duly 28, 1948.

Dear Ed:

Since your visit, something very waportemt has eventuated: namely,
the finding of a heterozygote in K-12. As I toid you then, I was
cleaning up some details on the inheritance of phaze resistance. ¥
is a mutation wich is resistant to 1 and Tlh (although there is adie
unexplained reaction, other than widespread lysis) and sensitive to 85.
fo test ite allelism with Vy» I set up the cross W416 x Y-64, or:

B-N- 27 in the usual wey Somewhat over 200
B+ - Lao- Vy" fT~

prototrophs were tested; they segregated nicely for resistance to T5
(V, r/s) but there was only a single Tl-sensitive prototroph. These
tests were done, as routinely, on synthetio Lao EMB, and the exceptional
culture was Tac+, Since the identification of the Vy, locus as distinct
from ¥y rested on this lone culture, I set out to purify it and check-~--
and it's a lucky thingt When streaked out on EMB Lac (complete), the
prototroph (call 1% W-465), three kinds of colonies were noted: typical
Lac-, Lac+, and an essentially + culture with numerous - streakg#iand
sectors. The - and + gultures were both phage-resistant (11); the
"mosaic" type was sehsitive on EMB-synth. On complete; it continued to
segregate out + and - resistants. After a few passages of W-465 on
minimal agar, 1% was plated out on complete, amd individual + and -
segregents studied in detail. To make a long story short, W-465 is re-
peatedly throwing off a very high proportion of recombination types,of
all classes, and including multiple-requiring types such as B-M-?-I-
Which would be impossible to obtain by the earlier methods. A broth
culture of W-465 will amanttm eonteain as many segregants as hetero-
zygotes, md these segregants are of all the classes expected on the
basis of the linkage relationships. Naturally, ¥-465 has to be preserved
on minimal agar, and at that I dontt know how long I can keep it. At
present, it is still possible that W-465 1s a synkaryon with karyogamy
at least h-1f the time, but the recombination frequencies seem too high
even for this. I don't know what may be responsible for this exceptional
delay in recuction of the zygote; previous data indicate that no more
than .1% of the week prototrophs can be heterozygotic, and probably
much less.Now that I know what to look for, however, I an setting up
crosses where the muktiple dominant is the rarest crossover class, and
may be able to repeat it, perhaps also bj using segregants from 465.
Aside from the obvious conclusions, it is interesting to note that
phage“resistance is recessive. This supports the feeling I hae always
had that the Fdelayed effect" is a matter of segregution, probably of
nuclei. Single-cell work, and probably some oytology, are under way, and
of course I'll let you know what develops.

Best regards,