176 



FUNDAMENTALS OF CYTOLOGY 



sive condition b or v. In the dominant conditions B and V these genes 

 produce, respectively, normal gray body and normal long wings. When a 

 fly, homozygous for both dominant factors, is mated to one with all 

 the corresponding factors recessive, the offspring all have normal body 

 and wings, because of the dominance of B and V over h and v, respectively. 

 If the females of this Fi generation are backcrossed to the homozygous 

 recessive, flies of four types appear in the next generation: gray-long, 

 black-vestigial, gray-vestigial, and black-long. Those flies mth the 

 original combinations (gray-long and black-vestigial) together comprise 

 83 per cent of the total number; only 17 per cent are of the new t^^pes 



B"' V 



V V -» 



b 



Back-cross 



b b 



V V 



% 



Fig. 124. — Linkage in Drosophila. One pair of chromosomes in each fly represented by 

 vertical lines; chromosomes in gametes, by diagonal lines. Explanation in text. {Adajited 

 from T. H. Morgan et al.) 



(gray- vestigia,! and black-long). It thus appears that if the two char- 

 acters, gray body and long wings, are contributed to the offspring b^^ 

 the same parent, they tend to appear together in the majority of the 

 individuals resulting from the backcross; in other words, they are 

 linked. This is explained by the fact that the differential genes con- 

 cerned are located in one chromosome of a pair. The same is obviously 

 true of the allelic characters, black body and vestigial wings, for their 

 genes are carried in the other chromosome of the pair. Hence in the Fi 

 fly one chromosome of a homologous pair carries B V, while the other 

 carries h v, and they tend strongly to continue thus into the next genera- 

 tion. Were the two pairs of genes in question, B b and V v, carried by 

 different pairs of chromosomes instead of by the same pair, there would 



