40 I The Process of Evolution 



one for the recessive. (In other words, the Fi is heterozygous.) In 

 the formation of the F2, these factors are segregated, and the off- 

 spring are produced in the approximate ratio of three with the 

 dominant trait to one with the recessive. The individual showing the 

 recessive condition is homozygous for the factors. Further crossing 

 (inckiding backcrosses to the parental types) shows that, of the 

 three with the dominant trait, one will have like factors, and the 

 other two, difiFerent factors, as in the Fi individuals. 



When parents differing in, and homozygous for, several characters 

 were crossed, Mendel found that the factors for the different traits 

 he was studying behaved independently. In the Fi both dominant 

 traits were observed, and in the Fo each trait segregated by a 3:1 

 ratio. If the homozygous parents differ in two traits, for example, the 

 proportion of types in the Fo is the square of a 3:1 ratio or 9:3:3:1. 

 By backcrossing offspring to the parental types, verification of the 

 number of factors and their independence may be obtained. 



It is clear that the behavior of these factors parallels the behavior 

 of the chromosomes now known to bear them. The factors affecting 

 the traits in peas studied by Mendel were on nonhomologous differ- 

 ent chromosomes. Later studies showed that factors on the same 

 chromosome were linked (tending to occur together more fre- 

 quently than would be expected if assortment were independent). 

 When numerous traits are studied, their factors are found to fall 

 into as many linkage groups as the haploid number of chromosomes. 

 Within a linkage group the amount of recombination varies from a 

 very low percentage for genes close together to 50 percent for genes 

 far apart (which genetically is indistinguishable from independence 

 —occurrence on different chromosomes). 



RECOMBINATION 



The cause of genetic recombination of linked genes is cytological 

 crossing-over in meiosis (diagrammed in the previous chapter). In 

 studies of inheritance at a gross level, the factor presumably affect- 

 ing a particular characteristic is the minimum distance between two 

 points of crossing-over. (This is the operational definition of a gene 

 in this instance.) Crossing-over occurs in all organisms in which 

 meiosis and sexual recombination have been found. The basic mech- 

 anism appears to be the same wherever it occurs, and some workers 

 have postulated that meiosis cannot properly take place in the ab- 

 sence of crossing-over (or a specialized substitute). 



The precise mechanism of cytological crossing-over is not known. 

 Presumably when the chromosomes are synapsed and twisted to- 



