CYTOGENETIC CORRELATIONS AND CROSSING OVER 93 



From the \Vhite-E\ed From the Red-Eyed Male Parent 



Female Parent X Y 



XX XXX lethal females XXY uhite-eyed females 

 O XQ sterile males YO lethal males 



Bridges proved that the white-eyed female strain he had been study- 

 ing, \\ hich gave about 8 per cent unexpected progeny, was actually XXY 

 in chromosome constitution, and that in its meioses secondary nondis- 

 junction occurred, gi\ing rise to four lands of eggs with the frequencies 

 shown : 



From the Uhite-Eved From the Red-Eved Male Parent 



Female Parent X 



X ca. 46*^) XX red-eyed female XY white-eyed male 



XY (ca. 46%) XXY red-eved female X^*^' white-eved male 



XX (ca. 4^^) XXX lethal female XXY wh:;f-r,ed 'fnalf 



Y (ca. 4^^ X^' 'rd-rsed male W Ie:ha: 



Thus, in this nondisjunctional strain, about 4 per cent of the \iable females 

 received both X chromosomes from their mother, and about 4 per cent 

 of the \iable males received their one X chromosome from their father. 

 This demonstration of nondisjunction of the X chromosome in Droso- 

 phila represents the first instance in which the aberrant segregation 

 of a genetic factor was correlated with a chromosomal aberration, 

 therebv rigorously establishing the chromosomal basis of gene segrega- 

 tion. In maize, the linkage groups were first correlated v^ith particular 

 chromosomes by McChntock with the use of trisomic {2n — 1 stocks, 

 i.e.. plants in which one of the ten chromosomes is present three times 

 instead of twice. It should be appreciated that these first c\togenetic 

 correlations were not onl\" difficult to estabfish experimentally but also 

 represented an important conceptual advance, lasing the foundation for 

 the intricate and sophisticated analyses of chromosome rearrangements 

 w hich followed. 



CHROMOSOMES AS SEEN WITH THE LIGHT MICROSCOPE 



Chromosomes were first described in the 1570 s. and c\tologists ha\e 

 been obser%"ing them ever since. There is bv now a tremendous descrip- 

 tive literature based upon studies with the fight microscope. The 

 detailed morphology of each chromosome is itself an aspect of cell 

 hereditv. Chromosomes have an indi%iduahtv at the le\el of the fight 



