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ZOOLOGY: T. H. MORGAN 
the two open X's in the diagram) and if she were fertiHzed by a red eyed 
male (whose single X is represented by the black X in the diagram) 
the offspring will be expected to fall into the eight classes shown in the 
same diagram. The exceptional sons (4) arise when a Y bearing egg is 
fertilized by a female producing sperm (which here carries the factor 
for red). Thus a female producing sperm gives rise to a male, because 
maleness results from one X and not from anything else in the nature of 
the sperm that made him. The exceptional daughters (7) are the white 
eyed XXY females which arise when an XX egg is fertilized by a male 
producing sperm. Here again we see that femaleness is due to the oc- 
currence of two X chromosomes, and it makes no difference whence 
these two X's have come. 
There are many ways in which the hypothesis that non-disjunction 
is due to the presence of a Y chromosome, in the females that give 
these unique results, can be tested. For instance, the exceptional white 
FIG. 7. DIAGRAM TO ILLUSTRATE INTERFERENCE: THE DISTANCE BETWEEN THE 
POINTS WHERE THE HEAVY LINES CROSS REPRESENTS THE AVERAGE DISTANCE BETWEEN 
TWO POINTS OF CROSSING OVER; THE LIGHTER LINES INDICATE THE LESS FREQUENT 
POINTS OF CROSSING OVER IN THE PROTECTED REGION. 
females should repeat the non-dis junctional process. In fact, all such 
white females give non-disjunction. On the other hand the exceptional 
red males should neither give, nor transmit, non-disjunction, and this 
has been found to be true. The red females should be of two kinds; 
half (2) are expected to behave like normal females, and half (l) should 
show non-disjunction. These two kinds of females are found, and in 
equal numbers. There are two kinds of white males, XYY (5), and XY 
(6). The former should produce some XY spermatozoa. Such sperm 
would produce daughters, that would be XXY in composition, and 
from them non-disjunctional offspring are to be expected. This also 
has been tested and it has been found that half of the white males trans- 
mit through their daughters the peculiarity of non-disjunction. A dia- 
gram of a chromosome group of an XXY female is shown in figure 4. 
Interference. If there is an average length of loop between crossing- 
over points, it would seem to follow that the region of the chromo- 
somes on each side of the point where crossing over occurs would be pro- 
