CHROMOSOMES 629 



Here is what Professor H. S. Jennings, geneticist, of Johns 

 Hopkins University says concerning the genes : 



" Every pair of human parents contains thousands of pairs of the 

 packets (genes) of chemicals on which development depends. From 

 these a set is drawn almost at random (subject to the condition that 

 one packet is taken from each pair possessed by each parent) ; this 

 constitutes the heritage of the child. Any pair of parents may thus 

 produce not merely thousands but millions of different combinations, 

 each yielding a child of different characteristics. There is no way of 

 controlling the combinations that shall enter into a child of given 

 parents ; there is no prospect that there ever will be. It is, therefore, 

 impossible to predict what kind of offspring will be produced by a given 

 pair of parents — save in a few respects, in cases where the constitution 

 of both packets of a particular pair are known for each parent. If both 

 parents have the corresponding pairs defective in the same manner — 

 lacking, for example, something required for producing a normal 

 mind — then their children will be all defective like the parents ; 

 feeble-minded parents will produce feeble-minded children. But if, 

 as may well be the case, the feeble mind is due to defects in different 

 packets in the two parents, then all experimental breeding shows that 

 the two parental stocks may supplement one another, so that the 

 defect will not appear in the offspring. These characteristics that 

 are predictable are extremely few." — From Prometheus by Jennings, 

 published by E. P. Dutton & Co. 



More recently we have found that heredity is complicated by 

 another factor. We have already learned that the hormones play 

 a very important part in regulating life activities and that the over- 

 activity or underactivity of certain glands may produce profound 

 changes in an organism. The under- or overdevelopment of the 

 pituitary gland, for example, causes a dwarf or a giant to result, 

 and other of our ductless (endocrine) glands have almost as 

 startling results. The use of the X-ray in experimental biology 

 has brought about strange results. Professor Muller of the 

 University of Texas has recently produced about 100 new 

 varieties of fruit fly, which breed true, simply by keeping the flies 

 under the X-ray for certain periods of time. We are finding out 

 that our problems of breeding are not as easy to solve as we first 

 hoped. 



Practical Exercise 12. Make a report on the action of some one endocrine 

 gland. Get material from Harrow's " Glands in Health and Disease," or 

 use the Readers Guide to find titles of articles on the endocrines. 

 h. bio — 41 



