Hayes and Garber - Cucurbit Breeding

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 f\ 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,
Pucdnia glumarum, is a simple Mendelian character. Susceptibility
is dominant over resistance and in F 2 ,
ratios of 3 susceptible
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 intermediate
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 probable
reason for conflicting reports regarding inheritance of resistance
to black stem rust of wheat, Pucdnia graminis tritid. 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