112 



CHAPTER 15 



322 : 127 (a 3:1 ratio). Yet these 3 : 1 

 ratios are not produced independently of 

 each other, for if they were they would give 

 a 9 : 3 : 3 : 1 ratio, which is certainly not 

 found here. Instead, there are in F2 relatively 

 too many plants phenotypically like the Pi 

 parents (wrinkled, no tendrils; round, ten- 

 drils), and relatively too few, new recombina- 

 tional types (round, no tendrils; wrinkled, 

 tendrils). 



Examine also the phenotypic results ob- 

 tained from backcrossing the dihybrid in 

 question (-{- w -\- t X w w 1 1): 



Phenotypes No. Individuals 



round, tendrils 516 



round, no tendrils 9 



wrinkled, tendrils 7 



wrinkled, no tendrils 492 



The expectation according to independent 

 segregation is a 1 : 1 : 1 : 1 ratio for each 

 of the types obtained. Actually, there are 

 relatively excessive numbers of gametes, pro- 

 duced by the dihybrid, containing the old 

 combinations (+ + and w t) — that is, the 

 combinations which came from the parents 

 to form the dihybrid — and relatively too few 

 new combinational or recombinational types, 

 just as is true in the case where two of these 

 dihybrids are crossed. We conclude again, 

 therefore, that independent segregation does 

 not obtain here. 



The fact that we get some recombinational 

 types in the present case confirms the earlier 

 statement (which was really an assumption) 

 that we are actually dealing with two separate 

 pairs of genes. Let us assume that the two 

 pairs of genes involved are located on the 

 same pair of homologous chromosomes, a 

 possibility which was introduced in Chapter 6 

 (p. 46). In this event, the genes are said to 

 be linked to each other because they are on 

 the same chromosome. Recall that the phe- 

 nomenon of sex-linkage, already discussed 

 in Chapter 12, dealt with a single gene (such 

 as that for white eye) and its location on or 

 linkage to a particular chromosome (the X 



chromosome). We are now concerned with 

 studying intergenic linkage, which involves 

 all the nonallelic genes presumed to be located 

 on the same chromosome. (Evidence bearing 

 on this can only be obtained by studying the 

 transmission genetics of more than one trait, 

 other than sex, at a time.) For the first time, 

 then, we shall be testing the hypothesis that 

 a chromosome contains more than one gene. 



Let us re-examine, by means of Figures 

 15-2 and 15-3, respectively, the results of 

 the two kinds of crosses described. In these 

 Figures we will use a horizontal line to repre- 

 sent a chromosome and indicate the presence 

 of one member of each gene pair on each 

 chromosome. In those cases where the genes 

 could be either the normal or the mutant 

 allele, a question mark is placed in the appro- 

 priate position. If linkage had been complete, 

 that is, if the chromosome carrying w t or 

 H — \- was forever unchangeable (except for 

 the rare event of mutation), then all results 

 in Figure 15-2 down through the genotypes 

 of the P2 would be consistent with this view. 

 However, the occurrence of seven recombina- 

 tional individuals shows that linkage is not 

 complete. These recombinational individuals 

 have a chromosome which has kept one allele 

 and received the nonallele present in the 

 homologous chromosome. Moreover, the 

 reciprocal type of recombinant is apparently 

 equally frequent, as though a given pair of 

 genes had switched positions in the homologs 

 — that is, as though they had reciprocally 

 crossed over. For this reason such recom- 

 binational individuals are said to carry a 

 crossover chromosome produced by a process 

 called crossing over. So, complete linkage 

 between genes is prevented by a genetic proc- 

 ess, crossing over, which produces genetic 

 recombinations called crossovers. 



What other rules, if any, can we establish 

 for the crossing-over process and the cross- 

 overs it produces? Among the progeny ob- 

 tained from backcrossing the dihybrid (Figure 

 15-3), 16 received crossovers in the gametes 



