An Introduction to a Biology 



the difference between the results of mating two kinds of 

 hybrids differing not in any visible character but in their 

 pedigree. The two kinds of hybrids that were used were 

 (i.) a hybrid produced by the union of two hybrids, and (ii.) 

 a hybrid produced by crossing a hybrid with an albino ; we 

 may call the former HH and the latter HA. Three kinds of 

 matings can be made with this material namely, HH x HH, 

 HH x HA, and HA X HA. In each of these types of 

 union a hybrid is mated with a hybrid. So that I argued 

 that according to the Mendelian theory a quarter of the 

 population produced in each of the three cases should be 

 albino ; but that according to the commonly accepted view 

 of heredity, known as the law of contribution, one would 

 expect the proportion of albinos to be greater in proportion 

 as the number of albinos in the pedigree of the hybrid parents 

 was greater. This was found to be the case. But it was 

 pointed out to me that this result was not evidence against 

 Mendel's theory, unless I had established the hybrid nature 

 of every individual used in the experiment. " Have you 

 done this ? " I was asked. " Are there any cases of families 

 in unions of type (i.) where no albinos have been produced ? " 

 " Yes," was my reply " Then one or both the parents of 

 these families were really dominants," was the answer to me. 

 I did not think that this theory was true, because it 

 seemed to me that to say that unless a quarter of the family 

 produced by an animal consisted of recessives it was not 

 a hybrid, was a very easy way of establishing the fact that 

 hybrids always produce recessives in the proportion of one 

 in every four. It seemed that an argument of that kind 

 was not likely to be based on anything having any existence 

 in nature, but it is a strong warning not to be led away by 

 appearances that when I tested this theory I found it to 

 be true. The animals falling into the category CC in F 2 

 are sharply distinguished into two kinds : (i.) hybrids that 

 will produce albinos in the proportion of one in every four ; 

 and (ii.) dominants which, when mated inter se, produce no 



213 



