Chap. 20 THE PHYSICAL BASIS OF HEREDITY 403 



In his experiments, Mendel luckily dealt with no such genes. Linkage works 

 as a check on the independence of genes, a hold-back on too much scattering. 



Arrangement of Genes on Chromosomes 



Linear Arrangement. Genes are located throughout the length of each 

 chromosome in precise and standardized arrangement (Fig. 20.8). Maps 

 have been made of certain chromosomes of Drosophila showing the locations 

 that have been worked out for a comparatively large number of genes. How- 

 ever, such maps give little idea of the number of genes on a chromosome. In 

 one species of these little fruit flies it is well established that there are certain 

 chromosomes that contain some 2500 genes. 



Crossing Over. This change in location of chromosomes occurs during 

 meiosis in the prophase stage when similar (homologous) chromosomes unite 

 in pairs with equivalent genes opposite to one another. A part of one of the 

 pair may change places with a corresponding part of the other (Fig. 20.12). 

 It is as if the residents of a section of one side of a street changed places with 

 those of a corresponding section of the other side. 



Crossing over may occur in more than one section of the partner chromo- 

 somes in some species and not in others, and in some species only under 

 certain conditions. It may take place in one sex and not the other, as is the 

 case in the female but not ordinarily in the male of Drosophila, although it 

 may be induced by exposing the latter animals to high temperature or x-rays. 

 In most plants and animals, however, crossing over occurs in both sexes. 

 Thus the position of genes on different pairs of chromosomes results in their 





B3 c^-—^ 



■BZ) 





B 



D 



I 2 



Fig. 20.12. Crossing over of corresponding sections of the homologous partner 

 chromosomes during the "4-strand" phase, in the prophase stage of division of 

 sperm or egg cells. Upper — 1,2,3,4; example of single crossing. Lower — 1,2; ex- 

 ample of double crossing. Letters represent sections of chromosome strands. In 

 this early phase of synapsis, each member of a future pair of chromosomes has 

 doubled, thus forming 4 chromosomes. In these cases, crossing over occurs in 

 only two of them. Each of the four chromosomes will be distributed into a separate 

 cell in the two later meiotic divisions. 



