296 INTRODUCTION TO CYTOLOGY 



It should be pointed out that if the original cross in the foregoing 

 example had been made between a fly homozygous for black-long and 

 one homozygous for gray-vestigial, these combinations would have 

 appeared in about 83 per cent of the F2 individuals after back-crossing 

 to a pure recessive, while gray-long and black-vestigial would have 

 formed about 17 per cent of recombinations because of crossing-over. 

 In other words, one combination is as likely to appear as another in a long 

 series of generations, barring detrimental effects which may attend one 

 of them. The condition rather arbitrarily called "normal" is usually 

 the one which is most prevalent in healthy flies; sometimes it is the one 

 which happens to be observed first. 



Because the Mendelian factor pairs so far outnumber the chromosome 

 pairs, it is evident that each of the chromosomes must carry a consider- 

 able number of genes. Extensive researches on linkage relations in 

 various plants and animals have brought out the fact that the genes and 

 hence the Mendelian characters fall into linkage groups, the members of 

 each group being linked to one another in various degrees but showing 

 independent assortment with the members of other groups. Moreover, 

 when the linkage relations of enough genes are known, it is found that the 

 number of such linkage groups in an organism is the same as the number of 

 its chromosome pairs. This is well illustrated by Drosophila melano- 

 gaster and Zea Mays, the two organisms concerning whose genetic 

 behavior our knowledge is most advanced. In Drosophila melanog aster 

 there are four pairs of chromosomes; pair I is rod-shaped, pairs II and III 

 are longer and bent, and pair IV is very small (Fig. 196). The genes in 

 this species fall into four linkage groups, and it is noteworthy that three 

 of the groups are large while the fourth comprises only a very few known 

 genes (Morgan, Muller, Sturtevant et al.). In Drosophila Willistoni there 

 are three chromosome pairs and three linkage groups (Metz) and in D. 

 obscura five pairs and five groups (Lancefield) . In Zea Mays, where there 

 are normally 10 pairs of chromosomes, 10 linked groups have been 

 identified (Emerson et al.). In Pharbitis Nil, which has 15 pairs of 

 chromosomes, there have been identified 10 groups and several independ- 

 ent factors probably representing the five other groups (Imai, Hagiwara). 

 It is an interesting fact that Mendel, in his classic researches on Pisum, 

 which has only seven chromosome pairs, happened to select for special 

 study seven pairs of characters evidently belonging to as many different 

 linkage groups and so did not detect the phenomenon of linkage. In 

 view of such facts, it is possible to look upon a chromosome as a body 

 containing a definite group of genes which influence the development of a 

 definite group of characters; but it is not to be concluded from this that 

 the chromosome in question is solely responsible for these characters. 



Assignment of Linkage Groups to Chromosomes. — The first character 

 found to be definitely associated with a distinguishable chromosome was 



