CLASSIFICATION AND INHERITANCE IN WHEAT 85 



Presence or Absence of Beards. Wheats have been classified 

 as bearded and awnless but this is not genetically correct. The 

 awn is an extension of the flowering glume. The common wheats, 

 like Marquis and Bluestem, are not truly awnless for there is a 

 short extension of the awn particularly in the spikelets at the top 

 of the spike. Three to one ratios have generally been obtained in 

 crosses between bearded and so-called awnless (tip-awned) 

 wheats. The Howards (1915) have carefully worked out the 

 inheritance of these characters. They have explained results by 

 supposing two factors, A and B, to be present in a homozygous 

 condition in bearded wheats. They have found two kinds of very 

 short-awned wheats, one like the tip-awned Marquis or Bluestem, 

 and the other with somewhat longer tip awns. Each of these 

 varieties was found to contain one of the factors A or B in a 

 homozygous condition. In crossing a tip-awned wheat like 

 Marquis with bearded varieties, theFi generation, as a rule, shows 

 an extension of the tip awns and it is frequently possible to separate 

 these FI plants from the tip-awned parent. In crossing bearded 

 with true beardless, the PI is apparently beardless and there is a 

 range in F 2 from completely bearded to awnless. Fully bearded 

 plants breed true for this character. 



Inheritance of Disease Resistance. Biffen (1907a, 1912, 1917) 

 has found that the inheritance of host reaction to stripe rust, 

 Puccinia glumarum, is a simple Mendelian character. Suscep- 

 tibility is dominant over resistance and in F 2 , ratios of 3 suscep- 

 tible to 1 resistant are obtained. Nilsson-Ehle (191 Ib) in a 

 similar study found the FI generation resembled the susceptible 

 parent in some cases, the resistant in others, and was inter- 

 mediate in still others. Complex segregation for resistant 

 versus susceptible forms was obtained in later generations. 

 Results were explained on the multiple factor basis. 



Studies by Stakman and others (1919) have shown the prob- 

 able reason for conflicting reports regarding inheritance of resist- 

 ance to black stem rust of wheat, Puccinia graminis tritici. They 

 have demonstrated the fact that there are a number of biological 

 or racial forms of rust roughly analogous to pure lines. These 

 forms can only be differentiated surely by their specific reaction 

 to pure-line wheat varieties. Studies of their constancy indicate 

 that they are not easily modified, i.e., that the parasitic reaction 

 of each form is constant. At the Minnesota Station (Hayes and 

 others, 1920) studies of inheritance of resistance were made in 



