Letter from Barbara McClintock to Charles R. Burnham
McClintock's reply to Burnham's letter of January 26, 1931. In it, she conveyed some of her findings on the occurrence
of genetic and cytological crossing-over, on which she was then working with Harriet Creighton at Cornell.
Item is handwritten. Item is a photocopy.
Number of Image Pages:
6 (547,431 Bytes)
1931-02-06 (February 6, 1931)
Burnham, Charles R.
Original Repository: University of Minnesota, University Archives. Charles Burnham Papers
Reproduced with permission of the University of Minnesota, University Archives. Charles Burnham Papers
Medical Subject Headings (MeSH):
Education and Research at Cornell, 1925-1931
A Correlation of Cytological and Genetical Crossing-Over in Zea Mays (1931)
Box Number: 3
Folder Number: 4
Feb. 6, 1931
I had a long typewritten leter [sic] partly completed but got to feeling bum and have come home to bed. After several days
here I am nearly fully recovered but must apologize for being so late in writing. I realized there was so much to relate that
I couldn't do justice except for a long letter giving you the facts. I have'nt [sic] my cards home here but will send
you some of the dope.
Many thanks for your letter. I have already heard from Anderson and have sent in my application. Stadler, not knowing of my
desire for a fellowship, asked me to work at Missouri this summer at a salary of 500 dollars for the two months. I can work
on what I please but he wants me to tackle some of the deficiency stuff. I rather think I shall go there. Things may be in
a mess here but with the new building conditions our group work will be lost. It will be a good time to go. I am sorry for
Harriet working on nearly alone. After Missouri this summer I expect to come back to C.U. and get my corn ready -- that which
will be grown and pollinated for me here. I want to get the trisomes along
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I have grown material (about 25-30 kernels from each ear) of 460 and 464 which was giving peculiar ratios for pr. I did not
get any 2n plus 1 individuals. Looks bad for pr and [diagram]. However I have the other chromosome isolated I think. Your
B11 (13) [line to, ([diagram] plus c-sh-wx chr.)] crossed with c-sh-wx gave 270(11) which was trisomic and 12 percent sterile.
When (x) it gave peculiar ratios for c-sh and wx. I have'nt [sic] my cards here but the interchange chromosome was found
to be C Sh wx[ . . . ]. The kernels of 270(11)(x) were grown; gave 388. There were about 12 2n plus 1 individuals. All but
one of these were partially sterile (12 percent or so). One individual (388F1) was 2n plus one, 3.5 percent sterile, and gave
disomic inheritance for sh and wx. One would expect 2 types of 2n plus 1 from 27011 which was probably a 2n and one interchange
chromosome ( I will let you know which one by March, I hope). Trivalents in 27011 would give 16 plus percent sterility. [diagram]
would give no sterility, etc. I shall send you my calculation when I have a chance at my cards. Any way, I thought 388F1 might
be a mistake but the kernels were giving 2n plus 1 individuals. This looks as if there were no mistake. I shant [sic] feel
confident until I cross a 2n plus 1 to an x-n2. I have crossed 388F1 to granule chromosome which was pr. If pr is involved
I ought to get results this spring. Shall I send you
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some kernels of this ear? It should be worked right away for its trisome and [diagram] sterile value. Since 388 culture contains
2n and interchange chr. which has c sh wx on it I have crossed over the partial steriles and the granule chr. I hope to be
able to tell which piece carries the c-sh-wx genes. Has Beadle made any pollen counts on the c-sh-wx trisome I sent him? My
material hasn't come along yet. I am anxious for a verification.
I was very truely [sic] pleased about the [diagram] plus the 4-th smallest chromosome number 7. A nice correlation! Were
these 2n plus 1 also partially sterile? Is there, therefore, a certain part of the chr. Which is responsible for the peculiar
appearance of the plants? With the different steriles involving this chromosome we might be able to relegate the cause to
a small part of the chromosome, not the whole chromosome. Were your gl1 counts made on these individuals which were non-sterile
or partially sterile, i.e., is there any chance that the gl1 might be on a piece not present in that particular trisome. I
am growing number 7 heterozygous for gl1 now. Also expect to cross chocolate pericarp to number 7. Do so at Cal. Tech if you
can. Also cross chocolate pericarp to number 5 (6th smallest) if you can. I shall try to cross it but Emerson has very little
room this year due to rearragement [sic] of the houses and the use of one house for Livermore's potatoes. We may not have
material ready at the
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I am much interested in the progress of the steriles. How is the cytology coming? I believe everything is OK from all your
reports. Things here are dead except Randolph finds there is some pairing (81 and 111) in haploids. I am going to ask to see
the slides. It should be worked in early prophase for nature and extent of synapsis. Is Beadle doing this for a synapsis and
the c-sh-wx trisomic chromosome and the "impaired" large-small pair? I hope he is. I believe also that the wheat situation
can be solved better that way than the way they are going about it. Have spoken to Lillian Hollingshead about it. I believe
that metaphase chromosomes are not altogether reliable indices of relation of chromosomes. That is, 2 species may have similar
chromosomes or quite dissimilar. Even in some of the "dissimilar" chromosomes the chromatin may be comparable but
the metaphase morphology of the chromosomes may be somewhat different. My idea is based on the satellite chromosome in corn.
At metaphase the relative length of the 2 arms is about [diagram] but at prophase it is nearer [diagram]. The deep staining
part attached to the nucleolus adds greater length to the chromosome at metaphase than a similar linear length of prophase
chromosome adds to the metaphase chromosome. If the granules on the chromosomes do the same thing the differences in the relative
length of chromosomes may be accounted
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for without assuming so much manufacturing of new chromatin or rearrangement of the chromatin of the complement. My idea being,
the amount of stainable substance is not necessarily an index of the amount or quality of substance which can be compared,
one chromosome for another. We are very ignorant of the nature of chromatin, chromomeres, etc. Especially are we ignorant
of the manner of contraction of the chromosome. Belling has made some shots at it but so far he is too erratic. Theory blinds
him slightly. I am going to try Feulgen reaction on Zea chr. this spring if I can. We have a terrible mob in cytology this
year -- 6 labs. Dr. Sharp is going to take 2 labs. The 6 labs we had last year were the cause of my punk [?] summer and lackadaisical
fall. I have done nothing for days. I am going to take it easier in lab. and save some energy for the corn.
My talk at the meetings went over beautifully. I showed lantern slides of the photographs. They were very convincing to the
geneticists most of whom are not accustomed to seeing chromosomes clearly. I have the slide showing the cross (permanent one
from which the 3 photos were taken) with me and used it as a demonstration. There was no difficulty in convincing them for
the slide showed up beautifully. I brought along the green filters. I spent many hours getting my lantern slides just right
but it paid.
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I think I have written enough. I am writing nearly flattened out in bed. My scrall [sic] is most unreadable. I don't expect
you to get it all.
I have one more idea which I think is new. It may work. I am wondering about the nature of the secondary constrictions. I
feel strongly that some of them are related to the pedicle of the satellite. A satellite is produced if the attachment of
the chromosome to the nucleolus is near the end of the chromosome. The nearer the end (or the smaller the chromomeres on the
end piece) the smaller the satellite. At metaphase, the pedicle of the satellite is similar to a secondary constriction. If
the attachment were partly down in the chromosome the metaphase chromosome would'nt [sic] have so much of a "satellite"
chromosome but would have a secondary constriction. I do not think all secondary constrictions are defunct spindle fiber attachment
regions. I spoke to Sharp about it. He went to his Vicier slides. The longest chromosome has a secondary constriction [diagram].
It looks more than suspicious that this secondary constriction is related to the nucleolus. It is a logical conclusion even
if it prooves [sic] to be untrue.
Harriet was just over with the mail. She is coming down with my complaint. We both were out to the same dinner party and probably
got the same intestinal bug. She is just succumbing tho and I am getting better. I have'nt [sic] half completed what I
wish to tell you but have tried your patience enough.