Mendel's experiments 683 



Mendel did not stop at experimenting, but tried to find a 

 theoretical basis for his results. He saw that this must lie in the 

 nature of the sex cells, and that these must have a constitution 

 which is connected with that of the parent which forms them and 

 with that of the offspring to which they give rise, but is not 

 necessarily the same as either of these. He enunciated a single 

 principle : 



' the pea hybrids form egg and pollen cells which, 

 in their constitution, represent in equal numbers 

 all constant forms which result from the com- 

 bination of the characters united in fertilisation '.^ 



He used this to predict the results of what are now called 

 back-crosses (i.e. crosses between hybrids and their pure-breeding 

 parents). Experiments confirmed the prediction, so that the 

 principle was verified in the classical way for testing a scientific 

 hypothesis. Mendel speaks of the principle as ' the law governing 

 Pisum ' , but he was much too good a scientist to generalise it to 

 other organisms, and states that this must be the subject of 

 further experiment. Unfortunately his own further work, on 

 the bean Phaseolus and the sunflower Hieracium, met difficulties, 

 and after promotion to the headship of his House he had no time 

 for investigation. 



Although Mendel's papers must have been seen by many of 

 • the leading biologists of the time, their importance was over- 

 'looked, and it was not until 1900, as a result of a deliberate search 

 through the Hterature for anything that might throw light on 

 the problem of heredity, that they were rediscovered. His results 

 were soon confirmed for other plants, and extended to animals 

 by Bateson in Cambridge. Absence of horns, as in the redpoll 

 and other breeds of cattle, is dominant to the horned condition, 

 normal fur in rabbit and guinea pig is dominant to Angora, and 

 normal plumage in fowls to the condition known as silky, in 

 which the barbules are deficient. It was, however, found that 

 dominance does not always occur ; the heterozygote may be 

 exactly intermediate between the two parents, or resemble one 

 rather more than the other. The bluish-grey Andalusian fowl, 



1 Quoted from the translation of Mendel's paper made by the Royal Horti- 

 cultural Society, and revised and republished by W. Bateson in his Mendel's 

 Principles of Heredity (Cambridge, 1909) p. 343. The fact that the pollen grain 

 was not strictly the male cell was not known in Mendel's time, and the term 

 gamete had not been invented. 



