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CHAPTER 8 



(after its discoverer) and is characterized by 

 the failure to mature as a woman. Turner 

 type females usually do not develop breasts. 

 ovulate, or menstruate. Because of vari- 

 ability in the genotypic details and in the 

 environment (including medical treatment), 

 considerable variation occurs in the pheno- 

 ls pic consequences of the X0 condition. In 

 fact, one woman of this constitution is known 

 to have given birth to a normal (XY) son. 

 The X0 mouse is apparently less variable 

 phenotypically since it always seems to pro- 

 duce a fertile female. The other single sex 

 chromosome type, YO. presumably lethal in 

 man, is known to be lethal in mouse. 



Otherwise-diploid individuals having three 

 sex chromosomes are of three types: XXX 

 is female (sometimes mentally defective); 

 XYY is male; XXY is male. The XXY in- 

 dividual who is characteristically sterile, may 

 have undersized sex organs, and may de- 

 velop various secondary sexual characteris- 

 tics of females, possesses Klinefelter's syn- 

 drome (named after its discoverer). Along 

 with the X0 female, he is phenotypically 

 variable; for instance, some Klinefelter males 

 are mentally retarded, others are not; al- 

 though all those presently known are sterile, 

 some show normal sexual drive and be- 

 havior. In the mouse, XXY is a sterile 

 male. 



Otherwise-diploid persons of the follow- 

 ing additional types are also known: XXXX 

 ( 9 ) ; XXXY ( $ ) ; XXYY ( $ ) ; XXXXX 

 ( 9 ) ; XXXXY ( $ ); XXXYY ( $ ). Con- 

 trary to the situation in Drosophila, it is 

 clear from all these results that the Y chro- 

 mosome is the primary sex-determining 

 chromosome in man and mouse. Presence 

 of a single Y determines the sex as male; 

 absence of a Y produces the female. All 

 individuals require an X in order to be 

 viable. 



The Y versus no Y sex-determining mech- 

 anism in human beings and mice implies 

 that the Y must carry one or more genes 



for maleness in that portion which makes 

 it cytologicaUy unique, the X having no cor- 

 responding allele(s). Admitting that the 

 presence of gene(s) for maleness on the Y 

 makes for male, what is genetically responsi- 

 ble for the femaleness produced in the ab- 

 sence of the Y? Clearly other genetic factors 

 are present — not limited in location to the 

 Y chromosome — which affect sex and, there- 

 fore, femaleness. The female tendency often 

 shown by the human XXY suggests that the 

 X contains genes affecting normal sexual 

 differentiation which, when present in excess, 

 cause a shift toward femaleness. Presum- 

 ably, the X also has this capacity when Y 

 is absent. 



All cases in which the entire body seems 

 to contain an abnormal number of sex chro- 

 mosomes can be explained as the result of 

 nondisjunction leading to chromosome loss 

 or gain which occurs either during meiosis 

 or at an early cleavage division — probably 

 the first — of the fertilized egg. Such nondis- 

 junctions are correlated in human beings 

 with the mother's advanced age at the time 

 of pregnancy. 



By following the distribution of X-l inked 

 mutants, it has been shown, however, that 

 the nondisjunction which produces an ab- 

 normal sex-chromosome number sometimes 

 involves the paternally contributed sex-chro- 

 mosome material. This origin is exemplified 

 by a red-green colorblind father having an 

 X0 daughter of normal vision. Since cer- 

 tain aged Drosophila eggs cause the loss of 

 paternal chromosomes after fertilization, it 

 is important to recognize the possibility that 

 the loss of a paternal chromosome in man 

 can occur post- as well as pre-meiotically. 

 Due to a premeiotic paternal nondisjunction 

 colorblind women can, of course, have XXY 

 Klinefelter sons of normal vision. 



A considerable number of persons having 

 different chromosomal compositions in dif- 

 ferent body parts are mosaic for sex chro- 

 mosomes. These include the following 



