UNIVERSITY OF LONDON

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POSTGRADUATE MEDICAL SCHOOL
OF LONDON

Telegrams

POSGRADMED CHISK LONDON DUCANE ROAD

Telephone
SHEpherds Bush ERUCHeX 2030, Ext. 222. LONDON W.12

Dr. W. Hayes,
Department of Bacteriology.

19th. June, 1957.

Dear Josh,

Many thanks for your letter about the question of dominance
Of Azr. I thought, and I think said so at the time, that your
criticism of my experiment was a perfectly valid one, i.e. that
although multiplication of the zygotes is inhibited, nevertheless
this does not say that expression cannot take place in its presence;
I think it quite likely that your diploid result is the correct one.

With regard to your query about the M/1600 azide, as you suggest
I found that this was optimally differential by plating dilutions
of sensitive and resistant prototrophs from the test cross on
minimal agar containing varying concentrations: M/800 gave no
colonies of the resistant strain, whereas M/3200 gave colonies of
each. My reference to M/500 azide in nutrient agar was both a
quotation and a confirmation of your work. I always use this
concentration and find it perfectly satisfactory. About the
0.1% Sodium aspartate, this is expressed in terms of concentrations
of aspartic acid, but is not at all critical.. Asparagine is not,
I think, so effective, but I have no quantitative data to support
this.

I am leaving here ina fortnight and am very busy clearing
up all sorts of matters as well as changing house. I am then
going to the Mediterranean for a IO. weeks! holiday, and will not
be returning to London wmtil September 10. ‘I hope if you do come
that I will not miss you. If you write ‘to me at my present
address it will be forwarded to me. —

. Best Wishes to Esther and Yourself,

~

| Yours, js