Letter from Barbara McClintock to Charles R. Burnham
McClintock informed Burnham that she believed she had isolated all the linkage groups with particular chromosomes and denoted
most of the letter to her data. Letter included several diagrams.
Item is handwritten. Item is a photocopy.
Number of Image Pages:
10 (671,390 Bytes)
1930-04-04 (April 4, 1930)
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):
Crossing Over, Genetic
Education and Research at Cornell, 1925-1931
Box Number: 3
Folder Number: 3
Have'nt [sic] hard from you in some time and wondered what was up. Jenkins came in to-day saying he had talked with you.
I didnt [sic] get too much from him so would appreciate a direct report (hint, command or what have you?).
I have been busy as can be. Classes have been terrible this term. There are 3 sections meeting twice a week or 6 lab periods
for me. So far it has been almost my total energy consumer but lately I have found time to work on yours and my material.
Loads of new things have turned up and I will try to tell you some.
To start with -- I believe I have all (probably) of the chromosomes isolated. This spring I isolated 2, 3, 4, 6, and know
where 8, and 9 are. I assume 10 is f-br. and will turn up in your trisomes. Chromosome number 6 makes the plants look like
br plants -- dwarfish, thick tassels, thickish wide leaves which are very small near the top.
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This varies, however, from such an intense expression of the characteristics of lack of intermodal elongation that the tassel
hardly comes out of the upper leaves to almost normal plants. I had the satellite chr. and number 6 in a trisomic in the garden
(219A5), I think and was able to distinguish trisomic satellite plants from trisomic number 6 plants. I have (x) the most
striking plants to see if there is a segregation of 2n: 2n plus 1 for this group of characteristics.
I believe also that the satellite chr. will turn out to be the y-pl group. I have negative evidence for na and some very positive
evidence for y. That should be finished quite soon but you can plan somewhat on y-pl for your satellite sterile.
Have some good evidence on the satellite -- quite different but I shall not be certain of all for a time yet -- The end of
the sat. chr., not the satellite is the important part. [diagram] The satellite is made up of [diagram] 4 chromomeres, a large
[diagram] chromomere, behind two smaller granules and behind this a larger granule. From this granule to the end of the chromosome
is a region I have'nt [sic] made up my mind about as yet. The end of the satellite chr. attached to the nucleolus is reticulate
are quite different from the rest of the chromosome. It forms two bodies visible in late no. of spores if detached from the
nucleolus. (see photograph). The satellite proper "wags" in early
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prophase of meiosis. I do not believe it is the important part of attachment of chromosomes to nucleolus. I have studied,
slightly, trisomic satellites at early prophase and they are very interesting. Two threads are together and the third is free
but all seem to be together at this reticulate area of the chr. in most cases anyway. I have more plants growing for this
in the greenhouse now which will be along after you arrive so you can see it all. So much for the satellite.
Now for the constrictions. I have just touched on them but they are easily workable and very good. In early prophase I the
chr. can be followed in some cases totally and the constriction area observed. It is a homogeneous area with a granule in
the center. The stainable chromonema does not seem to be thru it but stops short at this region. That is probably why we have
square corners at the constriction area -- [diagram] The granule, not like a chromomere, is probably the point of spindle
Now for the nature of bivalent chr. They are 8 parted, not 4 parted. That is, there are 8 strands, four to each chromosome.
I hope to get some photos of this.
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Now for heteromorphic chrs:
The chromosomes you brought from Wisconsin are very heteromorphic when crossed to some of our material. This may not be totally
so but in your stains (and undoubtedly in some of ours) there are no granules at end of the satellite chr. as in some or most
all of the material I have looked at in our material. I have a slightly different method that shows dk chr very much better.
In crosses of two types there are heteromorphic granules. [diagram] These are very evident in early pro. I and clear at DK.
They are the things that caused our confusion last summer and all my confusion of position of granules and presence on different
chrs. They are present on any chrs. possibly from different sources but not on the same chr. from all sources. That is, there
is not a chromosome morphology in corn but chromosome morphologies. At first I was pretty well confused but things are clearing.
For instance there are the following types of number 4 chrs:
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I have crossed type 1 with type 3 to get [diagram] which will prove crossing-over, cytologically, I hope. I may not get any
seed but will repeat the cross in the field. I have 2 types of number 2, 2 types of satellite chrs, 3 types of number 6, 2
types of number 9 and 3 types of number 10. I can find heteromorphing extending even to chromomeres which is characteristic
and not haphazard, I believe.
Now for what you really are interested in and on which I have spent most of my efforts and am just seeing light. It has been
an aggrevation [sic] because I was working with this darn heteromorphism and could'nt [sic] distinguish correctly. I believe
I am half way along on it now. I dont [sic] believe I can directly tell how much of each chr. has been interchanged by direct
observation of spores but we may work it from pro plus minus by synapsis extents which do show up in some cases but (first
seen from looking at dk) the rings of c-sh-wx. is made up of 2 chrs, one belonging to (1 or) 2 and one to 3 or 4. I have taken
plants from your trisomic c-sh-wx plant and grown them. They should give 2 types of 2n plus 1 types. One with one of the chr.
and one with the other. I have found without any doubt that number 2 is involved and that the chrs. coming from your original
plant are identifiable from those coming from the pollen parent.
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Chr. number 2 seems quite large [diagram] where as the other number 2 seems different -- no granule and apparently not so
large at pro. due possibly to difference in condensation. This I don't know. The chromosome with which it forms a ring
is larger and possibly number 3. I have made some observations which indicate number 3. I have, however, crossed 2n plus 1
(number 4) with x-n2 and have the cross 2n plus 1 (number 1) crossed with [diagram]. In the rings in the material I grew there
is a decided difference in size of the two number 2 chrs. Whether this is due to heteromorphism or to unequal interchange,
I have nt [sic] as yet determined. There is one trouble which I will attack in the next few days. Do you remember the chr.
in Brink's [diagram] which I thought number 2 with a large terminal granule? [diagram] It is running thru your material
and still looks like number 2 but there is some conflicting evidence. It is involved, probably, in your interchange. I took
normal material, observed spaces and found some with this and some without. I then looked at dK and found a heteromorphic
pair which looked more like number 2 than anything else. However, in your material this chr. seems to be the larger one of
the 2 rather than number 2. In dK we have this: [diagram]
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Besides this ring there is a small chr. which is very probably number 1 or r-g, since number 2 is involved in the ring. I
have to look at the spores in this plant to be sure the terminal granule is the same or just what has happened. I hope to
know in a few days. You see, I have'nt [sic] solved the situation totally but have opened up some new things which are
very valuable, cytologically, and I sincerely think I can solve a lot more as it is turning out more interesting than I had
anticipated, even; at least I feel more confidence than I did 10 days ago when I felt I had hopelessly failed to solve anything.
This is only part of the story. There is more I could tell you but I shall wait till things are more coordinated and the story
is more complete, if I can make it so. As it is I have the terminal granule to solve.
This is what I believe we can find out:
(1) Differences in sizes of interchange
(2) How chrs are synapsed in ring, i.e, their constriction positions
(3) Possibly how far up the chr. the interchange goes by:
This is just a diagram and does not represent the case as I believe it to be. We should be able to orient ourselves by granules
-- there are several large enough to be identifiable -- and by constrictions.
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Also -- I believe that the chr. involved besides c-sh-wx may be su-tw on negative evidence. So far it looks as if 2 or 3 is
su-tw as it doesn't seem to be any of the others, i.e., if the chr. besides number 2 in your sterile is actually number
3 and not number 4. I see I am rambling again but you are familiar with my wanderings and know how much to judge them (Ha
I have had wonderful (and all superlative words) preparations of chrs. I have just begun taking a few photographs and will
enclose a couple. I have an [sic] series of photographs that show well all the chrs. in a single spore in pro. My original
diagram is all right, thank goodness. I don't know to what extent heteromorphism is going to change things but probably
So much for the news such as it is.
Let me know what you have --
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This is a post script but is much worth adding -- Have worked on the sterile and have gone ahead -- The situation is something
like this: [diagram] Apparently number 2 and number 3 have interchanged and number 3 has taken more than it has given. About
1/3-1/4 of the lower end of number 3 is involved, probably. In one batch of [diagram] number 3 has a huge terminal granule
and looks like this [diagram] at dK it forms as inked in above diagram.
As for the spores: I believe we can tell just how synapsis takes place and how disjunction occurs. The spores are of several
varieties. I have found that some of the sterile spores undergo the first division in the meiospore and should be able to
find figs with number 1, no number 2, two number 3 with granule etc. I have already seen this. Can calculate percent of figs
with 1 terminal granule and percent with 2. 1/4 should have 2, 1/2 only 1 and 1/4 none. However, one combination may not go
thru the first division as there is a percent which I have'nt [sic] calculated yet which do not go thru. Will have more
dope in a few days.
Have not decided to send you the raft of pictures but will only send one as you will be here so soon.
It is from one of your 30 percent steriles but contain only co-eu in this particular spore. I took it for the difference between
number 1 and number 2.