Locus of tf on chromosome 7.<p>

Rick, C. M.  <p>
   
Our 1957 test progenies yielded not only a trisomic identification of tf (see
note on trisomic tests) but also a three-point determination of the tf locus
on chromosome 7.  The latter was obtained in the F2 of a cross between wt-tf
and mc.  The following data were obtained:<p>
<pre>

Combi-                              Contingency
nation  Phase  + +  + t  m +  m t   chi square        co.
________________________________________________________________
wt-tf    C     198   11   28   39     92.34       15.0 +- 1.4
tf-mc    R     226   60   47    3      8.68       30.5 +- 3.5
wt-mc    R     156   53   57   10      2.56       40.5 +- 3.3
</pre>
Robinson and Rick previously reported a linkage of 13 units between tf and
what was assumed to be a new wilty gene.  The allele-test F1 between the
latter gene and the standard wt had been grown in the greenhouse, where it had
a wt+ phenotype.  The F1 and F2 hybrids of this cross have also been growm in
the field, where they all showed the wt phenotype.  Whether it was an effect
of the particular greenhouse environment on the notoriously sensitive
expression of wt or a mislabeled family that accounts for the negative results
in the first test is not known, but the balance of evidence now favors that
they are allelic.<p>
     
The data clearly indicate the order of these three genes and the estimates of
distances are consistent, taking double crossovers into account.  The
following arrangement is therefore suggested for chromosome 7: wt - 15 - tf -
30.5 - mc.  This order is also borne out by the fact that no less than 3
homozygous triple recessives were obtained in this relatively small F2,
requiring a single-crossover origin for the wt-tf-mc chromatids, and
eliminating the possibility that mc could be situated between wt and tf.  The
new material including seed of the triple recessiie segregants has been sent
to Harold Young, who is now keeper of chromosome 7.<p>
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