GENERAL ZOOLOGY 



hybridization bv the assumption that allelomorphic genes conditioning the 

 ahernate expressions of the character used are carried in homologous chromo- 

 somes and thus distributed to gametes and zygotes (Fig. 6.14). The theory of 

 the chromosomes as the physical basis of heredity can also be used to ex- 

 plain cases of di- and trihybridization. In Drosophila long wing {V) is domi- 

 nant to short or vestigial wing (?>), and gray body color {E) is dominant to 

 ebony {e) body color. A fly with vestigial wings and gray body (vvEE) is 

 mated with a long-winged fly with ebony body (Wee) (Fig. 6.15). At the 

 time of the first meiotic division the homologous chromosomes of each pair 

 undergo disjunction and pass into diflerent gametes. The gametes produced 

 by the parents contain genes vE and Ve, respectively. The zygotes of the 

 Fj generation will be VvEe and will develop into long-winged, gray-bodied 

 flies. Four different kinds of gametes can be formed by these Fy individuals, 

 since the diflferent pairs of homologous chromosomes assort independently 

 when thev undergo disjunction at the first meiotic division. Segregation and 

 independent assortment of genes yield the following four classes of gametes in 

 any F^ individual: VE, Ve, vE, and ve. Random combination of gametes con- 

 taining such genes gives rise to 16 possible zygotes that develop into the F2 

 generation as shown in the checkerboard at the bottom of Figure 6.15. There 

 are four phenotypes occurring in a 9:3:3:1 ratio and nine genotypes 

 (WEE, VVEe, VvEE, VvEe, Wee, Vvee, vvEE, vvEe, and wee). 



Linkage. The discussion of the chromosomes as the carriers of the genes 

 has been concerned so far with what could occur if each gene were carried 

 in a separate chromosome. It was discovered by W. Bateson and R. C. 

 Punnett in 1906 that certain characteristics were linked in inheritance. They 

 found, in crossing a sweet pea with purple flowers and long pollen grains and 

 a sweet pea with red flowers and round pollen grains, that the genes which 

 came from each parent tended to remain together instead of assorting inde- 

 pendently during meiosis. The study of inheritance of several hundred genes 

 in Drosophila shows that they fall into four groups; the genes composing each 

 of these groups are said to be linked (Fig. 6.16). Cytological investigation 

 demonstrates the presence of four pairs of chromosomes in Drosophila (see 

 Fig. 2.12/) and E, p. 42), and pairs of linked genes are known to be carried 

 by pairs of homologous chromosomes. Furthermore, there is evidence which 

 is interpreted to indicate that genes in a chromosome are arranged like beads 

 on a string, and relative distances between genes in the string have been com- 

 puted (Fig. 6.16). This linear order of the genes makes the longitudinal 

 reduplication of the chromosomes and subsequent separation of the sister 

 genonemata highly significant events (p. 43). 



The example of sex-linked inheritance described as a breeding experiment 

 (p. 181) can be explained if it be assumed that the X-chromosomes carry the 

 genes concerned with red and white eye color. It will be recalled that in 

 the cells of a female there are two so-called X-chromosomes, whereas in the 

 male only one X-chromosomc is found (see Fig. 2.12/) and E, p. 42) The 

 male Drosophila possesses one X-chromosome and a Y-chromosome which car- 



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