May 4, 1949.
Dear Max,
I have checked 5-239 snd ~240. They are identical, and :

BF) Mf TH Lf Bf Lac# Xyl- Mtl- Gal- ¥,.° V,

This class (Lee# others—, orotctroph) is the most common segregant Ok
from H~168. Therefore, its origin from one division of xkix ~59 is
not certain roof of ten-strand scrozsinc-ovar. Tt would be interesting
to find 2 rare segregant and see whether it were accompanied by a more
common type as a sib, which would indicate 4 strands. This is assuming
tt 59 ig a nni-melerte eei ena undermert reduction, and that
119 undergoes second division to give two identies1 segres-nts. Of course,
the lethality of ~120 might not be meaningful. Quite the ncst interesting
thing that has coma up is your comment on the predicttbility of the survival
of eertain cells, Which, of course, strongly supports the genetic inter—
pretation of the rutios that have come out.
Lately I heve been trying to map Mal mre rdamntely, becnuse of its
peculiarity An the heterozygotes. The results I got were that Mal was
linked to M (methionine) but neither to B, on one side, nor Lac on the
other. I think that this may turn out to be the key to the situation,
not that there is a branched chromosome, but that I am dealing with a $rans-
locution heterozygote. "Het" would be a factor which increases the rate
of unequal disjunctions of the translosation cross givging a $k 311 disjunction:
A crossover betreen M and TL would givs the possibility of a balanced proto-
troph complenent, if there is alternate segregation, If there#were an unequal
@isjunction, so that the X-Mal# arm went to one pole, and the other arms to

the other, we hive the makings of a heterozygote which would, however, be
dafintuntx hemizygous for the factors on the Mal-X arm. This theory needs

some more going over, but is so far the best explanation of the svallable

facts. Bub J.F.Cro# and I have just finished working it up, so it may have

many defects. If this is true, there is some hope of gatting normally segregating
heterozygetes from the right outcrcases.

I am afraid that I've hud some very bad buck in recovering heterosygotes
from complete medium. In the first place, I've lost H-168 for the time being
becuse I relied on 4 muitrient agur clant of 1t, and threw out the EMS plate
I had ciso been storing it on. Secondly, I havandt been able to get anything
bub segregants omit cf the cultures whieh yor gent me. J hed emch better luck
the first time with the slant cultures you mde directly from FUS Lac, rather
than with the agur-sclidified cultureskixtenemtiox, If you've gotten them out ,
I wonder if you could send me transfers of the reisolated 243-246 in the 5~ .
(4-24-49) series. I think I'll be ible to recover H-148, but I thought I'd
better warn you. I couldn't get anything but, — and # out of 196, 204, and 53
also, so they too were probably heterozygous. Sometimes Mel heterozygotes have
the appearance you desoribed, but they should be easily verified on Xyl EMB.

Sincerely,