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CHAPTER 12 



We have seen that the faikire of normal 

 separation of chromatids during meiosis, at 

 either the first or second meiotic division, 

 would have the consequence of producing 

 some eggs containing either no X or two X's. 

 Actually, this nondisjunction of chromosomes 

 would represent a failure of the members of 

 a pair of chromosomes to segregate. On our 

 hypothesis that the X chromosome carries 

 an allele for w, chromosomal nondisjunction 

 can provide the mechanism by which a pair 

 of genes can fail to segregate, so that after 

 meiosis eggs are produced containing two 

 members or neither member of the gene pair. 

 Any egg produced following nondisjunc- 

 tion will usually be fertilized by a sperm 

 carrying either an X or a Y in addition to a 

 haploid set of autosomes. (Nondisjunction 

 can also occur during meiosis in the male. 

 We can ignore this here, because it would be 

 an infrequent event, and the probability that 

 an egg produced after nondisjunction would 

 be fertilized by a sperm produced after non- 

 disjunction is so small as to be negligible.) 



If the hypothesis of chromosomal nondis- 

 junction is valid, it should be consistent with 

 the genetic results. After nondisjunction the 

 exceptional eggs produced by a white (X"'X"') 

 female would be either X^'X"' or (zero des- 

 ignating an egg carrying no X). The normal 

 sperm produced by a dull red (X""Y) male 

 would carry either X'"^ or Y. The expected 

 genotypes of Fi from random fertilizations 

 between these gametes are given in Figure 

 12-10. 



Ignoring the sex of these exceptional off- 

 spring momentarily, and classifying them 

 only for eye color, type 1 would be dull red- 

 eyed, type 2 white-eyed, type 3 dull red-eyed, 

 and type 4 of undetermined eye color. The 

 genetic observations would be explained if 

 types 1 and 4 were lethal, type 2 was female, 

 and type 3 male. On the hypothesis that XX 

 is female and XY is male, it is reasonable that 

 types 1 and 4 would be neither, and might 

 be, therefore, lethal. Even more specific re- 



quirements need be fulfilled before accept- 

 ing these hypotheses, namely, that each 

 exceptional white female must prove to be 

 XXY cytologically, that is, such females must 

 have, in addition to the normal diploid chro- 

 mosomes of a female, an extra chromosome 

 which is Y; moreover, each exceptional male 

 must have, besides the normal autosomes, one 

 X but no Y. When the diploid cells of ex- 

 ceptional females and males are examined 

 cytologically these chromosomal prescrip- 

 tions are found to be fulfilled completely. It 

 is also possible to show, moreover, that YO 

 zygotes are lethal and that X"''X"'X"' individ- 

 uals are dull red-eyed, but usually die before 

 adulthood. 



It should be noted that while XY individ- 

 uals are fertile males, XO flies are invariably 

 sterile males. This means that the Y chro- 

 mosome is necessary for male fertility, the 

 trait being attributable to a gene on the Y 

 which is absent from the X. Note also that 

 our sex chromosome requirement for male- 

 ness must be modified to include XY and XO 

 individuals while that for femaleness includes 

 XX and XXY individuals. The matter of 

 the chromosomal and genetic basis of sex 

 determination will be discussed in greater 

 detail in the two Chapters which follow. 



We should now take stock of our hy- 

 pothesis that the chromosomes serve as 

 the material basis for genes. In previous 

 Chapters, a number of cases of parallelism 

 had been found in the known or assumed 

 properties and behavior of genes and of chro- 

 mosomes, including the following: both come 

 from pre-existing counterparts, both occur 

 as pairs in all nondividing cells except gam- 

 etes, both are replicated each mitotic division, 

 both maintain their individuality from one 

 mitotic division to the next, both are ca- 

 pable of mutation and subsequent replication 

 of the new form, both segregate during 

 gametogenesis so that they occur unpaired 

 in the gamete, both combine at random at 

 fertilization, both show independent segrega- 



