CHAPTER XIX 



LINKAGE 



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 different 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. Cross- 

 overs 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 to- 

 gether 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 previ- 

 ous 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 in 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 in known 



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