PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 5 FEBRUARY 15. 1919 Number 2 
IS THE ARRANGEMENT OF THE GENES IN THE CHROMOSOME 
LINEAR? 
By W. E. Castle 
BussEY Institution, Harvard University 
Read before the Academy, November 18, 1918 
Every biologist is familiar with the remarkable discoveries of Morgan and 
his associates concerning the germ-cells of Drosophila. One of the most 
important of these discoveries is concerned with the phenomenon of linked 
inheritance. This kind of inheritance, while entirely conformable with 
Mendel's law, forms a very distinct and important class of cases whose 
existence has been brought to light since the rediscovery of the general law in 
1900. ^^rnder the general law it is found that characters which behave as 
distinct. units in heredity assort quite independently of each other. Thus if 
parents are crossed one of which possesses two characters, A and B, while the 
other lacks them, then the offspring of this cross will transmit A and B some- 
times associated in the same gamete, sometimes in different gametes, the two 
events being under the laws of chance equally probable. 
But in linked inheritance, a phenomenon first made known to us through 
the work of Bateson and his associates in England, later more fully explored 
and explained by Morgan, A and B are not wholly independent of each other 
in transmission. If they enter a cross together, they have a tendency to stay 
together in later generations ; and if they enter a cross separately, they have a 
tendency to remain apart in later generations. Morgan has suggested that 
what binds or links two characters together is the fact that their genes lie in 
the same body within the cell-nucleus. Such bodies he supposes are the 
chromosomes. The evidence for this conclusion is very strong. Morgan and 
his associates have demonstrated the existence in Drosophila of four groups of 
linked genes corresponding with the four pairs of chromosomes which the 
cell-nucleus of Drosophila contains. Morgan has further suggested (and has 
beyond doubt established the fact) that the genes within a linkage system 
have a very definite and constant relation to each other. He supposes their 
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