HEREDITY 



piece of chromosome which has been exchanged with the 

 piece belonging originally to its homologue is inverted. 

 In other words, the chromosome may sometimes have the 

 genes lined up as ABCihgjed. When such a thing happens, it 

 is known as a chromosome mutation, since it usually 

 produces a change in the traits exhibited by the 

 organism. 



Other chromosome mutations also occur in rare instances, 

 and each usually produces a detectable change in the 

 individual. A whole extra chromosome may be received 

 by a germ cell owing to the failure of a particular pair to 

 divide. This occurrence naturally leaves one germ cell 

 with n — \ chromosomes. In such cases the deficient germ 

 cell usually does not function. The redundant germ cell 

 may occasionally mate with a normal germ cell, however, 

 with the result that an individual is produced having 

 2« + 1 chromosomes. Other chromosome mutations have 

 also been traced, and their study has yielded significant 

 results. Pieces of chromosomes are lost; unequal crossing- 

 over occurs; and there are duplications and translocations 

 of chromosome parts. Following such instances in pedigree 

 cultures by means of known genes has established the 

 Chromosome Theory of Heredity beyond all shadow of 

 doubt, and has thrown new light on the way in which 

 evolution has taken place. 



This brief glimpse of one of the main phases of genetic 

 work gives fair support^ for the statement that the me- 



1 Detailed proof of the genetic principles mentioned (and many others unmen- 

 tioned), together with the consequences which flov/ from them, can be found in 

 numerous elementary textbooks. "The Principles of Genetics," by E. W. Sinnott 

 and L. C. Dunn (McGraw-Hill Book Company, Inc., New York, 1925), and 

 "Genetics in Relation to Agriculture," by E. B. Babcock and R. E. Clausen 

 (McGraw-Hill Book Company, Inc., New York, 2<^ ed., 1927), are recommended. 

 For information on the cellular basis of genetics, the reader should refer to "The 

 Cell in Development and Heredity," by E. B. Wilson (The Macmillan Company, 

 New York, 3"^ ed., 1925). For the effect of modern genetic doctrines on the evo- 

 lution concept, he should see "A Critique of the Theory of Evolution," by T. 



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