232 MUTATIONS 



arisen from nondisjunction in early cleavage. The complementary 

 chromosome constitution may have led to death of the blastomere, 

 or it may have formed extra-embryonic tissue or adult tissue not 

 accessible to chromosome analysis. 



Atwood: Have the Klinefelter and Turner cases been investigated 

 as to maternal age dependence? 



Stern: Klinefelter's have, and there is a maternal age dependence 

 which is not as strong as in mongolism. 



McKusick: In Turner's, there appears to be no maternal age 

 effect (11). 



Atwood: It ought to be, because it is simply the complementary type 

 to Klinefelter's. 



Glass: No, this is not necessarily so, since you have the possibility 

 of nondisjunction both in the male and the female. The frequencies of 

 Turner's and Klinefelter's syndromes are sufficiently different that 

 different mechanisms of nondisjunction might be involved. 



Atwood: They are differentially hard to detect, though. Some of 

 Klinefelter's individuals appear normal and might not be detected. 

 Isn't that true? 



McKusick: It is in the opposite direction. Klinefelter's are hard to 

 detect and they are frequent. Klinefelter's syndrome is as frequent as 

 1 in each 400 male births (23). The Turner syndrome probably occurs 

 about once in every 5000 births. 



Stem: We do not yet know whether "chromosomally Klinefelter" in- 

 dividuals occur who are phenotypically normal, fertile males, and 

 whether there exist also "chromosomally Turner" individuals who are 

 phenotypically fertile females. After all, XO mice are normal and 

 fertile. Unless we "survey 100,000 or so run-of-the-mill (human) fe- 

 males" (G. L. Walls, 1959, Peculiar color blindness in peculiar people, 

 A.M.A. Arch. Opthal. 62:13-32) we will not have a clear idea as to the 

 possible variation in the expression of the XO constitution. And the 

 corresponding considerations would apply to the XXY constitution. 



Neel: Is there further discussion? If not, I will turn the meeting 

 over to our summarizer. 



Stern: May I add one comment on the question of relative frequen- 

 cies of Klinefelter and Turner types? Assume a zygote starts out with 

 a normal XY constitution. Let there be mitotic nondisjunction of the 

 replicated X chromosomes at the first cleavage division. Then, two 

 cells will be formed, with XXY and OY constitution, respectively. The 

 latter will die; the former develop into a Klinefelter type. Thus, this 



