The
Path to the Chromosome Theory of Heredity
The notion of the
chromosome theory of heredity is very important to the understanding of
evolution and genetics. There were
many ideas, some correct and others not, which influenced the revolutionary
discovery of chromosomal heredity. The
main stones in the path to the chromosomal theory are the pangenesis hypothesis,
the germ-plasm theory, and Mendel’s Laws.
Pangenesis
Lamarck
expressed the idea that by simply using or not using certain organs they may be
developed or atrophied and their offspring can then inherit these acquired
characteristics. (Milner p. 375, 1993) It
should be noted that this theory was not widely accepted largely due to the fact
that the French word that Lamarck used in the sense of “must” was translated
as “wants to,” which makes it sound as though the organism decides to change
its body. Furthermore, Lamarck
provided no mechanism by which this could take place. (www.ridgenet.net/do_while/sage/v1i8f.htm)
Darwin was the one to give a mechanism for Lamarck’s idea.
This mechanism is known as pangenesis.
Darwin’s ideas of evolution were well founded in the beginning;
however, as his theory progressed he reverted to Lamarckian thought
to
explain his observations. To begin
his theory he started with the observations that there is variation in
offspring. He wrote, “no one supposes that all the individuals of the
same species inhabiting the same confined locality, are cast of the same
mold…I am convinced that the most experienced naturalist would be surprised at
the number of the cases of variability…as I have collected” (Chapter 2 of Origin
of Species). He sees that there
is a struggle for existence saying, “as more individuals are produced than can
possibly survive, there must in every case be a struggle for existence”
(Chapter 3 of Origin of Species). Finally,
Darwin recognized that there is a survival of the fittest in that the organisms
with the best variations for the continuance of the species live and those with
variations that are not as useful to the continuation of the species die
(Chapter 4 of Origin of Species). Darwin
begins to diverge into Lamarckian thought at this point.
Darwin wrote that, “ slightly different changes in the conditions of
life add to the vigor and fertility of all organic beings…the crossing of
forms which have been exposed to slightly different conditions of life or which
have varied, favours the size, vigor, and fertility of their offspring”
(Chapter 9 of Origin of Species). Furthermore,
“changed habits produce an inherited effect…with animals the increased use
or disuse of parts has had a more marked influence,” and an example that
Darwin uses is that of the development of cow udders.
He writes, “the great and inherited development of the udders in cows
and goats in countries where they are habitually milked, in comparison with
these organs in others countries, is probably another instance of the effects of
use” (Chapter 1 of Origin of Species).
This leads to Darwin’s hypothesis of Pangenesis.
The hypothesis describes the way in which heritable units are transferred
from generation to generation. At
all stages of development multicellular organisms’s component cells disperse
gemmules throughout the body which develop into cells like their predecessors.
The gemmules are collected as packets into the reproductive organs due to
a mutual affinity. A new organism
is created when two of these gemmule packets combine.
The combination of the elements of some of the gemmules in the packets
develop into a new organism. The
gemmules that do not develop are transmitted for subsequent generations. (Romanes
p. 2-3, 1899). The hypothesis
leaves open the idea of inheriting acquired characteristics because the
component cells can change due to the environment.
In the late 1860s
Francis Galton, the cousin of Charles Darwin, Galton went further into
Darwin’s theory of Pangenesis hypothesizing “that combination of gemmules
might be conveyed by the blood to growing parts of the body, or passed to other
bodies by reproductive organs, thereby passing on characteristics.”
In order to test his hypothesis Galton gave blood transfusions between
rabbits to see “whether changes of blood accelerated the appearance of
inherited characteristics
in offspring.” However, he could
not conclude that the transfusion altered successive generations. (www.tld.jcu.edu.au/hist/stats/galton/index.htm)
Actually, few people accepted Darwin’s notion of pangenesis, but George
John Romanes (1848-1894) did. Romanes
along with Darwin advocated this idea of pangenesis and communicated back and
forth their 
ideas
and struggles. In 1875, Darwin
wrote a letter to Romanes stating:
"I hope with all my heart
that you are getting on pretty well with your experiments; I have been led to
think a good deal on the subject, and I am convinced of its high importance,
though it will take years of hammering before physiologists will admit that the
sexual organs only collect the generative elements” (Romanes, 1896; Darwin and
Seward, 1903).
In the light of genetic research, Romanes proceeded with research after
Darwin’s death to sort out some of the problems with Darwin’s hypothesis.
Seeing that acquired characteristics cannot be inherited, Romanes
proposed a model that natural selection takes place using only inherited
characteristics, which is Neo-Darwinism and also follows Weismann’s ideas. (www.ridgenet.net/~do_while/sage/v1i8f.htm)
Germ-Plasm Theory
Romanes wrote
that Darwin and Weismann’s theories “constitute the logical extremes of
explanatory thought” (Romanes, 1). 
Before August Weismann (1834-1914) “it was assumed that acquired
characters, although not so fully—and therefore not so certainly—inherited
as congenital characters, nevertheless were inherited in some lesser degree; so
that if the same character continued to be developed successively in a number of
subsequent generations, what was first only a slight tendency to be inherited
would become by summation a more and more pronounced tendency, till eventually
the acquired character might be as strongly inherited as any other character
which was ab initio congenital.” (Romanes p. 6, 1899).
Therefore, the idea of congenital characters was well known and they were
thought to be “much more heritable than acquired characters” (Romanes p. 49,
1899). Of course, Weismann nipped
the idea of inheriting acquired characteristics in the bud so to speak when he
cut the tails off of rats and mated them, showing that the offspring still had
tails.
Sexual
propagation is important according to Weismann because it creates variation.
He believed that “[sexual propagation’s] function is that of
furnishing congenital variations to the ever-watchful agency of natural
selection, in order that natural selection may always preserve the most
favorable, and pass them on to the next generation by heredity.
That sexual propagation is well calculated to furnish congenital
variations may easily be rendered apparent…at each union there is a mixture of
two sets of germinal elements; that each of these was in turn the product of two
other sets”(Romanes p. 11-12, 1899). This
is actually the ideas of other scientists such as Darwin, but Weismann
elaborated (Romanes p. 13, 1899).
Ideas in Germ-Plasm
Theory include heredity being transmitted through a chemical or molecular
substance; and the soma and the germ plasm composing an organism, where the soma
makes the body of the organism and the germ plasm having the cells that become
gametes and is segregated from soma during the development of the egg.
The main importance of Weismann’s theory is that it asserts that
acquired characteristics are not passed to the next generation, leaving natural
selection the exclusive the explanation for biological evolution. (http://library.thinkquest.org/19926/text/tour/30.htm)
One can think of germ plasm kind of like the contents of a germ cell.
According to Weismann this germ-plasm is “passed on continuously
through generation after generation of successively perishing organisms,” and
furthermore, it has been continuous though the time since life first appeared on
this earth” (Romanes 9).
One important realization that needed to be made is that germ cells are
different from other somatic cells. In
1887, Walther Flemming observed that the production of sperm was different from
mitosis, which occurs in normal cells. The
chromosomes went through what looked like normal mitosis, and then the second
division resulted in half the number of chromosomes.
Weismann recognized that this
showed that the production of germ cells is different from normal cells (Farley,
p.170, 1982). He rationalized that
because germ plasm has been transmitted from generation to generation all along
and because in sexual reproduction two germ plasms are combined, each germ plasm
must be halved (Farley p. 172, 1982). Weismann
did not ever really say that the hereditary material is located on chromosomes,
however, he did believe that nucleus of the germ cells has ability to transfer
hereditary material (Romanes p.29-30, 1899).
Looking at
this information, one may see the natural correlation of chromosome and
heredity. The way in which the
chromosomes separate in meiosis is key to combining the ideas.
Although Weismann did not know Mendel’s Laws, his research was
instrumental in their rediscovery (www.nceas.ucsb.edu/~alroy/lefa/Weismann.html).
Mendel’s Laws
Johann
Gregor Mendel published a paper on inheriting “particulate factors” from
each parent that segregate when sex cells are formed.
Mendel’s Laws were ignored until around 1900.
This could be due to the fact that his ideas were in opposition to
Darwin’s ideas on heredity. Because
of Darwin’s pangenesis hypothesis upholding the notion of inheriting acquired
characteristics and his evolution theory maintaining continuous evolution,
Darwinians rejected Mendel’s theories. www.mpiz-koeln.mpg.de/~loennig/mendel/mendel02.htm
Using the pea plant, Pisum, Mendel was able to
show that hybrids form different kinds of pollen and egg cells, showing that the
reason the offspring are variable is due to the segregation of particulate
factors when sex cells are formed. Mendel
found that the segregation of heritable factors “follows a constant law, which
is founded on the material composition and arrangement of the elements which
meet in the cell in a vivifying union” (www.hpcc.astro.washington.edu/mirrors/MendelWeb/).
Additionally, there are dominant and recessive traits.
Mendel wrote, “if the reproductive cells be of the same kind and agree
with the foundation cell of the mother plant, then the development of the new
individual will follow the same law which rules the mother plant. If it chance
that an egg cell unites with a dissimilar pollen cell, we must then
assume that between those elements of both cells, which determine opposite
characters some sort of compromise is effected” (www-hpcc.astro.washington.edu/mirrors/MendelWeb/). He further clarifies this by asserting that there is no
blending of traits, rather the hybrid will have parental traits based on a fixed
scheme. This scheme is dominance
and recessiveness following a statistical method.
The Chromosome Theory of Heredity
At this point the pieces were in place to prove that chromosomes carry
hereditary information. Scientists
such as Weismann had suggested the idea, but possessed no conclusive proof.
After the proof came, Thomas Hunt Morgan was the one to truly bring
Mendel’s Laws and the chromosome theory together into a revolutionary,
believable idea.
The first proof that chromosomes carried hereditary material first came
from Walter Sutton through studying grasshopper cells.
Seeing that during meiosis chromosomes pairs split, making two daughter
cells. Sutton writes, “many
points were discovered which strongly indicate that the position of the bivalent
chromosomes in the equatorial plate of the reducing division is purely a matter
of chance—that is, that any chromosome pair may lie with maternal or paternal
chromatid indifferently toward either pole irrespective if the positions of
other pairs, --and hence that a large number of different combinations of
maternal and paternal chromosomes are possible in the mature germ-products of
the individual” (Sutton 1993). This
division of sex cells correlates with Mendel’s laws.
Thomas Hunt Morgan was critical of both Darwinism and Mendelism in the
beginning of his career as well as the chromosome theory of heredity.
When he began working with Drosophila in 1908, his goal was to
distinguish Darwinian and Neo-Lamarckian evolutionary theories via the
statistical analysis Mendel used on experimental breeding.
In this processed he became a Mendelian.
Moreover, he combined Mendelism with the chromosomal theory of heredity. clio1.cshl.org/History/morgan.html
According to Mendelian cytologists in Morgan’s time, the
“characters” that are passed from parent to offspring were physically
located on chromosomes (Farley, p. 227, 1982).
Through work done on aphids and phylloxerans, Morgan became convinced
that chromosomal differences in sperm and eggs correlated with sex
determination, concluding that “both eggs and sperm contain factors that
determine sex, and such factors are linked to the presence or absence of two
accessory chromosomes” (Farley, p. 229, 1982).
He later modified this statement saying that “sex is now treated by the
same methods that are used for Mendelian characters in general…the heredity of
sex can be best understood when one sex is regarded as the pure line, or
homozygous, and the other sex is treated as heterozygous” (Farley p.230,
1982). He came upon this idea
through the crossing of a white-eyed male fly with
a
red-eyed female. The offspring were
all red-eyed, but the second generation was 75% red-eyed and 25% white-eyed.
Furthermore, the white-eyed flies were all male, and there were twice as
many red-eyed females as red-eyed males. This
proved that “color was a typical Mendelian trait and that indeed the sperm did
carry sex factors,” an idea previously stated by Wilson and Castle (Farley p.
230, 1982). Seeing the linkage
between sex and another non-sex trait and further experiments showed that sex is
determined by chromosomes and the gene for eye color is on the same chromosome.
Thus, using Drosophila as a model
system
Morgan was able to show that genes are present in alleles that have a specific
order on the chromosome. Furthermore,
upon learning that another scientist observed the cytological crossing over of
chromosomes Morgan realized that this event could explain genetic exchange
between chromosomes and their segregation rate is a rate proportionate to their
distance apart. (Kohler p. 54, 1994; clio1.cshl.org/History/morgan.html)
From Darwin to Morgan there has been a huge change in the perception of
heredity. It has gone from abstract
collections of particle aggregates to specific transmission of distinct
molecules in a statistical fashion.
-
Darwin, F. and A. C. Seward, More
Letters of Charles Darwin.
London: John Murray, 1903.
-
Farley, John. Gametes and Spores: Ideas
About Sexual Reproduction 1750-1914. Baltimore: The Johns Hopkins
University Press,1982.
This really is a great book. It
is clear and understandable and slightly humorous at times.
-
Kohler, Robert E., Lords of the Fly.
Chicago: The University of Chicago Press, 1994.
This book is
highly recommended by many people.
-
Milner, The Encyclopedia of Evolution.
1993.
-
Romanes, E., The Life and Letters of George John
Romanes. London:
Longmans, Green & Co., 1896.
-
Romanes, George J., An Examination of
Weismannism. Chicago: The Open Court Publishing Company, 1899.
This
is a very good text to read. Romanes
was a big supporter of pangenesis but knew that there was something missing.
Weismann had some answers. Romanes
was a critical eye of Weismann’s work.
-
Sutton, Walter S. “The Chromosomes in
Heredity,” Biological Bulletin (1903) 4: 231-251
-
www.talkorigins.org/faqs/precursors/percur10.html
This is an excellent website.
It is all about Darwin’s precursors and influences.
It has many good links. The
information is broken down into digestible chunks and can easily be found
through the contents page.
9.
www.ridgenet.net/do_while/sage/v1i8f.htm
This
site is a critical look at the theories on evolution.
The actual purpose of the web site is to show all of the flaws in every
theory of evolution that has arisen. It
has concise descriptions of where the various theories are mistaken and shows
the progression of thought through time. I
highly recommend this website.
- post.queensu.ca/~forsdyke/evolutio.htm
This
is a wonderful website because it simple provides selected papers on evolution
that are extremely difficult to find. It
also provides commentary on the papers.
- honors.ccsu.ctstateu.edu/Honors/EText.html
This is great to find online text
for documents that are difficult to find.
- http://www-hpcc.astro.washington.edu/mirrors/MendelWeb/
This is the
MendelWeb Homepage. I am very
impressed with this site. There is
a vast amount of really good information.
- www.mpiz-koeln.mpg.de/~loennig/mendel/mendel02.htm
- www.cshl.org/
- http://library.thinkquest.org/19926/text/tour/30.htm