MENDEL'S LAWS OF HEREDITY 393 



ILLUSTRATIONS OF SIMPLE MENDELIAN INHERITANCE IN 

 BOTH ANIMALS AND PLANTS^ 



J. ARTHUR THOMSON 



How far has Mendel's experience been confirmed? — There has 

 been confirmatory work by Correns (on peas, maize, and garden- 

 stock), by Tschermak (on peas), by De Vries (on maize, etc.), by 

 Bateson and his collaborators (on a large variety of organisms), by 

 Darbishire (on mice), by Hurst (on rabbits), by Toyama (on silk- 

 moths), by Davenport (on poultry), and so on. There are some 

 difficulties and not a few discrepancies, but, as Bateson says, " the 

 truth of the law enunciated by Mendel is now established for a large 

 number of cases of most dissimilar characters." 



In experimenting with Lychnis, Atropa, and Datura, Bateson and 

 Saunders found that the phenomena conformed with Mendel's law 

 "with considerable accuracy, and no exceptions that do not appear to 

 be merely fortuitous were discovered. In the case of Matthiola 

 (garden-stock), the phenomena are much more complex. There are 

 simple cases which follow Mendelian principles, but others of various 

 kinds which apparently do not. The latter cases fall into fairly defin- 

 ite groups, but their nature is obscure." 



In experiments with poultry, the phenomena of dominance and 

 recession were detected; interbreeding of the hybrid offspring resulted 

 in a mixed progeny, " some presenting the dominant, others the reces- 

 sive character, in proportions following Mendel's Law with fair con- 

 sistency, though in certain cases disturbing factors are to be suspected." 



The general result, so far, is that Mendel's law has received con- 

 firmation in a number of very dissimilar cases. 



Dominant and recessive characters. — Let us first of all collect a 

 number of instances of contrasted characters which behave in relation 

 to one another as dominants and recessives. 



Dominant Recessive 



Pisum sativum Tallness Dwarfness 



Round seeds Wrinkled seeds 

 Coloured seed-coats White seed-coats 

 Yellow albumen in coty- Green albumen in coty- 

 ledons ledons 

 Purple flowers White flowers 

 Sweet pea Tall ordinary form Dwarf or "cupid" vari- 

 ety 



=" From J. Arthur Thomson, Heredity (copyright 1907). Used by special 

 permission of the publisher, John Murray, London. 



