Linkage and Crossing Over Between Genes 



117 



Note that each gene pair shows segrega- 

 tion in the F 2 since the ratio of round to 

 wrinkled is 323:126 (a 3:1 ratio), and the 

 ratio of tendrils to no tendrils is 322:127 

 (a 3:1 ratio). Had these gene pairs been 

 segregating independently, the resultant ra- 

 tio would have been 9:3:3:1. Instead, the 

 F L » has relatively too many plants pheno- 

 typically like the P x parents (wrinkled, no 

 tendrils; round, tendrils) and relatively too 

 few new recombinational types (round, no 

 tendrils; wrinkled, tendrils). 



Examine also the phenotypic results ob- 

 tained from test crossing the dihybrid in 

 question ( + w -\-t by w w t t) : 



Independent segregation would have 

 given a 1:1:1:1 ratio for each of the types. 

 But again the dihybrids produced relatively 

 excessive numbers of gametes containing 

 the old (parental) combinations (-\ — h and 

 w t) and relatively too few new combina- 

 tional or recombinational types. Based on 

 the results of both crosses, we conclude that 

 independent segregation does not occur in 

 this dihybrid. The very existence of re- 

 combinational types proves — what had pre- 

 viously been an assumption — that we are 

 dealing with two separate pairs of genes. 



Let us assume now that the two pairs of 

 nonalleles involved are located in the same 

 pair of homologous chromosomes, a possi- 

 bility already mentioned in Chapter 4 (p. 

 48). In this situation the nonalleles in the 

 same chromosome are linked to each other. 

 Recall that sex-linkage involves the linking 

 of a single gene (such as the one for white 

 eye in Drosophila) to a particular chromo- 

 some (the X chromosome). Our concern 

 here is with intergenic linkage, which in- 



volves all the nonallelic genes presumed to 

 be located in the same chromosome. We 

 can obtain evidence for this only by study- 

 ing the transmission genetics for at least two 

 traits simultaneously. Since no genetic re- 

 combination was detected between the ge- 

 netic material for sex and for a sex-inde- 

 pendent trait (like eye color) in the X chro- 

 mosome, the linkage between the two traits, 

 sex-linkage (or, more precisely in this case, 

 X-linkage) was complete and presented no 

 evidence that this chromosome contained 

 two or more separable nonalleles. Because 

 the present experiments with peas involved 

 two separable pairs of genes, we were able 

 to propose the hypothesis that a chromo- 

 some contains more than one gene. 



Let us reexamine the results of the two 

 kinds of pea crosses described. In Figures 

 9-2 and 9-3 a horizontal line is used to 

 represent a chromosome and to indicate the 

 presence of one member of each pair of 

 alleles in each chromosome. Where the 

 genes could be either the dominant or the 

 recessive allele, a question mark is placed 

 in the appropriate position. Down through 

 the genotypes of the P 2 the results in Fig- 

 ure 9-2 are consistent with the view that 

 linkage is complete; that is, the chromo- 

 somes carrying w t or + -f are unchange- 

 able (except by mutation). However, the 

 occurrence of seven recombinational indi- 

 viduals in F 2 shows that linkage is not com- 

 plete — that these recombinants have a chro- 

 mosome which has kept one allele and re- 

 ceived the nonallele present in the homolog. 

 Moreover, reciprocal types of recombinants 

 are approximately equal in frequency, sug- 

 gesting that a given pair of genes switched 

 positions in the homologs; that is, they had 

 reciprocally crossed over. For this reason, 

 such recombinational individuals are said to 

 carry a crossover chromosome produced by 

 a process called crossing over. Therefore, 

 complete linkage between genes is prevented 



