564 GENETICS AND EVOLUTION 



the members of the pairs of homologous chromosomes separate as units 

 and go to opposite poles. Hence, all of the genes lying in one chromo- 

 some go to one pole and become incorporated into one gamete, and all 

 of the genes in the other member of the homologous pair go to the 

 opposite pole and become incorjjorated in another gamete. 



The linkage between the genes in a given chromosome is usually 

 not complete. During the process of synapsis, when the homologous 

 chromosomes are twisted around one another and attached point by 

 point, they frequently exchange whole segments of chromosomal ma- 

 terial together with the genes located within that part of the chromo- 

 some. The exact mechanism of this exchange is still unknown, but it 

 appears to occur at random along the length of the chromosome. The 

 chance that an exchange of segments will occur between the loci of 

 any two genes in a chromosome depends on the distance between the 

 loci: the greater the distance, the greater the opportunity for exchange. 

 The exchange of segments between homologous chromosomes, called 

 crossing over, makes possible new combinations of linked genes. 



1 he genes for plant size and fruit shape in tomatoes are located 

 in the same chromosome and therefore are linked; they tend to be in- 

 herited together. The gene for tall plants (T) is dominant to dwarf (t) 

 and the gene for spherical fruit (S) is dominant to the one for pear- 

 shaped fruit (s). The mating of a homozygous TTSS plant with a homo- 

 zygous ttss plant yields an Fi generation all of which are TtSs, tall 

 plants with spherical fruit (Fig. 32.6). So far, there appears to be no 

 difference from the ordinary dihybrid cross in which the genes are 

 located in different chromosomes. The difference becomes apparent, 

 however, when one of these TtSs plants is crossed to a homozygous re- 

 cessive one, ttss. If the two pairs of genes were located in different 

 chromosomes, the four classes of offspring— tall, spherical; dwarf, spher- 

 ical; tall, pear; and dwarf, pear— would be found in equal numbers. 

 If the genes were completely linked, that is, if no crossing over occurred 

 between them, only two classes, tall plants with spherical fruit and 

 dwarf plants with pear-shaped fruit, would be found and these two 

 classes would occur in equal numbers. When the cross is actually made, 

 most of the offspring are either tall plants with spherical fruit or dwarf 

 plants with pear-shaped fruit (the non-crossovers) and only a few are 

 either tall plants with pear-shaped fruit or dwarf plants with spherical 

 fruit (the crossovers). Crossing over between these two pairs of genes 

 occurs in 20 per cent of the chromosomes; the offspring are found in 

 the ratio of 40 tall plants with spherical fruit: 40 dwarf plants with 

 pear-shaped fruit: 10 tall plants with pear-shaped fruit: 10 dwarf plants 

 with spherical fruit. The distance between two genes in a chromosome 

 is measured in units of the percentage of crossing over that occurs 

 between them; thus T and S are said to be 20 units apart on the 

 chromosome. 



The facts of crossing over provide proof that the genes lie in a 

 linear order in the chromosomes. If three genes. A, B and C, lie in the 

 same chromosome and tests show that crossing over between A and B 

 occurs 5 per cent of the time (A and B are 5 units apart) and cros.sing 



