrees are examined for 

 sex ratio, several consecutive births of the 

 same sex occasionally occur. This phe- 

 nomenon could, of course, happen purely 

 as a matter of chance when enough pedigrees 

 are scored. One family, however, is re- 

 ported to have only boys in 47 births and, 

 in another well-substantiated case, out of 

 72 births in one family, all were girls. In 

 both these cases the results are too improb- 

 able to be attributed to chance. 



We do not know the basis for such results 

 in man, but two different cases of almost 



exclusive female progeny production in Dro- 

 sophila might suggest an explanation for 

 those human pedigrees in which only one 

 sex occurs in the progeny. In the first case, 

 an XY male carrying a gene called sex ratio 

 is responsible. Because of this gene, the 

 X and Y fail to synapse, and the X replicates 

 an extra time to form a tetrad; since almost 

 all Y chromosomes degenerate during meio- 

 sis, almost all sperm carry an X. In the 

 second case, a female transmitting a spiro- 

 chaete microorganism to her offspring 

 through the egg is responsible. Such a fe- 

 male mated to a normal male produces 

 zygotes which begin development; soon 

 thereafter the XY individuals are killed by 

 the spirochaete, leaving almost all female 

 survivors. 



The sex ratio can be controlled if the 

 genotypes of the zygotes formed can be con- 

 trolled. Since X- and Y-bearing sperm of 

 men apparently differ in cytological appear- 

 ance (Figure 8-8), it should be possible to 

 separate them and thereby control the sex 

 of progeny. Using various animal forms, 

 such experiments have been performed with 

 some success by Russian, American, and 

 Swedish workers, using electric currents or 

 centrifugation. Although these experiments 

 have been encouraging, the results are not 

 yet consistent, and the techniques not yet 

 suitable for practical use. 



Hymenoptera 



In Hymenoptera (for example, bees, ants, 

 wasps, and saw flies) unfertilized eggs de- 

 velop as males (haploids) and fertilized 

 eggs, usually, as females (diploids). Hap- 

 loid males produce haploid sperm via suit- 

 able modifications of the meiotic process, 

 and all gametes of males and females have 

 morphologically identical chromosomal com- 

 positions. 



In the parasitic wasp, Habrobracon jug- 

 landis, when the parents are closely related, 

 some of the sons are haploid, but others are 



