MENDEL'S LAWS OF HEREDITY 245 



ILLUSTRATIONS OF SIMPLE MENDELIAN INHERITANCE IN 

 BOTH ANIMALS AND PLANTS 1 



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 Dwarf ness 



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 



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

 permission of ihe publisher, John Murray, London. 



