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was able to demonstrate that crossing over had taken place in one chromo- 
some and not in the other, while in two cases he showed that two crossover 
chromosomes were present in the exceptional daughter. He explained 
these facts as due to non-disjunction occurring at the equational division, 
and believed that they involved crossing over at the "four strand stage" 
in the primary oocyte. 
Of the two certain cases of "equational" non-disjunction in willistoni, 
one gave a result like that just mentioned. The exceptional fly in this 
case came from an XXY female (W 1955) which had stubby, orange, 
small-bristle in one X chromosome, and orange, rough, short in the other. 
This female was crossed to a two-bristle, short-3 male and produced the 
following offspring: 135 daughters and 119 sons of the regular classes, 2 
two-bristle, short-3 sons (secondary exceptions of the ordinary "reduc- 
tional" type), and 1 stubby, orange, small-bristle daughter — an "equation- 
al" exception. This last female produced secondary exceptions as ex- 
pected, but she also carried a sex-linked lethal so that her only surviving 
sons were stubby, orange, small-bristle. Her exceptional daughters, 
being of the same constitution, also gave this same lethal ratio, but breed- 
ing tests from her regular daughters revealed her constitution. By such 
means it was proved that she had one non-crossover chromosome carrying 
stubby, orange, and small-bristle, and one crossover chromosome carrying 
stubby, orange, small-bristle, rough and short, and the new lethal. A 
non-disjunctional strain was established from this female known as line 
D (table 1). 
TABEE 1 
Summary of Breeding Tests 
LINE 
TOTAL, FLIES 
EXCEPTIONS 
PERCENTAGE OP 
EXCEPTIONS 
A 
6671 
117 
1.8 
B 
1131 
13 
1.1 
C 
701 
3 
0.4 
D 
497 
22 
4.4 
Total 
9000 
155 
1.7 
The other equational exception Is not of special interest since both of 
her chromosomes were non-crossover chromosomes, as far as the region 
that could be followed was concerned. It is not certain whether her 
mother, a descendant of line B, carried a Y chromosome. She gave rise 
to lineC (table l). 
Additional possible cases of equational exceptions have been found but 
they could not be positively identified as such. 
To secure XXY chromosome groups for cytological study the daughter 
of exceptional females from line A (W 1894b) were used. Theoretically 
half of these daughters should be of the desired constitution. No attempt 
