September 16, 1949.

Dr. Howard B. Newcombe,
Atomic Energy Project,
Chalk River, Ont., Canada.

Tear Howard:

Dr. Michael Doudoroff, from the University of California, spent
some weeks here on a Guggenheim fellowkhip, and was interested to
try some experiments in K-12 recombination. We decided fo test for
the dominance of streptomycin-resistance in heterozygous diploids,
along the lines that I believe I already discussed with you. We think
that you may be interested in the results, of which please feel free
to make any use you wish.

A W-677 sr stock (W+1177] was produced by plating into nutrient
agar with 100 units streptomycin. As you may remember, W-677 is a multiply
marked atock: TLP]~ Lacl- Mal,- Xyl- Gal- Ara~ Mtl- V P which I have been
using for segregation and croSscver studies. By crossing With W478, a
Beit Het stock, a number of heterozygotes were obtained, identified by
their segregating Lac* and Lac-. However, all the heterozygotes tested
were pure SR, and gave no sensitive segregants. This behavior parallels
that of Male, as mentioned in my PNAS paper. SR was therefore tested
for linkage to Mal. In this cross, with parental couplings:
[Mal~ SR]; [Mal+ sr*] , 30 Malt and 52 Mal- were tested :
SR + whigh shows a distinct linkage of
Yal+ 8 30 Mal to SR.
Male 44 0
In these tests, a less intense linkage of Mal and of SR to Lae
was also noted, e.2.:
In analysing these fagures, take into

Lact Lace account an adjustment of the Mal segregaticc
Mal+ 17 ls which is about 20:1 Male:Yal+. In order
Yal— 6 23 to collect more material, Mal+ are picked

disproperticnately to the frequency with which they appear. Although a linkage
or interaction of Mal to Lac of some kind 1s apparent, I have not been able
to map it. I have come to the conclusion that there is some mechanism for

the elimination of part of the chromosome intwpoduced from the Mal* parent

and that this distorts the segregations both in the "normal" protctrophs,

and in the persistent diploids. All this does not Pexplain" the genetics of
sr, but puts it in the same dilemma as Mal, on which I have been working for
some times.

Yours sincerely,

Joshua Lederberg