CHAPTER XXXII 
LINKAGE AND CROSSING-OVER' 
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 wil] 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 (so 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 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 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 
tFrom W. E. Castle, Genetics and Eugenics (copyright 1920). Used by 
special permission of the publishers, The Harvard University Press. 
441 
