June 9, 1949.

Dr. L. Cavalli,
Deot. Genetics,
University of Cambridze,

England.
Dear Dr. Cavalli,

I hops you will forgive my dekay in sending you duplicate subcultures
of the stocks which were lost from my letter of Marcn 24. However, I have
been away from the laboratory part of this time, and also I have bean a
little perplexed about what would really be worthwhile to send you. You
may by this tine have noticed my paper in the April Proc. Nat Aaad. Sei,
which deserlbes soma of the perplexities of the aberrant heterozygotes.
in an attempt to elucidate the role of the Mal, region (since Mal has
alaays besn joctme hemizygous when other factors were ome or heterczygous)
I have been trying to map it, and the resukts so far indicate that it may
be off on a "branch" of the chromosome, 1.a., dadicating structural hetaro-
sygoaaty in the stocks used. However, further tasts on crosses of segre-
gants from the same heterozygote, which should be atructurally unifora,
have given the mum peculiarities of segregation, so that the situation is
not yat settled. I am writing you this, because I am afraid now that the
assumuptions,shich had been made previously,that segregation was essentially
random, and that the ratios recovered are the resuit only of the selection
imposed by the minimal medium, may not be true. In this connection, detailed
segregation sackx and crossover data will be very useful. We are doing a little
of this, and will send you a summary as soon as feasible. Meanwhile, any
information that you may have been able to accumulate in crosses involve the
complex fermentation stocks would be very useful.

I am sending you again a few stocks which may be useful.

W-377 (closely related to 7-533, but carries a non-apistatic, bus difficult-to-
score Gal~) Lac)~ Mal,~ Gal- Xyl- Are- utl- V," 1L3,-

W-945 (carries same Lac, Mal factors as W-677, but the other mutations may be
different, as they were obtained Andependently, and have not been
gested.) Lacj- Mal,- Gal- Ar~ Xyl- wtl- TLB|-. Judging from the occurrence
of soma weakened Lac/ recombinants’ in outcrosses, this stock also carries
a factor shich may be partially epistatic to Lacs, hut probably not
enough so to cause any confusion. I am of course continuing to.try to
develop the most suitable stocks. .

W-705 Beif-Lac,- Mal- Ara- Gal- V,". The Lacj— and V,” of this stock are
allelic to those in W-677,: di though of independent occurrence. I have
no further information on the other factors. .

W326 Histiddne-; Serine or Glycine— New mitant from K-12, using penicillin
W-336 methionine? lysine-; tryptophane~ "01, May be useful in "outcrosses",
There is very little else that I can report to you. I hope and trust
that you have had better luck with FMB. And, of course, I would be very
pleased to hear any later details on your stocks which show high rates of
recombination. | |

Sincerely,

Joshua Lederberg,
Assistant Professor of Genetics