Sestember 14, 19048

Dr, M, HL. ‘elle
Devartment of Bacterology
Cornell University
Ithaca, HL. Y,

Dear Jalle:

Tn the past several weeks, we have been working on a set of coli K-12
culturee of unusual genetic interest, It started with the realization that
one of the prototrophs which came up in a routine cross was, very unisuely,
heterozygous and sogreg ting for each of the factors which distinguish the
parents: 3B, H, T, 1, By, Lac, and a few ohage resistances, The heterozygote
can be maintained a$ such only on minimal medium; on complete medium it
rapidly segregates out giving an array of hrploid recombination types, Most
of theses, naturally, carry nutritions1 deficiencies and c-nnot therefore
compete with tne diploid on synthetic medium, The opacity to produce
heterozygotic prototrophs ia transmitted st lesst to the one segregnant tested
Crosses of it with standard tynes gave vrototrophs of which severnl percent
were heterozygous, ‘Some of these Fe heterozygotes, however, sre homozygous
for some factors in which the varents differed (and may be hemizygous, i, 6
heteroploid),

\s you might exoect we are pursuing the genstiec anclysis 2s vigorously
as we can, “Ve have a zygote of aorts since there is considerable recombi-
nation among the segregants, However, we sare probably dealing with some form
of chromosomal sberration in view of the suggestions of heteroploidy,

It would be very helpful to have single celt isolations made on thie
material, particularly by your famous technique of recovering products of
successive divisions, The rate of segregation is eo high that you would
have a fair chance of catching « cell in the act if both daughter celle
were taken, It would be especially important to know whether both products
of a segregating division were a) viable and b) haploid, and I have no means
now of attacking this point without your assistance,

I am writing to you now to ask whether your offer to do this kind of a
Jod on material that merited it were stil] open, and if you had the time and
facilities now to be able to undertake it in the reasonably near future, If
you will let me know whether you are interested, I'11 send the cultures and
further detaile,

Yours very sincerely,

Joshua Lederberg
Acsiatant Profeasor of Genetics

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