juliana

Mendel has assumed trihybrid segregation in phenotype as a
combination of three independent monohybrid distributions of the features,
mathematically denoted in the formula (3:1)³. The general formula of
polyhybrid segregation according to phenotype is expressed as (3:1)ⁿ where n
is equal to the number of the pair of features, that are traced in discerning the
individual traits. The genotype distribution has the general formula (1:2:1)ⁿ.
With the increase of the number of hereditary features the picture get
more complicated and in some cases the results considerably deviate from
Mendel’s rules. Later on the studies in this field have registered many such
deviations that were not only explained but have also served as starting
points in the clarification of a number of problems in the structure and
interaction of genes. That is why the results from his classical experiments
are viewed as the basis of the chromosome theory and he, himself — the
acknowledged founder of genetics.
If Mendel’s rules of inheritance of the features are applied to man
whose karyotype contains 23 chromosome couples with an unknown
number of alleles in them, then their distribution according to phenotype
would surpass the fantastic quantity of 2²³ and the one of genotype — 3²³.
That is why each man has a unique hereditary pool of information with
exception of only the identical twins (see Dubinin, 1976).

Discovery of Nuclein and Elucidation of the Biological Role of Nucleic Acids

Section 2.5. In the second half of the XIX century one more great discovery
has been made. By 1868—69 the Swiss physiologist Friedrich Miescher has
isolated an unknown substance from the nuclei of pus cells called by him
nuclein. At this time he has been working in the field of organic chemistry in
Hoppe-Seyler’s Laboratory at the University of Tübingen (Germany) and was
studying the chemical composition of leucocytes. After that he has continued
his research on fish spermatozoa in which the nucleus accounts for over 90%
of the whole mass of the sperm cell.
Miescher’s studies on nuclein in the leucocytes have been published in
1871 and the ones on spermatozoa — in 1874. As in many other cases (for
example the Mendel’s case) his name would have also remained in oblivion
if were not his friends, in particular his uncle W. His (Anatomy professor in
Zürich), who organized the publication of a collected edition of his works
and letters after his death (Die histochemischen und physiologischen
Arbeiten von Friedrich Miescher, 2 vols. F.C.W. Vogel, Leipzig, 1897).
Miescher has pondered the role of nuclein in the fertilization of fish but
has let the “golden fish” swim freely in the waters of the Rhine river where by
the help of fishermen he has ensured material for his studies. He has
displayed a physicochemical approach to the problem and has demonstrated
a dubious attitude towards the nuclein-chromosomes connection predicted by

Intermediary Inheritance of the Features

The rule of the dominant and recessive inheriting of the features established by
Mendel is not confirmed in all cases of hybridization. Some times the hereditary
features in heterozygotic phenotype (Aa) are displayed in an intermediary fashion.
Such a case was observed in crosses of pure lines of the garden flower plant
snapdragon (Antirrhinum majus). One of the varieties has got red flowers and the
other — white one. Their crosses have all displayed in F₁ a pink colouring of the
flowers. In F₂ the ratio of the plants with red, pink and white flowers is 1:2:1 (Fig. 2–10).
The observed intermediary inheritance of the features shows that the applicability of
Mendel’s laws do not always account for dominance and recessiveness.

image

Figure 2–10. Intermediary inheritance of the colour of the 
flowers in Antirrhinum majus. One of the parents is 
homozygotic for red colouring (AA) and the other – for white 
(aa). There is no dominance, that is why the flowers of the 
heterozygotic plants (Aa) from F₁ have a pink colour. The 
offspring in F₂ yields red, pink and white colours in a ratio of 1:2:1. 

Polyhybrid Crossing

The crosses of plants and animals differing in more than two hereditary
features is called trihybrid, tetrahybrid and polyhybrid. For example the
thiheterozygoticity (AaBbCc) is based on the assumption that the three allelic pairs
are situated in three homologous chromosomes. Their independent
combination leads to the formation of eight types of gametes — ABC, ABc,
AbC, aBC, Abc, aBc, abC and abc in F₁, which will yield in F₂ 64
combinations each with three hereditary features. In the case of Mendel
they are distributed according to phenotype as follows:

emerged — wrinkled yellow and round green ones. The ratio between the
different phenotypes was as follows: 9 round yellow; 3 round green; 3
wrinkled yellow and 1 wrinkled green (Fig. 2–9).
In order to give an explanation to these results, Mendel has assumed
that upon forming the gametes the genes of the different pairs determining
the given feature are segregated independently of one another so that the
plants of F₁ of the (AaBb) genotypes will yield gametes of the following
genotypes: (AB), (Ab), (aB), (ab). Of these four types of gametes 16
combinations are obtained. This regularity about the independent
segregation of the hereditary features in the progeny became known as the
second law of Mendel.

image

Figure 2–9. Illustrative presentation of the second law of Mendel on the 
independent segregation of the features in dihybrid crosses at a ratio 
of 9:3:3:1 in F₂. The inheritance of both the round (B) and wrinkled (b) 
forms, and the yellow (A) and green (a) colours of the seeds is traced. 
Alleles (A) and (B) are dominant; (a) and (b) — recessive. 

accomplished through the gametes four combinations are possible — 1 (AA),
2 (Aa), 1 (aa). Since the dominant gene for yellow colouring (A) participates in
three combinations it follows that in the offspring of F2 the ratio of the yellow
seeds to the green ones in phenotype is 3:1.

Not always the phenotype (totality of the displayed features
and properties of the organism) is a reflection of the genotype
(the totality of all hereditary features in the organism).
Individuals of an identical phenotype can be differing in the
genotype. In order to understand the genotype of a given
organism after a definite feature, a number of crosses are
required to be carried out in the course of several generations
until the so-called pure (inbred) lines are obtained. Phenotype
and genotype can be homozygotic when the father’s
and mother’s allelic couples are identical in the determination of
a given feature (AA or aa), or heterozygotic — when they differ (Aa).
The emergence of the recessive features in F2 shows
that the recessive genes in the hybrid individuals do not
disappear and are transmitted in
the progeny. Besides, dominant and recessive genes are transmitted in the
posterity independently of one another. This regularity became known as
the first law of Mendel.

image

Figure 2–8. Illustrative presentation of 
the first law of Mendel on the uniformity 
of individuals in F1 hybrids and the 
segregation of features in the progeny 
which determines the 3:1 ratio of the 
dominant to the recessive in the second 
generation (F₂).

Dihybrid Crossing

Mendel has carried out experiments for crosses between varieties differing in
two features as well. One of them was characterized with round yellow seeds
and the other one — with wrinkled green seeds. Since the genes determining
the yellow colour and the round shape have been dominant then the whole
posterity in F₁ was with round yellow seeds. When crosses in F₁ among
themselves were performed, then in F₂ except the initial phenotypes (round
yellow and wrinkled green seeds) two new (recombinant) phenotypes have

G. Mendel has succeeded in the correct interpretation of the results
obtained by him in the peas crosses differing in distinct features and he could
very explicitly express the idea of the existence of hereditary factors. The
Danish researcher W. Johannsen (1903, 1909) has termed these factors
genes and has introduced the notions of genotype, phenotype and pure
line. In 1905 W. Bateson has introduced the term genetics to denote the
science dealing with heredity of organisms (Greek: génesis — origin).
The classical experiments of Mendel have found place in almost all
books and manuals on genetics and biology. They are very popular but in
view of their great importance for the development of chromosome theory,
which will be the object of our further consideration, it is exigent to review
some of his principal formulations also known as the laws of Mendel.

Monohybrid Crossing

G. Mendel has crossed two varieties of peas differing in only one feature —
seeds of green and yellow colouring. In offspring of the first generation (F1)
there were registered only yellow seeds. The feature that has emerged was
called by him dominant and the suppressed one — recessive. The
meaning of these results has become clear when plants of the F1
generation have been crossed among themselves. It proved that in the
second generation (F2) 75% of the offspring is with the dominant feature
and 25% of the receive one, i.e. the ratio was 3:1. When crosses were
carried out within the F2 generation with the recessive feature, the entire
offspring has displayed the recessive one. When the F2 crosses have been
performed within the dominant line then 1/3 of the offspring has displayed
the dominant feature, and 2/3 were mixed thus preserving the 3:1 ratio
between the dominant and recessive features.
Mendel has interpreted the obtained results brilliantly. Schematically this
is shown in Figure 2–8. He has assumed that each hereditary feature is
determined by pair of factors. The hereditary factor for the dominant feature
he has designated by the capital letter (A) and the recessive — by the small
one (a). The obtained hybrids possessing both factors were designated as
(Aa). The pure dominants of a given feature have been received the symbol
(Aa) and the recessive ones — (aa).
For purposes of better explanation of the biological essence of these
symbols let us consider an example using terminology that has been
introduced later. The vegetative parental cells of the pure variety of peas with
yellow seeds contain each one a pair of genes (allelic couple) determining
their yellow colour (AA). The pure variety with green seeds has also got two
genes determining its green colour (aa). The one gene comes from the male
parent, while the other one — from the female. The parental cells of the hybrid
plants in F1 bear both genes (Aa). Their gametes (the sexual cells with a
reduced chromosome set obtained in the process of meiosis) contain only one
of the two genes (A) or (a). In the crosses of both varieties of peas which is

With the formation of cytology as a separate branch of biology
favourable conditions were created for its accelerated development and
improvement of the methods existing at that time, and the introduction of
new, more sophisticated ones. Such is microscopy for example.
Alongside ordinary light microscopy other types such as phase-contrast,
interference, polarization, fluorescent and electron microscopies have
gradually been created and applied. X-ray diffraction analysis has proven
very precious in the studies on high-molecular substances (proteins,
nucleic acids, carbohydrates) playing a decisive role in the structure and
function of the cell.

Discovery of the Discrete Factors of Heredity. The Experiments of Gregor Mendel

Section 2.4.

Heredity as a phenomenon has been known since antiquity. People have
noticed the similarities between parents and children and have practised
selection of animals and plants according to the qualities preferred. But the
material basis of this phenomenon and the mechanisms involved in it have
remained completely in the dark till the beginning of the XX century.
This problem has been occupying scientist minds for a long time. It was
considered that heredity in some way was transmitted through blood. Darvin’s
suggestion about the existence of discrete hereditary units in the blood of
animals and men called gemules by him is well-known. His cousin F. Galton
has expressed disagreement with that opinion being based on the “simple”
fact that transfusions of blood from white rabbits into black ones and vice
versa did not lead to a change in their colouring.
In 1865 the studious monk Gregor Mendel has discovered some
regularities in the inheritance of parental characteristic features in the process of
hybridization of peas (Pisum sativum). His work “Experiments on Plant-Hybrids”
(Mendel, 1866) has remained unnoticed. The symbols provided by him to
designate the hereditary factors and the “boring” figures of the results obtained
could not have possibly interested his contemporaries. Scientific thought at that
time was not mature enough to accept his results and give him his due. His work
has become known to the public after 1900, when C. Correns, H. de Vries and
E. Tschermak have indenpendently from one another rediscovered these
regularities. After Dobzhansky’s view (1955) “The work of Mendel is truly classic
and will be studied by all students of genetics as a brilliant example of the
application of scientific methods in modern experimental science”.
Mendel’s success was ensured by the combination of his skill to select
the exact object for studies, to choose well-discernible, contrasting and
quite simple hereditary features, to lead precisely the experiments and to
interpret the results correctly implementing mathematico-statistical
processing.

grains are formed, which by way of condensation form a nucleolus and a
proper nucleus after that. On the surface of the nucleus again from mucous
substances a membrane is formed, which confines the space holding the
nucleus. This space according to Schleiden is the newly formed cell.
Similar is the theory of cytoplasts by Schwann. The difference between
them is that according to Schleiden the cell-forming substances are found
inside the cell, while Schwann admits that they can also be found in the
intracellular spaces.
A common weakness of both theories is the assumption of a possibility
for spontaneous cell formation through crystallization of mucous
substances. This understanding was opposed by F. Unger (1841—55), K.
Nägeli (1842—46), H. von Mohl (1835—51), etc. Their studies gradually
have been leading to the viewpoint that the formation of new cells is via cell
division. Attribute in this trend is also the premise of R. Virchow (1855,
1858) — “Omnis cellula e cellula”. This formulation of his was accepted as
a law of biology by some authors since modern science does not know any
other way for cell reproduction except through division.
With the discovery of cell division (see Chapter 3, Section 3.2), and the
refuting of the incorrect concepts about the cell formation, the studies have
taken along the right path. The attention of the investigators was drawn to
the content surrounded by cell walls. It was already regarded as live
matter which was called sarcoda by F. Dujardin in 1835. The term
protoplasm was introduced by J. Purkinje (1839) in the animal cells and by
H. von Mohl (1846) for the plant cells. Several years after that F. Cohn
(1850) has arrived at the conclusion “that optical, chemical and physical
properties of these two substances are similar”.
The studies in this period were intense and versatile. They have
served for the rise of various viewpoints, hypotheses and theories which
have finally led to the summarized idea that morphologically the cells
consist of cell membrane, cytoplasm and nucleus. Ernst Brücke (1862)
has for the first time looked upon the cell as an elementary organism,
which has marked a new stage in the process of its study.
This concept about the structural organization of the cell has
strongly stimulated the studies on the processes and phenomena taking
place in the cytoplasm. Different pictures about this structure were
forwarded — fibrous, granular, reticular, etc. Some of its organelles have
also been discovered — the cell centre by E. van Beneden,
chondriosomes (mitochondria) by C. Benda, the Golgi apparatus, etc.
which will be described later (Section 2.8).
In the second half of the XIX century the studies on the cell have given
rise to an individual science — Cytology (Greek: kýtos — cell and logos —
science). The beginning was marked by the two capital books — “Biology
of the Cell” by J. Carnoy (1884) and “Cell and Tissue” by O. Hertwig
(1893—98).

image

Figure 2–7. Cells from different organisms with defined nuclei (After 
Schwann, 1839; From Katznelson, 1963). 
1 — gill cartilage from a frog; 2 — cartilage from a bone in the loins of 
a pig embryo; 3 — enamel fibres from the teeth of a pig embryo; 4 — 
cells from the surface of the enamel coating; 5 — fibres from a human 
tooth isolated by maceration with HCl; 6 — fibrous cell from a cell 
wall; 7 — better developed cells from a cell membrane; 8 — cells from 
the mucous substance of the membranes of a pig embryo; 9 — cell 
from the orbital cavity of a pig embryo; 10 — mast cells from the skull 
cavity of a fish; 11 — cells from the Achilles’s tendon of a pig embryo; 
12 — cells from the middle layer of pig embryo aorta; 13 — muscle 
cells of a pig embryo.

Later Concepts on the Structural Organizacion of the cell. Discovery of Some Its Organelles

Section 2.3. The cell theory has confirmed the principle of uniformity in the
structure of plant and animal organisms. The cell was accepted as a basic
structural unit. The problem of the formation of new cells from the already
existing ones i.e. their genesis has come into the limelight.
An answer to this very important question was attempted to be given
by M. Schleiden and Th. Schwann. Schleiden has created the theory of
cytogenesis which is expressed in short by the following: from the mucous
substances attached to the inner layer of the cell wall of the existing cells,

Cell Theory

Section 2.2. Due to improving of microscopic technique and as a result
from a number of studies carried out in the first half of the XIX
century, the concepts concerning the very essence and structure of cells
are also subjected to change. They are already accepted as structural
elements with a cell membrane, which can be linked to one another thus
forming tissues and organs. During this period the cell nucleus has been
discovered. This subject will be in detail treated in Chapter 3 (Section 3. 3).
The accumulation of a lot of experimental material in the studies on both
plant and animal cells gives rise to the idea of uniformity in the morphology of
the plant and animal kingdoms. Such an idea has been launched in a purely
contemplative manner by some philosophers and researchers at that time
while in 1838—1839 it has been finally shaped as a cell theory.
In literature the names of a number of authors have been indicated
as creators of the cell theory — C. F. Wolff, L. Oken, P. F. Goryaninov,
R. Dutrochet, J. E. Purkinje, G. Valentin, M. Schleiden, etc. It would only
be just a historical point of view to state that they all had a certain
tribute, especially Schleiden. The priority however for this great
discovery is due to the German physiologist and histologist Theodor
Schwann (1839) who has not only very clearly formulated the thought of
the uniformity in the building of plant and animal world, but has also
proved it with a convincing experimental material (Fig. 2–7).
The rise of the cell theory has met varying estimates. Some
investigators have accepted it as fully proven, while others were sceptical.
As a logical consequence from its basic premises the world of organisms
has been divided into two large groups: multicellular and unicellular. This
has on its part raised a number of questions: are protozoa cells or are they
organisms; what is a cell and what is an organism; are multicellular
organisms, their tissues and organs a mechanical sum of autonomous cells
or they display a specific wholeness; are there smaller units than the cells,
etc.
Some of these questions are quite reasonable and some of them have
already been answered in a satisfactory way, while others still remain
unanswered. Nevertheless, cell theory has passed the test of time and has
been acknowledged as the only right one in the explanation of living
organism structure.
Cell theory is one of the most fundamental generalizations in biology.
Very justly it is classified by F. Engels as one of the three remarkable
discoveries of the XIX century, together with the discovery of the law for the
preservation of energy and the role of the chemical evolution for the
creation of the living organisms.

them the future organism is found in a ready strongly reduced state in
the cells (ovists in the eggs and spermatists — in the spermatozoa),
which later on undergoes its individual development. Extremal
preformists such as the Swiss philosopher and naturalist Charles Bonnet
were holding the view that in the embryo of the ovary is already
contained the embryo of the next generation, in it the next and so on.

image

Fig. 2–6. Schematic imaging of cell walls (After Link, 1807; From 
Katznelson, 1963).

A decisive rebuff to the preformists was rendered by the theory of
epigenesis. Its more well-known supporters are R. Descartes, D. Didrot,
C. F. Wolff, P. Maupertuis, W. Harvey, etc. On the basis of the epigenetic
concepts and insights of that time which have admitted formation and
development of the embryos, the studies on the cell are driven in the right
direction and the cell theory has been conceived.