THE METHOD OF CYTOLOGY 479 



Drosophila, long wings (F) are dominant to short or vestigial 

 wings (v), and gray body-color (£") is dominant to ebony (e) 

 or black body-color. A fly with vestigial wings and gray bodj^ 

 (vvEE) is mated with a long-winged fly with ebony body (Wee) 

 (Fig. 256). At the time of maturation the chromosomes of a pair 

 separate and pass into different gametes. The gametes of the 

 parent generation wdll be (vE) and (Ve), respectively. The zygotes 

 of Fi will be (VvEe) and will develop into long-winged, gray-bodied 

 flies. Four different kinds of gametes can be formed by these Fi 

 individuals, since the different pairs of chromosomes pass indepen- 

 dently into gametes at the time of reduction. The possible segre- 

 gation of genes in any one individual is as follows: (VE), (Ve), 

 (vE), and (ve). Recombination of such gametes to form the 

 zygotes of F2, according to the laws of chance, is shown in the 

 checkerboard at the bottom of Fig. 256. 



Linkage. — The discussion of the chromosomes as the carriers 

 of the genes, in the previous section, has been concerned with what 

 could occur if each gene mentioned were carried in a separate 

 chromosome. It was discovered by Bateson and Punnett, in 

 1906, that certain factors are coupled or linked in inheritance. 

 They found, in crossing a sweet pea carrying genes for purple 

 flowers and long pollen grains with a sweet pea in which the genes 

 were for red flowers and round pollen grains, that the factors that 

 came from each parent were inherited together instead of being 

 segregated and recombined independently. The study of the 

 inheritance of some 400 factors in Drosophila shows that they fall 

 into four groups. The genes composing each of these groups are 

 said to be "linked." Cytological investigation demonstrates the 

 presence of four pairs of chromosomes in Drosophila, and linked 

 genes are beheved to be carried by the same chromosomal pair. 

 Furthermore, there is evidence which can be interpreted to mean 

 that genes in a chromosome are arranged like beads on a string. 

 This linear arrangement of the genes makes the equal longitudinal 

 splitting of chromosomes during mitosis significant as a qualitative 

 division (c/. p. 139). 



The example of sex-hnked inheritance described as a breeding 

 experiment (p. 470) can be explained if the chromosomes associated 

 with sex determination are assumed to carry the factors concerned 

 with eye-color. It will be recalled that in the cells of a female 

 there are two so-called X-chromosomes, while in the male one 



