BIOLOGY IN HUMAN AFFAIRS 



the number of independent assortments of gene packets 

 which can be made, since the genes carried by a single 

 chromosome would tend to be inherited together. Experi- 

 mentation has shown that such a principle actually does 

 exist. It is known as the Limitation of the Linkage Groups. 

 I If « chromosomes characterize a species, then only 2" 

 different germ cells can be formed through the choice of 

 chromosomes at the reduction division. 



The chromosomes do indeed pass as entities to the daugh- 

 ter cells at the reduction division, which is the essential 

 feature of germ cell maturation; yet this is not the only way 

 that genes can be distributed. If it were, we should have 

 to consider the whole chromosome as the unit of heredity. 

 Actually, any given chromosome contains a packet of genes 

 which tend to be inherited together; but individual mem- 

 bers from this packet may be exchanged for homologous 

 genes from the mate of this chromosome at the period when 

 like pairs of chromosomes come together, just before the 

 reduction division occurs. This phenomenon is known as 

 crossing-over; and by studying its peculiarities intensively, 

 an extraordinarily precise conception of the architecture 

 of the germ plasm has been obtained. 



Individual chromosomes can be tagged by the possession 

 of particular genes, and their behavior followed in hybrid- 

 ization experiments. Such investigations have shown that 

 the genes are placed in the chromosomes in a linear order, 

 like beads on a string. The Linear Order of the Genes is, in 

 fact, one of the basic tenets of genetics. Moreover, when 

 crossing-over occurs, the genes ordinarily recombine in the 

 same linear order. If, for example, a pair of homologous 

 chromosomes contain genes ABCDEFGHI and abcdefghi, 

 then a cross-over may occur which will result in chro- 

 mosomes having the genes lined up as ABCdejghi and 

 abcDEFGHI; but the order of the genes is usually retained. 

 If any change in order does occur, it is such that the whole 



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