Chromosome Maps 269 



to one of the transposed traits — that is, 99 per cent of the 

 wild type red-gray and mutant white-yellow, and i per 

 cent of white-gray and red-yellow. In other words, the 

 linkage groups are broken up in i per cent of the cases. 

 If the same characters are introduced into a cross in a dif- 

 ferent combination, and the dominant hybrid is again 

 crossed back to a recessive, the four types will again appear, 

 and again the grandparent types will be represented by 99 

 per cent of the progeny, and the cross-overs by i per cent. 

 These facts are interpreted as showing a breaking up of the 

 linkage group, by the exchange of genes between the two 

 members of a pair of chromosomes. 



Chromosome Maps 



Such cross-overs have been found to occur for many 

 pairs of characters, but the percentages are not always 

 the same, ranging from 1-99 to about 50-50, at which 

 point the effect is the same as independent unit characters 

 of Mendel. 



The theory of the gene assumes that the genes are ar- 

 ranged along the length of the chromosome. Before the two 

 chromosomes in a pair separate (at the time germ cells are 

 formed) they first come together, and in many cells they 

 are seen to become twisted about each other. It is sup- 

 posed that in this situation there is an opportunity for the 

 two corresponding genes at one point or another to cling 

 together and cause a transposition of portions of the chro- 

 mosome (Fig. 81). Assuming that this is the behavior of 

 the chromosomes and genes, that explains the observed facts 

 of cross-over in breeding. 



Morgan and his associates have constructed a ** map " 

 of the fruit fly chromosome (Fig. 82) . This is based on the 

 facts of linkage and grouping of pairs of contrasting char- 

 acters or allelomorphs, on the facts of cross-over, and on the 

 relative proportions of cross-over, for various pairs of linked 

 characters, or the frequency with which the various linkages 

 are broken up. 



