MCB 142
MAJOR ADVANCES IN UNDERSTANDING EVOLUTION AND HEREDITY
FALL 2015
Week 4: September 29 and October 1
SEPTEMBER 29: THE CHROMOSOMAL BASIS OF MENDELISM
Morphological individuality of chromosomes. Meiotic pairing, segregation and assortment of
chromosomes and the parallel behavior of Mendelian factors.
OCTOBER 1: SEX CHROMOSOMES AND FURTHER EVIDENCE FOR THE CHROMOSOME THEORY
Accessory chromosomes and sex determination. Evidence from non-disjunction. Polytene chromosomes
and rearrangements.
Readings to be Discussed on Tuesday September 29

Thomas H. Montgomery, Jr. (1901) A Study of the Chromosomes of the Germ Cells of Metazoa.
Transactions of the American Philosophical Society 20: 154-236.
Read pages 154-155 up to “The material…” and pages 221-224, starting with “Now for the bearing
of all this...” up to "It is quite possible..."

Walter S. Sutton (1902) On the Morphology of the Chromosome Group in Brachystola magna.
Biological Bulletin 4: 24-39.
Read pages 11-16 in the posted paper, starting with “To sum up...”

Walter S. Sutton (1903) The Chromosomes in Heredity. Biological Bulletin 4: 231-251.
Read pages 1-6 in the posted paper ending with "...half the gametes produced"

Matthew Hegreness & Matthew Meselson (2007) What Did Sutton See?: Thirty Years of Confusion
Over the Chromosomal Basis of Mendelian Genetics 176: 1939-1944.
Read the article.
Readings to be Discussed on Thursday October 1

Clarence E. McClung (1902) Notes on the Accessory Chromosome. The posting on the course
website is an ESP reprint of the original article in Anatomischer Anzeiger 20: 220-226.
Read the article.

Edmund Beecher Wilson (1905) The Chromosomes in Relation to the Determination of Sex in
Insects. The posting is an ESP reprint of the article that appeared in Science 22: 500-502.
Read the article.
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Study Questions
Please hand in Tuesday September 29
1. What is the question about the behavior of chromosomes in meiosis that Montgomery asks in the
assigned section of his 1901 paper? What is his conclusion?
2. Elsewhere in the paper, Montgomery reports his detailed observations of spermiogenesis in 42
species of order Hemiptera, suborder Heteroptera. These are so-called "true bugs" and
characteristically have piercing, sucking mouthparts. Examples include stink bugs, squash bugs and
milkweed bugs. Of the 42 species examined, 13 species have spermatogonial chromosome numbers
that are not divisible by 4. Explain why such cases support the conclusion that synapsis is not
between chromosomes of like parentage. (Spermatogonia are the cells that precede meiosis in the
male germ line and accordingly are diploid.)
3. In addition to the evidence cited in the above question, what additional arguments does Montgomery
give in support of the same conclusion about synapsis?
4. What does Montgomery believe to be the biological significance of synapsis? What two functions
are now attributed to the synapsis of maternal with paternal chromosomes in meiosis?
5. What are the arguments in Sutton’s 1902 paper that chromosomes preserve their individuality
through cell divisions?
6. What observations in male meiosis in the grasshopper Brachystola does Sutton cite in his 1902 paper
as evidence that a particular chromosome determines particular characters of the organism? What is
his argument?
7. Sutton concludes in his 1903 paper that the segregation and independent assortment of Mendel's
"factors" can be explained as the result of the chromosome behavior he describes in Brachystola male
meiosis. What are his arguments in this regard?
8. As he lists on the second page of his 1903 paper Sutton believed that the maternal and paternal
chromosomes disjoin in the second meiotic division--or in his words that "The second postsynaptic
division is a reducing division". If this incorrect belief were true, in what division would Sutton have
seen what he calls "direct cytological evidence" to support his chromosomal explanation of
Mendelian independent assortment? What sort of "cytological evidence" could allow one to see
evidence for independent (random) assortment? Does Sutton make any mention of any features of the
chromosomes that would have allowed him to make this distinction?
9. According to McClung, what distinguishes accessory chromosomes from other chromosomes?
10. What arguments does McClung present in support of his (correct) view that the accessory is
nevertheless a chromosome?
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11. What is the behavior of the accessory chromosome that causes there to be "two forms of spermatazoa
in equal numbers".
12. At what places in his paper does McClung state that the accessory chromosome goes to only one pole
("remains undivided") without stating whether this takes place in the first or in the second division of
meiosis? In what places does he specify the division of meiosis in which the accessory goes to only
one pole? Which division is that?
13. Most of the known chromosomal mechanisms of sex determination are of either of two types. These
may be called the genic balance type and the dominant Y type. Which type is Wilson's "type A" and
which is his "type B"?
14. Wilson knew that males of the Hemipteran insect Protenor have six pairs of ordinary chromosomes
(autosomes) plus an accessory chromosome (also called an idiochromosome). Suppose, however, that
he had mistakenly thought the total number of chromosome pairs in Protenor females was 6 instead
of 7. How would this have changed Wilson's conclusions regarding the chromosomal basis of sex
determination in Protenor?
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Some Background (Chromosomal Basis of Mendelism)
Theodor Boveri
1862-1915
Boveri's Demonstration of the hereditary role and functional individuality of
chromosomes in the sea urchin. The theoretical analysis of mitosis by Roux
suggesting that chromosomes carry hereditary determinants and the evidence
obtained by van Beneden in Ascaris that both parents contribute equal numbers of
chromosomes to the zyglote suggested the conclusion that chromosomes might be
carriers of heredity. If so, does each chromosome carry all the information needed
for development or is each chromosome functionally different from the others, so
that at least a full haploid set is required for development to proceed? That the latter
is the case and therefore that all the chromosomes in a haploid set are functionally
distinct was shown in a brilliant series of experiments by the German embryologist
Theodor Boveri with sea urchin eggs fertilized with not one but two sperm.
Boveri’s demonstration of the functional individuality of chromosomes took advantage of Oskar
Hertwig’s discovery that sea urchin eggs exposed to a high concentration of sperm are occasionally
fertilized by two sperm instead of one, as he mentioned in the paper we read last week. Such doublyfertilized eggs undergo first cleavage to give four (not the normal two) cells (blastomeres). These
tetrafoils or simultaneous fours, as they were called, almost always die very early. Boveri had earlier
shown that in addition to injecting its nucleus into the egg, the sea urchin sperm contributes the
centrosome, and that, upon fertilization, the centrosome divides into two. These then separate and
become the two poles of the spindle. The two sister chromatids of each chromosome then go to
opposite poles during anaphase of the first mitotic cell division. In the normal case, with only two
poles, each sister nucleus receives one of each kind of chromosome. When two sperm contribute
centrosomes there are four poles. Boveri conjectured that it would simply be a matter of chance
which two of the four poles made attachments to a given pair of sister chromatids in the first division
of the zygote. Considering that there are 18 chromosomes in the haploid set of the sea urchin with
which Boveri’s experiments were done and therefore six chromatids of each kind, the chance that
every one of the four cells of the tetrafoil would receive at least one chromatid of each kind would be
exceedingly low, consistent with the observation that such tetrafoils almost always die at a very early
stage.
While the early death of nearly all tetrafoils was consistent with the supposition that chromosomes
have functional individuality, much more decisive evidence came from experiments with doubly
fertilized sea urchin eggs in which only three poles compete for chromosomes. Thomas Hunt
Morgan, of later fruit fly genetics fame, had found that vigorously shaken dispermic sea urchin eggs
occasionally produced three cells rather than four at first cleavage, embryos called trefoils. (Shaking
can prevent one of the two centrosomes brought in by the two sperms from dividing, therefore
providing only three poles instead of four.) With six chromatids of each kind and only three poles to
which chromatids might go during anaphase the chance that each of the three cells of a trefoil would
have at one chromatid of each kind would clearly be much greater than the corresponding chance
that each of the four cells of a tetrafoil would have least one chromatid of each kind.
Boveri and his American (from Boston) wife, Marcella O’Grady the first woman biology graduate from
MIT and subsequently a student of Edmund Wilson at Columbia, began the dispermy experiment in the
winter of 1901. As related in his 1902 paper (at page 5) Boveri wrote “I have found, among the 695
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tripolar eggs cultured as wholes, 58 almost normal plutei, that is 8.3% compared with the expected 4%.
Among the ten larvae considered to be plutei obtained from 1170 quadripolar eggs, there was not a
single one as normal as the 58 obtained from the tripolar eggs and just mentioned.” Thus, it appeared
that at least one of each of the 18 chromosomes in the haploid set of this sea urchin is required for
development to the plutal stage. In subsequent experiments with individual separated cells from
trefoils and from tetrafoils, Boveri obtained results similarly in accord with calculated expectations, as
described in last week’s reading from the 1928 edition of Wilson’s book “The Cell in Development and
Heredity”. (note: to obtain a reasonably normal pluteus from a tetrafoil (or a trefoil), all four (or all
three cells from the first cleavage must have at least one of each kind of chromatid. But if the cells of
the tetrafoil or trefoil are disaggregated, any single cell with at least one of each kind of chromatid can
develop into a pluteus.)
Sutton's evidence that the behavior of chromosomes in meiosis parallels that of Mendel's factors. This
was a great advance, uniting genetics with cytology and opening the way to the study of the
chromosomal basis of inheritance. But showing that the segregation and random assortment of
characters observed by Mendel have their basis in the behavior of chromosomes in meiosis required a
correct understanding of the meiotic behavior of chromosomes. This was achieved only gradually, over
several decades, with much dispute and confusion along the way.
August Weismann accepted the theoretical argument of Roux (1883) that the behavior of chromosomes
in mitosis as described by Flemming (1879) must constitute a mechanism for exactly equal partitioning
of diverse "qualities" between sister cells. Assuming that such qualities were in fact the determinants of
heredity and that, as van Beneden (1883) had shown, both parental gametes contribute the same number
of chromosomes to the zygote, Weismann saw that some provision was required to keep the number of
qualities (and chromosomes) from increasing indefinitely over generations as a result of successive
fertilizations. He speculated that in each generation there must be a special cell division the function of
which was to reduce the number of qualities in the germ plasm by half.
It was known that the last two cell divisions leading to egg production, the female "maturation
divisions", the two divisions of meiosis, were unusual in several respects, including the long duration of
the process and the production of polar bodies. At each of the two female meiotic cell divisions in
animals and plants, one of the two sister cells degenerates. These cells, called polar bodies, contain a
nucleus but very little cytoplasm, an arrangement that maximizes the amount of cytoplasm in the egg
available for embryonic development. (Consistent with this explanation, all four meiotic products of
male meiosis and of meiosis in fungi develop into gametes--there are no polar bodies.) Weismann
argued that the second of these two divisions was the predicted "reducing division" and that in both
sexes "there must be a form of nuclear division in which the ancestral germ plasms contained in the
nucleus are distributed to the daughter-nuclei in such a way that each of them receives only half the
number contained in the original nucleus." While the conclusion is correct, polar bodies have nothing
to do with reduction. Weismann did not know at the time that no polar bodies are produced in males.
The change from diploidy to haploidy does, of course, take place in meiosis, but in association with the
separation of homologs at first meiotic division, not by discarding anything.
Although he believed that chromosomes are the vehicles of inheritance, Weismann's conception of
chromosomes differed from what is now known. He thought that each chromosome included qualities
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(information) specifying the entire organism and that each individual chromosome had qualities from
various ancestors. At the time, not yet knowing about the work of Mendel, this was not inconceivable.
A fundamental advance in understanding meiosis was the publication in 1901 by
Thomas Montgomery of clear evidence that the reduction in the apparent number
of chromosomes seen in the first maturation division (older term for first meiotic
division) that results from the coming together of chromosomes in pairs (synapsis)
is a pairing of paternal with maternal, not paternal-paternal and maternal-maternal.
Working as an assistant professor at the University of Pennsylvania, he also
presented in the same paper evidence that chromosomes retain their individuality
through successive cell divisions: certain morphologically distinct chromosomes
reappeared in successive divisions. But much remained uncertain. At the time it was
a matter of dispute, for example, whether the synaptic pairing of chromosomes at
the first division of meiosis was transverse (end-to-end) or longitudinal (side-byside) and whether the separation of maternal and paternal chromosomes took place
in the first or in the second meiotic division. (Owing to crossing-over, unknown at
Thomas Montgomery
the time, it is only the centromeres that, strictly speaking, can be said regularly to
1873-1912
segregate reductionally in a particular division of meiosis--actually the first
divison.) In 1901, opinion was divided, with some cytologists mistakenly believing that synapsis was
end to-end and, also mistakenly, that the first division was transverse, therefore arriving at the correct
conclusion that reduction takes place in the first division.
Additional and more influential evidence for morphological individuality and
persistence of individual chromosomes was brought forward in 1902 by Walter
Sutton, pursuing his Ph.D. at Columbia University with Edmund Wilson. Under
Wilson’s guidance, Sutton continued studies of chromosomes in male grasshoppers
of the species Brachystola magna that he had begun as a student of Clarence
McClung at the University of Kansas. Sutton begins his 1902 paper with a reference
to the “recent remarkable paper” of Boveri, showing that different chromosomes
are functionally different in development. Sutton then raises the question of
whether the differences in chromosome sizes within a species “...as usually taken
for granted... are merely a matter of chance.” He then presents his own detailed
Walter Sutton
1877-1916
evidence, adding importantly to that brought forward by Montgomery the year
before, in support of chromosome morphological individuality, based on
measurements of the relative sizes of chromosomes of Brachystola at various stages of spermatogenesis.
In addition, he noted that morphologically indistinguishable pairs of chromosomes paired, concluding
that homologous maternal and paternal chromosomes come together in meiosis, in agreement with what
had been shown the year before by Montgomery.
In his 1903 paper, Sutton presents no new data but summarizes the conclusions of his 1902 paper and
then argues that if the different chromosomes determine different aspects of development, there must be
an exact correspondence between the behavior of chromosomes in meiosis and that of the "factors"
studied by Mendel. There are two different points at issue here, segregation and independent assortment.
Segregation. Is the segregation of character pairs observed by Mendel in the first generation from
hybrids (the F2 in modern notation) in Pisum heterozygous for that character (Mendel's “law" of
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segregation) attributable to the segregation of the two members of a given chromosome pair during one
of the meiotic divisions? Sutton deals most explicitly with this question only after discussing random
assortment.
Independent Assortment. Is the independent assortment of different character pairs observed by Mendel
(Mendel's "second law", holds only if there is no linkage) attributable to independent segregation of
paternal and maternal chromosomes in meiosis? Sutton concludes that both segregation and the
independent assortment of factors described by Mendel are indeed attributable to the behavior of
chromosomes in meiosis. This realization was of great conceptual importance, as it provided a physical
explanation for Mendel’s findings and linked the previously separate fields of genetics and cytology.
Deep and important as Sutton’s insight was, the seldom-mentioned fly in the ointment is that he
concludes that independent assortment happens in the second division of meiosis when it actually
happens in the first division. Sutton was looking at the wrong division to have seen what he claimed.
Ironically, 11 years later, working with the glass slides of Brachystola that Sutton
himself had made and left behind at Kansas, Eleanor Carothers, working as a graduate
student of McClung, noticed what Sutton had not noticed or at least did not report, a
heteromorphic pair of autosomes, one slightly longer than the other and therefore
distinguishable from its partner. Looking at anaphase in 300 first spermatocytes, she
found 146 in which the accessory chromosome (the sex chromosome in the XO
Brachystola magna male) was going to the same pole as as the smaller autosome and
154 in which it was going to the same pole as the larger one, demonstrating that
assortment of the heteromorphic pair of autosomes is random with respect to assortment
of the sex chromosome, like the assortment of pairs of Mendelian factors. While seeing
Eleanor Carothers
clearly that the heteromorphic pair segregated in the first division of meiosis, Carothers
1883-1957
wrongly believed, like Sutton, that all the other pairs of autosomes segregated in the
second meiotic division. This and the eventual resolution of the problem is described in the HegrenessMeselson paper.
Some Background (Sex Chromosomes)
The X-chromosome was first described by Hermann Henking in Germany in 1891 as a "peculiar
chromatin-element" in the nuclei of males of the hemipteran insect Pyrrhocoris apterus (the fire bug)
that stains like a chromosome, divides in the first meiotic division and, after lagging behind the other
chromosomes at anaphase of the second meiotic division goes undivided to only one pole, while each of
the eleven other chromosomes split longitudinally and is equally divided between the two poles. (Such
first division equational and second-division reductional segregation is a peculiarity of hemipteran X
chromosmes, not generally found in other organisms with XO or XY sex determination. In these, as
Sutton saw in the grasshopper Brachystola magna, the X goes entire to one pole in the first division).
Not sure it was a chromosome, Henking called it "X". One early view was that "X" is a chromatin body
that once was a chromosome but is in the process of degeneration and loss. Another view was that it is a
nucleolus, not a chromosome. (A nucleolus is a conspicuous RNA-rich structure found in higher
eukaryotes that forms around tandemly repeated genes for ribosomal RNA and in which ribosomal
subunits are assembled before their exportation to the cytoplasm.)
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That the presence of the X chromosome in only half the sperms is associated with the
determination of sex was first proposed by Clarence McClung, at the University of
Kansas in his 1901 paper "Notes on the Accessory Chromosome". In this, he was
essentially correct. But both he and his student William Sutton miscounted the
number of chromosomes in the females of the locust species (Brachystola magna),
the species on which their principal observations were made, counting two fewer
chromosomes than are actually present (22 instead of 24). This erroneously implied
that the female has no X-chromosomes -- that females are OO and males are XO-where O means no chromosome. This led McClung to conclude that X imparts the
quality of maleness. Because the females of these locusts have
one more pair of chromosomes than McClung thought, they
are actually XX and the males are XO. So it is not the presence
of the X that determines maleness but rather the number of X
chromosomes that determines it: One X determines maleness;
Clarence McClung
1970-1946
two X chromosomes determine femaleness, a so-called dosage
effect. It all got straightened out with the publication in 1905
of papers by Nettie Stevens at Bryn Mawr and Edmund Beecher Wilson at
Columbia, as explained in Wilson's short paper of 1905 in Science. Stevens and
Wilson recognized two types of male sex-chromosome constitutions in the
various insect species that had been studied by then: in some species there is only
an unpaired X and in other species the X is associated with a morphologically
different chromosome, subsequently called Y, making females XX and males XY.
In humans, as you know, the mere presence of a Y chromosome determines
maleness.
Nettie Stevens
1861-1912
By 1913-1914 when Calvin Bridges published his "Direct Proof" that sex-linked
genes of Drosophila are on the X-chromosome, the "chromosome theory" had
become generally accepted as being able to explain a wide range of genetic and
cytological phenomena. The distinctive aspect of Bridges' "Direct Proof" was that
instead of merely providing an explanation for an observation that had already
been made, it made a startling prediction and the prediction was then found to be
correct. Ordinarily, Drosophila males receive their X-chromosome from their
mothers and females receive one X from each parent. White eyes being recessive
to red eyes, a cross of white-eye females by red-eye males should give only redeye females and white-eye males. The anomalous observation was the rare
occurrence from such a cross of white-eye females and red-eye males. To explain
Calvin Bridges
the unusual flies, Bridges conjectured that the white-eyed females arose from eggs
1889-1938
having two X-chromosomes instead of one and that the red-eyed males arose from
eggs having no X-chromosome and that such XX and no-X eggs resulted from the occasional failure of
the two X-chromosomes to separate in female meiosis, giving eggs with two X-chromosomes and eggs
with no X-chromosome. A cross with a normal (XY) red-eye male would then give white-eye females
and red-eyed males, as well as XwXwXr red-eye females, which are sterile. The fourth type of egg from
the cross, OY, fails to develop.
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Eggs: XwXw
O
------sperm-----X
Y
XwXwXr♀
XwXwY♀
OXr♂
OY
Bridges then demonstrated by direct cytological examination of the chromosomes of the white-eyed
females and the red-eyed males that his conjecture was correct.
Further work showed that such occasionally arising XXY females, when mated to ordinary males, give
rise to progeny of which 4-5 percent of the females inherit sex-linked alleles only from their mothers
and are, like their mothers, XXY. This Bridges explained as the result of further ("secondary") nondisjunction resulting from the fact that, although in XXY oocytes the Y chromosome always disjoins
from one of the X chromosomes, that X-chromosome is occasionally accompanied by the other X,
giving a small percentage of XX eggs. Altogether, this gives four kinds of eggs: XX, XY, XO, and YO,
where "O" means no chromosome. Satisfy yourself that fertilization by an ordinary male would give six
different kinds of zygotes. Of these, only XX, XXY (both female) and XY, and XYY (both male) are
viable. The XXY females, if mated to ordinary males, then repeat the behavior of their mothers, etc. In
two long publications 1917 in the first two issues of Genetics Bridges reported his verification of all the
genetic and cytological predictions of this scheme.
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