380 INTRODUCTION TO CYTOLOGY 



flies of four types appear in the next generation, as shown in the dia- 

 gram: white-yellow, white-gray, red-yellow, and red-gray. Since one 

 parent in this last cross is a double recessive, these results show that 

 the FI red-gray female must produce eggs of these four genotypic 

 constitutions. 



All four types, however, are not produced in equal numbers. Those 

 flies with the same combinations as were shown by their grandparents, 

 white-yellow and red-gray, together comprise 99 per cent of the total 

 number; only 1 per cent show the other possible combinations, white- 

 gray and red-yellow. It thus appears that if the two characters, red 

 eyes and gray body, enter a hybrid together (i.e., are contributed by the 

 same parent) they come out together in the'next generation in nearly all 

 cases: they are definitely linked, and this is explained on the hypothesis 

 that their genes are located in the same chromosome. The same is 

 obviously true of their respective allelomorphs, white eyes and yellow 

 body: their genes are carried in the other chromosome of the homologous 

 pair. Were the two allelomorphic pairs of genes carried in different pairs 

 of chromosomes no such linkage would occur : the two characters red and 

 gray, and similarly the two characters white and yellow, would then be 

 present together in 50 per cent of the flies, the chance frequency, rather 

 than 99 per cent. How it is that the linkage is broken in some cases, 

 giving 1 per cent with exceptional combinations, is a question to which 

 we shall return later in the chapter. 



Sex-linkage. A very interesting special case of linkage is seen in 

 the phenomenon of sex-linkage, which may be illustrated by the following 

 example (Fig. 146) (Morgan 1910). If a red-eyed wild male is mated to a 

 white-eyed female (a member of a race descended from white-eyed 

 mutants) the FI individuals are white-eyed males and red-eyed females 

 each eye color has been transferred from one sex to the other, a case of 

 "criss-cross" inheritance. If these FI flies are bred together, the F 2 

 generation comprises four types: red-eyed males and females, and white- 

 eyed males and females. Turning now to the chromosomes, if the dis- 

 tribution of the sex-chromosomes is followed through these generations 

 this striking fact is revealed : red eye color appears in every fly, male or 

 female, which possesses the .XT-chromosome of the original red-eyed male ; 

 and white eyes appear in every male which receives one of the X-chromo- 

 somes of the original white-eyed female, and in every female receiving 

 two of them. This is taken by Morgan to mean that the original male's 

 X-chromosome carries the dominant factor for red eyes, while each of 

 the X-chromosomes of the original white-eyed female carries a recessive 

 factor for white eyes. In all the flies it is seen that the presence of one 

 X-chromosome is correlated with maleness, and that of two X-chromo- 

 somes with femaleness (compare Fig. 140); and that the two types of 

 eye color under consideration follow the distribution of these chromo- 



