CHAPTER VI 



ARE MUTANT RECESSIVE GENES PRODUCED 

 BY LOSSES OF GENES? 



MENDEL did not consider the question of the 

 origin or the nature of the genes. He repre- 

 sented in his formula the dominant gene by a 

 capital letter and the recessive gene by a small letter. The 

 pure dominant was AA and the recessive was aa and the 

 hybrid, or Fj, was Aa. The question as to origin did not 

 arise, because the characters yellow and green, tall and 

 short, round and wrinkled, were already present in the 

 peas selected for the experiment. Only later, when the 

 relation of the mutants to the mid species from which 

 they were supposed to have come was considered, did 

 their origin arouse interest. A specific case, that of rose 

 comb and pea comb in domestic fowls, seems to have had 

 something to do with the reasoning that led to an inter- 

 pretation of recessive genes as losses or absences. 



Certain breeds of domestic poultry have a comb called 

 rose (Fig. 43c). They breed true to this type of comb. 

 Other races have a comb called pea (Fig. 43b). They also 

 breed true to their type. If these breeds are crossed, the 

 Fi has a new form of comb, called walnut (Fig. 43d). If 

 two F^ fowls with walnut combs are mated, the offspring 

 show 9 walnut, 3 rose, 3 pea, to 1 single. The numerical 

 result shows that two pairs of genes are involved, rose 

 and not-rose, pea and not-pea. The single comb is not- 

 rose, not-pea, which was then interpreted to mean the 

 absence of pea and of rose genes. But the not-presence 

 of pea and not-presence of rose genes does not prove 



