5 8o BOTANY OF THE LIVING PLANT 



Bateson and Punnctt on the Sweet Pea. In this plant, purple flower colour is 

 dominant to red, and erect standard to " hooded " (lax) standard. The experi- 

 ment which led to the discovery of linkage was a so-called " back-cross " 

 between a Purple, Erect plant heterozygous for both characters with a Red, 

 Hooded plant homozygous for both recessive characters. The genotype of 

 the Purple, Erect parent may be represented as AaBb and that of the Red, 

 Hooded parent as aabb. The gametes of the former will be of four types, viz. 

 AB, Ab, aB and ab ; those of the latter can be of one type only, viz. ab. The 



O U e D i N Rl » G P x C t < A V 



O U «.' 4 1 N « L' SGP X«TF 



Fig. 440. 

 Diagram of two homologous chromosomes, maternal and paternal, indicating that 

 similar genes are placed at the same level in the length of the chromosome. Compare with 

 Fig- 433- (From Shull's Heredity, by courtesy of the McGraw-Hill Book Co., Inc.) 



next generation would therefore be expected to consist of the four categories 

 Purple, Erect (AaBb), Purple, Hooded (Aabb), Red, Erect (aaBb) and Red, 

 Hooded (aabb) in equal numbers. Actually, the parental types (Purple, 

 Erect and Red, Hooded) were found to be greatly in excess, each forming 

 49*5 per cent, of the whole, whereas the two re-combinations (Purple, Hooded 

 and Red, Erect) appeared to the extent of only «5 per cent, of each. This 

 result was difficult to understand at the time. The view was therefore enter- 

 tained by geneticists that the phenomena of linkage and crossing-over were 

 present at gametogenesis. Meanwhile the cytologists were able to show that, 

 in chiasma-formation (see above, p. 565), a related chromosome mechanism 

 was, in fact, present. The explanation depends upon two additional assump- 

 tions, viz. (1) that every gene occupies a definite position (locus) on a chromo- 

 some and (2) that the genes are arranged along the chromosomes in a linear 

 series like beads on a string, allelomorphic genes occupying corresponding loci 

 on the chromosomes of a homologous pair, the " dominant " and " recessive " 

 genes of each allelomorphic pair thus being " opposite numbers " (see Fig. 440). 

 These assumptions are supported by adequate cytogenetical evidence which 

 cannot, however, be discussed here. 



The cytological basis of linkage is illustrated diagrammatically in Fig. 441. 

 Here three pairs of genes are involved A-a, B-b, C-c, the ' dominants " 

 A, B and C having come from one parent and the " recessives " a, b and c from 

 the other. A and B are situated close together, whereas the locus of C is far 

 removed from both. If crossing over occurs more or less at random along 

 the chromosomes, the chances of it occurring between A-a and B-b are 

 small, but there is a much greater chance of its happening somewhere between 

 B-b and C-c. A and B, and similarly a and b, will therefore tend to pass to 

 the gametes in association with one another. Crossing over may not take 

 between B-b and C-c either ; in that case the gametes produced will be 

 ABC and abc. When, however, crossing over does happen in this region, the 

 gametes will be ABC, ABc, abC and abc, the proportion of ABc and abC 

 gametes — the " cross-over types " — depending on the frequency of crossing 

 over between B-b and C-c. The further apart two genes are on a chromosome, 

 the greater will be the frequency of crossing over ; conversely, the closer the genes 

 lie together on the chromosome, the more often will they be transmitted to the same 



