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NATIONAL RESEARCH COUNCIL.
CANADA

ATOMIC ENERGY PROJECT

Biology Branch CHALK RIVER. ONT.

June 28th, 1949,

Dre Joshua Lederberg,
Department of Genetics,

The University of Wisconsin,
Madison 6, Wisconsin.

Dear Joshua,

Iam at last getting down to the crossing experiments with the
strains you sent. In both crosses however (58-161 x W-677, and Y=40 x Y=-53)
& proportion of the recombinant colonies contain a mixture of more than one
typee In view of your recent work on heterozygote formation this was not un=
expected in the case of the first of these (I have assumed that W-677 contains
the factor Het) but I am surprised that it should occur in the Y=-40 x Y=53
CrOSSe

All crosses are made on minimal ager plus thiamin and the
parent strains are spread uniformly over the surface of the agare Two cc of
melted minimal agar is then poured over some of the plates, and other are in«
cubated without this additional layer. Colonies are picked, after 5-5 days,
into two cc minimal medium plus thiamin, incubated overnight, and streaked.

To determine whether there is appreciable parent type contamination streaking
has been done in parallel on EMB-Lactose and EMS-Lactose. The results indicate
that this is negligible. However, about ten percent of the cultures contain
an approximately 50:50 mixture of Lac* and Lac™ (% Lact = 35-80),

I would be inclined to suspect bad technique on my part were it
not that I get these mixed cultures/even when the greatest care is exercised,
(well spaced colonies, 2 ce layer of agar, picking small inocula from the
portions of those colonies which have grown onto the surface).

If you have run into the same thing and have the answer, I
should like to let it drop and get on with the crosses between streptomycin
resistant forms. However, if no one has done anything with it, it ought to be
followed ups I have been wondering if it represents the recovery of two single
crossover strands such as one would expect if two chiasmata were formed in the
four strand stage. This interpretation would, of course, only explain mixtures
of two types, and I am at the moment testing to see if we get mixtures of
three.

Sincerely,
?

Howard Be Newcombe

HBN:ar