Chapter *15 

 INTERGENIC LINKAGE 



I 



't was reported in Chapter 6 

 (pp. 46-41) that each of the first 

 .seven pairs of genes studied in the 

 garden pea segregated independently. This 

 can be attributed to each pair of genes being 

 located on a different pair of the seven pairs 

 of chromosomes which this organism carries. 

 What we should consider now are the results 

 obtained when an eighth pair of genes, show- 

 ing dominance and affecting an unrelated 

 trait, is studied simultaneously with each of 

 the other seven pairs in turn. When a dihy- 

 brid for a particular one of the seven and for 

 the eighth gene pairs is made, a 9 : 3 : 3 : 1 

 phenotypic ratio is obtained when the dihy- 

 brid is self-fertilized, and a 1 : 1 : 1 : 1 phe- 

 notypic ratio is obtained when this individual 

 is backcrossed to the double recessive. This 

 demonstrates in two independent tests that 

 the two pairs of genes involved are segregat- 

 ing independently and are located on non- 

 homologous chromosomes. The same ratios 

 are obtained, from the same types of crosses, 

 with dihybrids made with each of five other 

 genes and this eighth gene pair, so that the 

 same conclusion also applies for them. How- 

 ever, the results obtained from the dihybrid 

 made with the remaining gene pair and the 

 eighth will be described in some detail, since 

 these results are radically different. 



The seventh pair of genes happens to be the 

 one concerned with the seed coat, a trait we 

 have already studied in Chapter 6, whose 

 allelic alternatives are round (R) and wrinkled 

 (r). As mentioned earlier, we may use vari- 

 ous symbols to represent genes. In the pres- 



ent case, the first letter (or so) of the pheno- 

 type produced by the dominant allele (the 

 one normally found in nature) has been used 

 in upper and lower case to represent the domi- 

 nant and recessive alleles, respectively. In 

 other conventions (see Figure 15-1), the first 

 letter (or so) of the recessive mutant (not 

 normally found) allele is used in lower case 

 for the recessive allele (wrinkled = h-), while 

 the normal dominant allele is given either the 

 same symbol in upper case, or is given a + 

 symbol as a superscript or base to the lower 

 case symbol, or is given as + alone (so that 

 round may be, respectively, W, w+, +«', +). 

 Sometimes the hybrid +1^ is represented as 



= or — or +/vv to show that these alleles 



w w 



are on different members of a pair of homol- 

 ogous chromosomes. 



W 



w 



w 



w 



+ 



— +/ w 

 w 



FIGURE 15-1. Various ways of representing t/ie 

 round-wrinkled hybrid by gene symbols. 



The eighth pair of genes deals with the 

 presence (+) and absence (/) of tendrils, 

 threadlike structures used for attachment as 

 the plant climbs. When a double recessive 

 pea plant (wrinkled, no tendrils = w w 1 1) is 

 crossed to a pure double dominant (round, 

 tendrils = + -f + +), all Fi are round with 

 tendrils (+ vv + /), as expected. (Notice we 

 are using the + system for symbols here.) 

 When the Fi are self-fertilized (dihybrid X 

 dihybrid) the following results are obtained 

 in F2: 



wrinkled, no tendrils 



123 



Note that each gene pair shows segregation 

 in the Fo since round : wrinkled as 323 : 126 

 (a 3 : 1 ratio) and tendrils : no tendrils as 



