After MoFRan 



ADULTS AND CHROMOSOMES OF THE MALE AND FEMALE FRUIT-FLY, DROSOPHILA 

 MELANOGASTER 



This species has been more thoroughly studied than any other animal, with the pos- 

 sible exception of man himself. Thousands of inheritance experiments have been 

 made on the ordinary traits of the wild forms of the species, and hundreds of muta- 

 tions have been traced through dozens of generations 



the same in males as in females. Although we speak of all the chromosomes 

 as paired in body cells, in one of these pairs the two members are not quite 

 matched. In some species these two unmatched chromosomes differ merely 

 in size (see illustration, p. 491). In some species the smaller one may be so far 

 reduced as to be quite absent, or at least invisible. In other species there is a 

 difference in shape (see illustration above). Associated with this inequality 

 is the fact that in some species the sex of the individual is determined by the 

 constitution of the sperm, whereas in other species it is determined by the 

 chromosome character of the tgg (see illustration, p. 492). 



Now it is well known that among human beings a form of color-blindness 

 in which a person cannot distinguish red and green is seldom found in females. 

 If we suppose that this characteristic results from the presence of a special 

 gene in the sex chromosomes, we can explain the actual distribution of color- 

 blindness. Color-blindness "skips a generation" in inheritance, being trans- 

 mitted not from fathers to sons, but from grandfathers to grandsons, and only 

 through the daughters (see illustration, p. 493). This is a familiar sex-Hnked 

 character. 



In studies on the fruit fly about two hundred characters have been found 

 to be sex-linked. The genes which determine these characters have been 

 assumed to be in the sex chromosomes, the so-called X-Y pair, shown in the 

 illustration above as a short, straight chromosome and one sharply bent. 

 Then there are two larger linkage groups which have been assigned to genes in 

 the two larger chromosomes. There is a much smaller group of linked char- 

 acters which have been assigned to the smallest chromosomes. 



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