158 



LIFE: ITS NATURE AND ORIGIN 



»V 



m 



<j 



v 



k 



k 



m 



Bl 



SINGLE CROSSING-OVER 



y* 



m 



B i. B 



DOUBLE CROSSING-OVER 



Disjunction Crossing-over 



of the gamete 



chromosomes tyI)e 



k 



Figure 19. (From paper in "Colloid Chemistry," Vol. 5, by J. Alexander, Rein- 

 hold Publishing Corp., N. Y.) 



duplicate themselves and the side-by-side partners separate, one 

 set going to each of the two cells formed when the original cell 

 divides. However in meiosis, the process of formation of the sex 

 cells or gametes, the members of each chromosome pair separate, 

 and the chromosome number in the resulting daughter cells is 

 reduced to half the number in the somatic, or ordinary body cells. 

 In 1909, Janssens (a Jesuit) in investigating meiosis, discovered 

 what he called chiasmata (from the Greek letter X, chi) involving 



:^ 



jy 



Before 

 break 



X 



y 



u 



y 



Broken Rejoining 



INVERSION 



a 

 b 



10 



y Uz 



Broken section 



transferred to 



another chromosome 



TRANSLOCATION 



u 



y 



n 



w 



U 



Broken section 

 entirely eliminated 



DELETION 



Figure 20. (From paper in "Colloid Chemistry," Vol. 5, by J. Alexander, Rein- 

 hold Publishing Corp., N. Y.) 



