628 IMPROVEMENT OF LIVING THINGS BY MAN 



that some of these characters are linked together in the same sex. 

 This would explain some of the characters common only to male 

 or to female animals. 



It is plain that with forty-eight chromosomes, each of which is 



probably made up of a 



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 tnittiature*AiV 



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 completed 



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large number of genes 

 or determiners, hered- 

 ity in man is a very 

 complicated matter at 

 best. For a number 

 of unit characters have 

 been found which are 

 inheritable according 

 to Mendel's laws, and 

 undoubtedly others 

 will be added as we 

 learn more about the 

 working of these laws. 

 But this mechanism 

 of heredity is not as 

 simple as it seems. In 

 the first place the 

 genes are ultra-microscopic. One scientist has said : " If we 

 magnified a hen's egg to the size of the world (which would make 

 atoms rather larger than eggs and electrons barely visible), we 

 could still get a gene into a room and probably on a small table." 

 Moreover, each chromosome probably contains hundreds if not 

 thousands of genes. It is clear then that experiments which will 

 attempt to separate the genes and make new characters appear in 

 the offspring will be extremely difficult, to say the least. 



Practical Exercise 11. Make a diagram of a chromosome and place in it 

 genes or determiners for some of the unit characters given on pages 630-631. 



Diagram of chromosomes of the fruit fly, showing that 

 genes for certain characteristics are found in different 

 parts of the chromosomes. 



At the time of fertilization of the egg cell the genes or determiners 

 in the sperm cell are added to those in the egg cell and are then 

 handed down as unit characters. Thus a child inherits characters 

 from both parents. 



