602 



CONTINUITY OF LIFE 



that the process is, unfortunately, not as changed. What actually did happen is an- 

 simple as was once thought. other unsolved riddle in biology. 



Genie balance and sex determination 



One of the first experiments that upset 

 the theory of sex determination by the X 

 and Y chromosomes was the appearance in 

 about 1922 of a fly in Dr. Calvin Bridges' 

 cultures at Columbia University which pos- 

 sessed three sets of autosomes (9 chromo- 

 somes instead of only 6) and 2 X chromo- 

 somes. According to the theory, this fly 

 should have been a female but actually it 

 was an intermediate between male and fe- 

 male, or an intersex. Apparently the pres- 

 ence of an extra set of autosomes upset the 

 "genie balance" in such a way that the two 

 X chromosomes could not completely con- 

 trol the situation in order to produce a 

 female. This and subsequent experiments 

 made it clear that other genes scattered 

 throughout the autosomes were also influ- 

 ential in determining the sex. The distribu- 

 tion of the genes in the autosomes and X 

 chromosome seemed to be the deciding 

 factor, as indicated in Table 3, below. 



From these experiments it is evident that 

 the X chromosomes carry a preponderance 

 of genes for femaleness while the auto- 

 somes contain a preponderance of genes for 

 maleness; how much "maleness" or how 

 much "femaleness" the offspring exhibits is 

 determined by how great a dose of either 

 one or the other it receives. If the dose is 

 intermediate, the offspring displays the ex- 

 ternal characteristics about evenly divided. 

 This revised view of sex determination 

 does not explain all of the observed facts. 

 Earlier (p. 438) we saw that the sex of a 

 hen chicken could be reversed when the 

 ovary was destroyed because it received 

 male hormone from an activated residual 

 testis. Certainly the genetic balance was not 



Sex linkage 



In addition to carrying genes for sex the 

 X chromosomes also contain other genes 

 that have nothing to do with sex, such as 

 those that control eye color in Drosophila. 

 The Y chromosome is virtually without 

 genes, that is, it seems to be a genetic blank, 

 and at least some insects {Anasa — the 

 squash bug) do not have it at all and ap- 

 parently get along satisfactorily. Since the 

 X chromosome carries certain genes that 

 control traits besides sex, it follows tliat 

 those genes will be tied in some peculiar 

 fashion to the sex, in other words, they will 

 be sex-linked. In man the genes responsible 

 for red-green color blindness and the blood 

 abnormality, hemophilia, are carried on the 

 X chromosome so that one might expect 

 these anomalies to be associated with sex. 

 This is definitely true and has been recog- 

 nized for centuries, particularly witli re- 

 spect to hemophflia. Because this condition 

 has occurred in royal families whose his- 

 tories are well known, it was learned very 

 early that the dread disease passed from 

 mothers to sons but never from father to 

 sons. For example. Queen Victoria of Eng- 

 land apparently carried the defective gene 

 because she herself did not have the disease 

 but transmitted it to one of her sons and 

 through two of her daughters to succeeding 

 generations so that it has occurred again 

 and again in her male descendants, particu- 

 larly in the Russian and Spanish royal fami- 

 lies. 



This great enigma in man's inheritance 

 remained a mystery until a mutation pro- 

 ducing white eye color occurred in some of 

 T. H. Morgan's fly cultures. When the in- 

 heritance of this trait was worked out it 



TABLE 3 



2 A (autosomes) plus 1 X = Normal male characters 



2 A " 2 X = Normal female characters 



3 ^ " 1 X = Over-developed male characters 



2 A " 3 X = Over-developed female characters 



3 ^ " 2 X = Intersex 



