was in the direction of Tripsacum. Work on these hy- 
brids is still in progress, and more data will be needed to 
explain what is occurring in them, But a tentative con- 
clusion is justified: that all of them up to and including 
the sixth backcross generation, with rare exceptions, had 
one extra Tripsacum chromosome and showed phenoty- 
pic effects of it. 
INTERCHANGE BETWEEN CoRN AND TRIPSACUM 
CHROMOSOMES 
With respect to crossing over between corn and Trip- 
sacum chromosomes, Maguire’s (21, 22) observations, 
confirming our own of 1939, showed clearly that some 
form of exchange had occurred. Maguire (22) concluded 
that the end of the long arm of a Tripsacum chromosome 
and the end of the short arm of corn chromosome 2 are 
sufficiently homologous to allow apparently normal pair- 
ing, and that the terminal knob of the Tripsacum chro- 
mosome was occasionally transferred to corn chromosome 
2 by some mechanism other than normal crossing over. 
Our earlier publication (26), which reported a gene ex- 
change in asimilar hybrid at the sw; locus of corn accom- 
panied by cytological evidence of crossing over, made no 
claim as to the frequency or the exact nature of such ex- 
changes, and our hypothesis of the hybrid origin of teo- 
sinte did not, and still does not, require a decision on 
these questions. According to this hypothesis, a single 
successful hybrid between corn and Tripsacum, followed 
by a few exchanges at such positions in the chromosomes 
as to replace certain blocks of corn genes with blocks of 
Tripsacum genes, is all that is required. Yet Randolph 
(32) continues to be perturbed by the apparent fact that 
the association between corn and ‘T'ripsacum chromo- 
somes is not followed by the ‘‘expected frequency”’ of 
crossing over. 
[ 364 ] 
