284 GENERAL ZOOLOGY 



concerned. The strength of linkage tending to keep genes in the 

 same chromosome is in inverse proportion to the distance sepa- 

 rating the genes. What these distances actually are is unknown 

 — the conclusions drawn from crossover data merely indicating 

 relative locations of genes in a linear order. 



Exhaustive breeding experiments have demonstrated that in 

 Drosophila four linkage groups occur, and only four, which, 

 fortunately for the theory correspond with the four pairs of 

 chromosomes. Characters belonging to different linkage groups 

 are independently assorted in breeding experiments because their 

 genes are located in different chromosomes. Of the four groups, 

 the sex-linked group may be assumed to have its genes in the sex 

 chromosomes, the locations being determined by the crossover 

 percentages obtained when the genes are tested in combinations 

 of two and two. The assignment of the genes of the non-sex- 

 linked groups to the autosomes is more difficult, since there is no 

 simple way of determining why a character should be assigned to 

 any particular autosome, except that there may be a correlation 

 in the size of the linkage groups and the size of the chromosomes. 

 A more accurate basis for answering this question is found in 

 what is known as translocation or transport of a part of one 

 chromosome to another, aside from that occurring in crossing 

 over. This sometimes occurs spontaneously, but may be 

 induced by heat and X rays. The alterations produced in the 

 linkage groups, when correlated with the cytological conditions, 

 enable one to arrive at a fairly accurate conclusion as to the 

 chromosomal distribution of the linkage groups. Figure 171 

 illustrates such a chromosome map for Drosophila. The 

 numerals opposite each character indicate the relative distances 

 of the genes from the tops of the chromosomes. Similar maps 

 have been prepared for other forms, including plants. 



The Gene and Development. — The gene concept provides an 

 internal mechanism governing the inheritance of characters that 

 distinguish members of the same species. In the fertilized eggs 

 these genes are present in a biparental series, the entire com- 

 plement of which is distributed to every somatic cell by quan- 

 titative mitotic divisions. Since the nucleus of each body cell 

 contains all the genes for the heredity of the entire body, the 

 actual differentiation of the body cells would seem to be the result 

 of an interaction between genes and environmental factors, 



