CHAPTER XXXH 

 LINKAGE AND CROSSING-OVER 1 



WILLIAM E. CASTLE 



In ordinary Mendelian inheritance, if two characters, A and B, 

 enter a cross in the same gamete (either egg or sperm), it will be 

 wholly a matter of chance whether they continue together or are found 

 apart in the following generation. If in the formation of gametes by 

 the cross-bred, A and B separate from each other and pass into differ- 

 ent gametes, it is evident that one of them has crossed over from the 

 gametic group in which both originally lay to enter the alternative 

 group. This event may be called simply a crossover. Crossovers and 

 non-crossovers will be equally numerous (50 per cent each) where no 

 linkage occurs. Also, if A and B enter a cross in different gametes, 

 one in the egg, the other in the sperm, it will in ordinary Mendelian 

 inheritance be a matter of- chance whether they emerge from the 

 cross together or apart. If together, it is evident that a crossover 

 has occurred; if apart, a non-crossover, that is a persistence of their 

 previous relations. Again, crossovers and non-crossovers will be 

 equally numerous '(50 per cent each) if no linkage occurs. 



Linkage may be defined as the tendency sometimes shown by genes 

 to maintain in hereditary transmission their previous relations to each 

 other. Thus if two linked genes, A and B, enter a cross together in the 

 same gamete, they will oftener than not be found* together in the 

 gametes formed by the cross-bred individual. Crossovers hi that case 

 will be less than 50 per cent, and non-crossovers more. And if the 

 same two genes enter the cross separately, one in the egg, the other in 

 the sperm, then oftener than not they will be found apart, in different 

 gametes formed by the cross-bred individual. Again crossovers will 

 be less than 50 per cent. 



The number of genes in a linkage group varies hi known cases from 

 2 to 50 or more. However many genes there are in a linkage group, 

 each gene shows linkage with every other gene belonging to the same 

 group, but the apparent strength of the linkage varies greatly. Under 

 uniform environmental conditions, A and B show a fairly constant 



1 From W. E. Castle, Genetics and Eugenics (copyright 1920). Used by 

 special permission of the publishers, The Harvard University Press. 



