November 10, 1949.

Dr. M. Demerec,

Dept. Genetics,

Carnegie Institation,

Cold Spring Harbor, L.I., N.Y.

Dear Demsrec:

I am glad to hear that you are conténuing genetic studies on
streptomyein-resistance in K-12. I wish you luck in attempts to
cross K~12 with B, but judging fron my own axperiences, I could
not be too hopeful.

The napping of sr (which I propose should be designated as the
St locus, with alleles s, d, andr, 4 ,) st®, st3, stt. oo.) proved

to be very difficult, in Newcombe's experience as well as our om.
Doudoroff here, this suomer, tried to obtaim diploids heterozygous

at St, but failed, as all the Lact/Lac- diploids receovered were

pure, presumably hemizygous St™, Since the ‘lal locus has behaved in
the same way, orosses were done to check on the linkage of St to Mal,
and a fairly close linkage was found. As I mentioned at Shelter Island,
there appears to be a group of loci, which now includes Mal, Gal and
St, which occupy a segment of chromosome which is regularly eliminated,
sometimes from one parent, rarely from the other, so that the "diploids"
are monogenic for these factors. Thus, these factors have not been
mappable on the standard linkage diagram. I am sending %-1177 for your
interest in wonfirming these findings. This stock is derived from
w-677, and ultimately from Y-10, and carries:

T-L-B,- V," Lac- Mal- Gal-(slow] Are [slow] Kyl- utl- st?
I am also sending Y-40, as requested.

I have not found any additional factors located in the V,-TL
region.

Your discovery of another sr mutation [ean we agree to call this

r
St2 ] is of great interest. If your location is correct, it should be

possible to obtain diploids which are heterozygous at St., to test domi-
nance. If you would care to, I would appreciate your sending at, for
just this purpose. So far, resistance to Tl, and to azide,scem to be
recessive. I have just checked that Az” x St* can be used in selecting
recombinants by plating &&th both agents, without too much confusion
from spontaneous mutation. 4S is in presa.

Sincerely yours,

Joshua Lederberg