426 ZOOLOGY: T. H. MORGAN 
interchange takes place is not as yet on the same footing as are the other 
relations previously described. 
Non-disjunction. In some experiments carried out by Mr. C. B. 
Bridges a race of flies was found that gave in 5% of cases exceptional 
results. There were 2|% of males that were like their father (patro- 
clinous) when the expectation was that all of the males should be like 
their mother, in the sex-linked character in question; and there were 
2|% of females that were like their mother (matroclinous) when the ex- 
pectation was that they should all be like their father. From certain 
evidence that I can not now consider, Bridges was led to suspect that 
an additional sex chromosome was present in the mother that gave these 
a ^ r 
FIG. 5. DIAGRAM ILLUSTRATING CROSSING OVER OF THE MEMBERS OF A PAIR OF 
HOMOLOGOUS CHROMOSOMES REPRESENTED BY A SERIES OF BLACK AND A SERIES OF 
WHITE CIRCLES. 
results; and from certain genetic evidence it was predicted that this 
chromosome could be none other than the Y chromosome. A cy to- 
logical study of these females has demonstrated that the inference was 
correct. 
These XXY females contain three elements that might be expected 
to come together during conjugation. The following situation then 
arises: X and X may conjugate and later pass to opposite poles (one 
going into the polar body), and Y would then be left to go to either 
pole. Two classes of eggs would result namely XY and X eggs. On 
the other hand X and Y might conjugate and pass to opposite poles 
(one going into the polar body) and the other X would then be left to 
go to either pole. The two classes of eggs that would result are XX 
