Octoder 4, 1948

Dr, Barbara McClintock
Department of Genetics
Carnegie Institution
Cold Svring Harbor
Long Island, HN, Y.

Dear Dr, McClintock:
Thanks very much for the set of your reprints, They will be very useful,

In about a month, I shall be in New Haven to give some lectures on bac-
terlal genetics, I'11 try to make % to Cold Spring Harbor, but it's rather
doubtful that I can make it. Could I see you in XN, Y.?

lately, I've been puzzling over a segregation phenomenon in E, coli K-12
that is still far from solution, You may be interested to read this precisa
of it, which I'm putting to you in hoves you may have some sugrestions,

“ork on standard stocks suggests one linkage group, with factors arranged:

B MOB Mal Lae Vv 7 L. Mal and Lae
1 1 1 1 1

1
relate to the fermentation of maltose and Inctose; V1 to phage-resistance and
the others to nutritional requirements, Ina cross using parents different

in these factors, one can recover h-ploid prototrophs as nreviously published,
These recombinants are regularly either Lace or Lact, v4" or v,°, i,e, they
have presumably already segregated from the diploid zygote, By chance, how-
ever, I noticed a single prototroph whose behavior ie quite aberrant, being
heterozygous for some of these factors, The heterozygote is maintained on
synthetic medium, on which the (for the most part) nutritionally exacting
segregants are uneble to compete with it; when plated on 2 complete medium,
the segregants are produced at a large proportion of cell divisions, and en
be purified by subsequent plating, To summarize very briefly:

1) Segregation is accompanied by crossing over, and all classes of
recombinants (including the hitherto elusive multiple mutants) are found with
varying frequencies, The segregants are apparently pure-line, and do not
segregate further,

2) When crossed with apvropriate standard stocks, segregante may yield
prototrophs 1-10% of which are heterozygous for Lac, One heterozygote gave
three segregants, which were tested, (I cnll these "H") ‘Two gave F2 hetero-
zygotes beyond question; I am not as sure as I'd like to be yet of the third,
but it seems to give heterozygotes in crosses with standard and definitely
does on crossing with one of the other "K" stocks,
Dr, Barbara McClintock 20 October 4, 1948

3) the heterozygotes may be heteroploid, Ina cross involving all the
factors listed above, where one of the parents is a "heterozygote producer"
derived from the Fl, heterozygotes were found digenic for 411 those loci
except. Neal, which was typically Male (as it hapoened, the allele contributed
by the "H* parent in one series, by the standard in another), This might be
interpreted as homozygosity, but from several such stocks, Mal+ reversions were
obtained, which would be expected to be Mal+/Nal- (from Mal-/Mal~) end therefore
to segregate, This was not observed, the reversions behaving like pure Mal+,
although still heterozygous for Lact-, From this, 1t is tentatively concluded
that the aberrant heterozygote is hemizygous for Mal, and therefore heteroploid,
By successive mutations, we have just made up stocks eo that four additional
fermentation characters can be introduced in such crosses, Unfortunstely,
these are all more or less close to Mal, and so far have shown comvareble
behavior, I do not xnow whether some loci cen be homozygous in these hetero=
zygotes, but am trying to check,

4) As one might expect from (3) (although this came first) segregation
from the heterozygote is not random, but biased to different extents for dif--
ferent factors, NM shows this especially, There seems to be, for a given
heterozygote, a predominant class of segregant, with a minority of other
types, Different hetero zygotes have a different sredominant segregant, This
is consistent with the notion that the presumed deficiency in the Mol region
acts ar a recessive lethal, and that crossing over occurs to form those less
frequent scgregztions which sro originally coupled with it,

I badly need some testable hypotheses which will explain how a hetero=
zygote can split off segregants appsrently cspabdle of effecting (=) a defi-
ciency for the Mal region in many of the F2 zygotes which it forms vith normal
stocks, and (b) prolongation of the Aiploid phase, Of course, with more
detailed study, the difficulties may be resolved by correction of the data,
but as they stend they sre rether puzzling,

In the back of my head is the notion that the answer to these perplexitirs
may be found in your corn work, but I haven't yet found it, However, although
the problem has not been solved, it hac already been 2 useful tool in my other
work on the gene enzyme relationshin which you may have noticed in the Genetics
Records, Judging by their recombination te give Tact phenctypes, quite a few
loci are connected with lactase (Setagalactosidase), which by the way, I have
gotten out cell-free, somewhat purified and have reasonably good evidence for
its "single enzyme" nature, Some of these loci (e.g. Lae, and Lacy) are linked
so closely that the Lact recombinants are exceedingly rare, Crossing an H Lacy-
with a standard Lac), however, Lact heterozygotes were obtained which clearly
segregnted into the Lace components, confirming their nomallelism and the
dominance of Tact, Phage sensitivity is nlso dominant,

With best wishes,

Yours sincerely,

Joshua Lederberg

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