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ZOOLOGY: T. H. MORGAN 
The unequal distribution of the X chromosomes in the two sexes has 
furnished an opportunity to examine the theory that the chromosomes 
are the bearers of the hereditary factors, because it can be shown that 
the inheritance of a certain class of characters follows the known i s- 
tribution of the sex chromosomes. For example, color blindness in man 
is inherited in the following way (figs. 2 and 3). A color blind man mar- 
ried to a normal woman (fig. 2) produces normal sons and daughters. If 
two individuals that have had this origin should marry each other, the 
daughters will all be normal, half of the sons will be normal but half will 
be color blind. In other words the color blind grandfather has trans- 
mitted his defect to none of his granddaughters, but to half of his grand- 
sons. If we assume that color blindness is due to factors carried by the 
X chromosomes — in this case by the single X chromosome of the grand- 
father — the inheritance of the chromosome (to the right) and of the 
defect (to the left) is the same, as shown by the scheme (fig. 2). 
Conversely if a color blind female is married to a normal male (fig. 3), 
all of the sons will be color blind, but all of the daughters will be normal, 
If such a color blind male (or any color blind male in fact) is mated with 
a female having this origin, half of the daughters will be color blind, half 
normal; half of the sons will be color blind, half will be normal. In 
other words the color blind grandmother transmits her defect to all of 
her sons, and to half of her granddaughters and to half of her grandsons. 
The inheritance of the chromosomes and of the defect is strictly par- 
allel in this case also, as seen in the diagram (fig. 3). In the fruit-fly 
Drosophila ampelophila there have been found over 40 sex-linked 
characters all of which are inherited according to the scheme given above. 
Moreover all possible combinations and tests that have been made — 
and there are many ways of testing such a conclusion — have given 
results that are entirely consistent with this scheme. 
Linkage. If the hereditary characters are carried by the chromosomes 
we should expect that there would be great groups of characters that go 
together in inheritance, because while the number of the chromosomes 
is limited the number of the hereditary characters must be very great. 
In Drosophila the number of pairs of chromosomes is only four, while 
over one hundred mutant characters have been studied. These char- 
acters fall into four great groups. The names of some of these char- 
acters that involve nearly all parts of the body, are given in the following 
list: 
