242 



NATURE 



[February 23, 1922 



Many examples of this sort could be given, and 

 further tests of the different kinds of individuals 

 that appear in such crosses could also be cited to 

 show that the distribution of the sex-linked char- 

 acters follows the distribution of the X-chromo- 

 somes. This evidence is so significant that it may 

 be further illustrated by a concrete case. If a 

 white-eyed female of the vinegar fly, Drosophila 

 melanogaster, is bred to a red-eyed male (Fig. 2), 

 the sons are white-eyed, and the daughters are 

 red-eyed (red dominates white). If these are in- 

 bred there appear in the next generation white- 

 eyed daughters, red-eyed daughters, white-eyed 

 sons, and red-eyed sons in the ratio of 

 1:1:1:1. 



The. distribution of the X- and Y-chromosomes 

 is illustrated by the rods in the middle of the 

 diagram. The white rod stands for the X that 

 carries the differential for recessive white eyes. 

 The black rod stands for the X that carries the 



differential for dominant red eyes. The Y-chromo- 

 some is represented by that letter. It is obvious 

 from the way in which these chromosomes are 

 distributed that there should be both red-eyed and 

 white-eyed grandchildren in equal numbers. 



The reciprocal cross gives a different result 

 (Fig. 3). Thus, when a white-eyed male is bred 

 to a red-eyed female, both the sons and the 

 daughters have red eyes. If these are inbred, 

 there appear in the next generation red-eyed 

 daughters, red-eyed sons, and white-eyed sons in 

 the ratio of 2 : i : i. Here also it is evident from 

 the distribution of the X's why, in the second 

 generation, the only white-eyed flies present are 

 males. These carry a single white-producing X 

 that traces back to the grandfather. All the grand- 

 daughters have red eyes, but are of two kinds, 

 one pure for red, and the other carries both a 

 red and a white rod. If these second-generation 

 females are tested it is found, in fact, that half of 

 NO. 2730, VOL. 109] 



them carry two red-producing chromosomes, and 

 the other half a red and a white one. Evidence 

 lijk,€ this from sex-linked inheritance, where both 

 the genetic and the chromosomal histories are 

 known, furnishes by itself very strong evidence in 

 favour of the chromosomal interpretation of 

 heredity, but there is further evidence that makes 

 the case even stronger. This evidence may now 

 be briefly stated. 



(2) Individual females of the fly Drosophila are 



Fig. 3. 



sometimes met with that break the rule for sex- 

 linked inheritance. A genetic study by Bridges of 

 this exceptional behaviour led to the prediction 

 that they must have an extra sex-chromosome. 

 Cytological examination showed, in fact, that 

 there is in these females an X- and another X- 

 and a Y-chromosome (Fig. 4). The genetic be- 

 haviour of the " non-disjunctional " females is so 

 important for the chromosome theory that it must 

 be followed through carefully. It will be simpler 

 to give the genetic and the chromo- 

 some histories together. 



When an egg containing the three 

 chromosomes XXY matures, the 

 two X's may conjugate, leaving 

 the Y free to go to either pole of the 

 polar spindle (this happens in 92 per 

 cent, of the cases), or else an X and 

 the Y may conjugate, leaving the 

 other X to go to either pole. As 

 shown in the diagram (Fig. 5), four kinds of eggs 

 result (and four kinds of polar bodies are ex- 

 truded). If the non-disjunctional female in ques- 

 tion has white eyes, the history of her white-bear- 

 ing X's can be followed when she is fertilised by a 

 male with a red-bearing X-chromosome. Con- 

 sidering first the fertilisation of her four kinds of 

 eggs by the red-producing X-sperm of the male, it 

 is evident that there will be produced four kinds of 

 individuals, viz. XXY, XX, XXX, and XY. 







/I 



Y 



xxy 



Fig. 4. 



