CONSTITUTION OF THE CHROMOSOMES 139 



X-chromosomes are of the usual type, so that she has the 

 dominant normal red eyes and gray body (see figure 32). 

 Their daughters (C) get, according to the usual rule, one 

 normal X-chromosome ("red-gray") from the mother, one 

 modified "white-yellow" chromosome from the father. In 

 the cells of these daughters (C), therefore, these two di- 

 verse kinds of X-chromosomes are together side by side in 

 the same cell. And while they are together, as the results 

 show, the two may exchange parts, so as to form a new 

 combination of characteristics, thus proving that each is 

 made up of diverse parts. 



This is discovered in the following way. Such daughters 

 (figure 32, C), carrying the two types of X-chromosomes 

 are mated with normal males (D), having normal X-chro- 

 mosomes, and therefore having red eyes and gray body. 

 Observe the sons that they produce (E, F, G, H). These 

 sons, as we know, receive their single X-chromosome from 

 their mother only, getting only the Y from the father. The 

 two X-chromosomes of the mother separate, as we know, 

 into different eggs. The eggs that contain the normal or 

 "red-gray" X-chromosome, when fertilized by a Y sperm 

 of course produce sons with red eyes and gray body (E, 

 figure 32). The eggs with the recessive "white-yellow" X- 

 chromosome (which came from the original male B, fig- 

 ure 32), produce sons with white eyes and yellow body 

 (F). Most of the sons produced are of these two types, in 

 equal numbers. But in addition, there are a few sons (about 

 one per cent of all), that have a new combination of char- 

 acteristics. There are a few sons that have red eyes and 

 yellow body (G, figure 32). And there is an equal number 

 that have white eyes and gray body (H). It is clear that 

 in the single X-chromosome of these sons a new combina- 

 tion has been made, composed of a part of the X-chromo- 

 some of the original father (B), and a part of the X- 

 chromosome from the original mother (A). The mother's 



