Download TEXT Mendel`s Study of Heredity A. Gregor Johann Mendel

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Mendel’s Study of Heredity
A.
Gregor Johann
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Death date :
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Mendel- an amazing monk
Johann Gregor Mendel
:
July 22, 1822
Hyncice, Czechoslovakia
January 6, 1884
:
Brünn, Bohemia
Moravian
Male
Natural Scientist
The first person to gain some understanding of the
principles of heredity was Gregor Johann Mendel (Fig.
1). He was born in an ethnic German family on July
22,1822 in the village of Heizendorf in north Moravia,
Austria (now Hyncice, Czech Republic). Mendel was son
of a peasant (farmer) and the grandson of a gardener.
As a child, Mendel benefited from the progressive
education provided by the local vicar, and he was
eventually enrolled at the Philosophical Institute in
Olmutz (now Olomouc). During his childhood, he had to
suffer on account of financial and health problems, which
seriously interfered with his education. Unable to
continue his studies due to worsening financial condition,
he joined the Augustinian Monastery of Saint Thomas at
Brünn (now Brno) in 1843 and became a friar. Born
Johann Mendel, he took the name Gregor upon entering
monastic life and eventually was placed in charge of the
monastery’s experimental garden.
Although Mendel felt no personal vocation at the
time, he believed that the monastery would provide him
the best opportunity to pursue his education without the
financial worries. In 1851, he was sent to University of
Vienna to study zoology, botany, chemistry, physics,
mathematics, etc. Although Mendel was a sincere and
hard working student, he did not do very well in his
studies, particularly in physics and mathematics.
After completing his studies, he returned to Brünn in
1854, where he was appointed as a substitute teacher in
Brno Technical School. His performance as a science
teacher was excellent, and it was here that he, for the
first time, showed the glimpses of the genius within him.
In addition, he worked as a priest in the local church; he
lived in a house located within the premises of the
church.
Mendel believed that species are resistant to
change, because characters are inherited without
alteration throughout generations. This was a novel idea
to breeders of the day. No one knew just how
characteristics were inherited. Common experience
showed that children resembled their parents, but it was
not known how do various traits sort out in the union of
sperm and egg. Likewise, it was not clear why some
crosses of plants or livestock are sterile, and the others
fertile. Darwin (Fig. 2) toyed with a hypothesis he called
Pangenesis, which assumed that traits from all over the
body somehow flow into the gametes. A common
misconception of the time was that traits were blended in
the offspring, rather than remaining as discrete units (by
analogy, compare mixing two fluids versus mixing two
jars of colored marbles). Darwin’s theory demanded that
variations be heritable, and that traits be fluid enough to
evolve so that they could be acted upon by natural
selection. If the traits remain unchanged, like the
colored marbles, how could new variation arise? Each
generation would just get a different ratio of static,
unchanging characteristics.
Mendel performed experiments with several species
of garden plants, and he even tried some experiments
with honeybees. His greatest success, however, was with
peas. He conducted all his experiments within the kitchen
garden of his house with the help of his own resources
(Fig. 3). He began to collect pea seeds for his
experiments in 1857 from commercial growers all over
the Europe. His work is often cited as a textbook example
of the experimental method. It required patience,
attention to detail, careful record keeping, and
interpretive insight. After seven years of pains-taking,
sincere, devoted and exhaustive experimentation, he
presented his findings before the Natural History Society
of Brünn at two of its meetings on February 8 and March
8, 1865. This paper entitled “Experiments in plant
hybridization” was presented in German language and
was published in the annual proceedings of the Society in
1866.
Later, Mendel became more and more involved with
the work of monastery, and he was made an abbot in
1868. But he still found time to continue his studies on
honey bees, some other plants, and on climatology.
Gregor Mendel died on January 6, 1884 at the age of 62
years.
Shortly before his death, Mendel said, “My scientific
labors have brought me a great deal of satisfaction, and I
am convinced that before long the entire world will praise
the results of these labors”.
Sixteen years after his demise, his paper languished
in obscurity until 1900, when this paper was rediscovered
by three botanists, Hugo de Vries (Fig. 4) in Holland,
known for mutation theory and studies on the evening
primrose and maize; Carl Correns (Fig. 5) in Germany,
who investigated maize, peas and beans; and Eric von
Tschemark–Seysenegg (Fig. 6) in Austria, who worked
with several plants, including garden peas. As these men
searched for the scientific literature for data supporting
their own theories of heredity, each found that Mendel
had performed a detailed and careful analysis 35 years
earlier. Mendel’s ideas quickly gained acceptance,
especially through the promotional efforts of William
Bateson (Fig. 7), an English man, who gave this
developing science the name “Genetics” in 1905. He
coined the term from a Greek word meaning “to
generate”. Gregor J. Mendel is appropriately called the
father of Genetics.
B.
Selection of the experimental Material
Mendel’s experiments were designed to investigate
the most widely accepted model of inheritance, blending,
which held that the traits of an offspring would be a
blend of the parental traits. For example, the theory of
blending predicts that a black and a white horse parents
would give rise to a grey-colour offspring. Mendel’s
results showed that for many simple traits, at least, this
model was wrong. Instead, the offspring displayed traits
in exactly the same form as they appeared in one or the
other parent.
One reason for Mendel’s success is that he chose his
experimental material intelligently. The garden pea,
Pisum sativum (Fig. 8), is a dicot, a type of plant that
sprouts two leaves, or cotyledons, from a germinating
seed. Garden pea offered several obvious advantages as
an experimental material, about which Mendel was
aware; these advantages are summarized below:
i)
In the garden pea varieties available commercially,
several characters had two contrasting forms, which
were easily distinguishable from each other (Table
1). This permitted an easy classification of F2 and F3
progeny from various crosses into clear-cut classes
on the basis of the contrasting forms of different
characters.
Table 1: Description of the seven pairs of contrasting
characters of pea studied by Mendel
Character
Dominant form
Recessive form
Seed form
Round (smooth)
Wrinkled
Seed colour
Yellow
Green
Flower colour
Purple
White
Pod colour
Green
Yellow
Pod shape
Full (Inflated)
Constricted
Flower position
Axial
Terminal
Stem
height
/ Tall
Dwarf
length
ii)
One peculiarity of pea reproduction is that the petals
of the flower close down tightly, preventing pollen
grains from entering or leaving (Fig. 9). This
enforces a system of self- fertilization, in which the
male and female gametes from the same flower
unite with each other to produce seeds (Fig. 10). As
a result, the individual pea strains are highly inbred,
displaying little, if any, genetic variation from one
generation to the next. Because of this uniformity,
we say that such strains are true-breeding.
iii)
Pea flowers are relatively large. Therefore,
emasculation and pollination of pea flowers is quite
easy, which allows easy artificial hybridization in pea
(Fig. 11).
iv) They have short life cycle, i.e. produce many
generations in a short time (Fig. 12).
v) Pea seeds are large and present no problem in
germination. Peas are easily grown in experimental
gardens or in pots. This permits a relatively large
number of pea plants to be grown in a relatively
small area.
vi) They are easily grown in experimental garden or in
pots in a green house.
vii) They are very productive, which means they produce
many seeds.
C.
Reasons for Mendel’s success
The success of Mendel cannot be ascribed to a single
factor. The following several factors acted together to
bring Mendel the astounding success, where many of his
predecessors had failed.
a)
b)
Diagnosis of predecessors weaknesses: The
most important factor that contributed to his success
was his ability for an accurate and keen analysis of
the reasons for the failure of earlier workers. He
accurately diagnosed the weaknesses of their
experimental materials, techniques and approaches
and he carefully avoided in his own experiments.
Identification of contrasting characters: Mendel
selected varieties that had clearly different forms of
one or more characters, e.g., smooth or wrinkled
seeds, grey or white seed coat colour, yellow or
c)
d)
e)
f)
green cotyledons, etc. (Fig. 13). The two distinct
forms of a character, e.g. smooth and wrinkled
seeds, are termed as contrasting characters. The
difference between the two forms of a pair of
contrasting characters was so clear-cut that the
individuals of a population could be easily and
accurately classified as having one or the other
contrasting form.
Study of unit character: At first, Mendel studied
the inheritance of only one pair of contrasting
characters at a time. He took advantage of these
contrasting traits to determine how the characters of
pea plants are inherited. His focus on these singular
differences between pea strains allowed him to study
the inheritance of one trait at a time, for example,
plant height (Fig. 14). Other biologists, had
attempted to follow the inheritance of many traits
simultaneously; but because the results of such
experiments were complex, they were unable to
discover any fundamental principle about heredity.
True breeder: The exclusive use of pure lines
ensured the predictable outcomes by preventing
masked traits in the parents.
Maintenance of pedegree records: Mendel kept
careful records of the experiments for every
generation, which allowed him to determine the
ratios.
Introduction of mathematics: Mendel’s knowledge
of
mathematics
was
instrumental
in
the
interpretation of his findings. As a result, he was able
to accept that ratios ranging from 2.82:1 to 3.15:1
were all estimates of 3:1, and not separate ratios.
g)
h)
Large number of progeny per experiment: This
gave Mendel the strength of numbers that enabled
him to make a statistically significant conclusion.
Used an experimental approach: Mendel was able
to devise appropriate hypothesis on the basis of the
explanation he offered for his experimental findings.
Further, he tested these hypotheses experimentally
to prove the correctness of his explanations.
In spite of the brilliance of experimental approach
and procedure, unquestioned genius in analysis and
interpretation of data, and the greatest care in the
selection of material and carrying out of experiments,
Mendel was, undoubtedly, lucky because of the following
facts:
i)
The seven characters selected by Mendel showed
qualitative inheritance, and not a single one was
inherited quantitatively.
ii) The contrasting form of each of the seven characters
was governed by a single gene, and in each case one
form was completely dominant over the other.
iii) Based on the genetic maps of pea chromosome, two
of the characters out of the seven characters studied
by Mendel have genes on chromosomes 5 and 7 i.e.
they are located on non-homologous chromosomes.
The genes for other two characters are located on
chromosome 1and the genes for remaining three
characters are present on chromosome 4. Out of
these only two genes, alleles of which determine pod
shape and stem height, were close enough to distort
the normal dihybrid ratio. Luckily, Mendel did not
investigate these characters. Had he done so, he
would have run into the confusing phenomenon of
linkage.
D.
Reasons for the neglect of mendel’s findings
A number of reasons were responsible for the
neglect of the findings of Mendel:
a) Mendel used mathematical principles of probability
and distribution (binomial) to explain a biological
phenomenon. This was something new and not
readily acceptable to biologists, since they believed
that biological phenomena are too complex to be
reduced to a mathematical treatment.
b) Mendel’s result did not match those of other
scientists who also used plants to try studying
heredity.
c) Most scientists of Mendel’s time focused on things
they could actually see in their research. They
wanted to know exactly what Mendel’s “factor” was?
What did they look like? Where they were located?
No one, not even Mendel, could answer these
questions.
d) Mendel failed to demonstrate the validity of his
conclusions in other species. He was distinctly
unlucky in selecting Hieraceum (a facultative
apomict) and honey bees (having haploid males) as
experimental materials. As a result, he failed to
verify his conclusions from pea in these organisms.
e) Mendel
corresponded
extensively
with
his
contemporary, the noted botanist, Carl Nägeli.
Mendel informed Nägeli about his failure to verify his
conclusions in Hieraceum and other plants. This may
have created a doubt about the applicability of
Mendel’s conclusions to plants other than pea.
E.
Highlights of the mendel’s paper
Mendel’s paper that launch the science of Genetics
had the title “Versuche über Pflanzenhybriden” which
translates from the German as “Experiments with PlantHybrids”. This paper was remarkable for its precision and
clarity. The main highlights of the paper are summarized
below:
i)
Mendel concluded a complete fusion of male and
female gametes, and equal contribution of the two
parents to the development of characters of the
hybrid (F1).
ii) He postulated the existence of factors, now known as
genes, which are responsible for the development of
various characters.
iii) He clearly stated that genes were particulate.
iv) He made a clear distinction between the appearance
(phenotype) and the genetic makeup (genotype)
(Fig. 15). He classified F2 individuals with the
dominant phenotype into pure and hybrid forms on
the basis of the character forms present in the F3
progenies produced by them.
v) He gave the formulae for determining the numbers
of:
a) Different types of gametes (F1),
b) Different genotypes,
c) Homozygous genotypes, and
d) Individuals in the perfect F2 for segregation of n
number of genes.
vi) He elaborated the effect of continued selfing on
homozygosity for a single allelic pair (a gene with
two alleles). Incidentally, this is the same as the
homozygosity for any number of allelic pairs.
vii) Mendel introduced the concept of dominance and
recesiveness and also gave these terms (actually
their German equivalents)
viii) Mendel elucidated the laws of segregation and
independent assortment.
ix) He confirmed his own findings in pea with those in
rajma. He studied the inheritance of pod colour
(green dominant over yellow), pod shape (inflated
pods dominant over constricted) and plant height
(tall dominant over dwarf) in rajma.
x) He studied the inheritance of flowering time (and
peduncle length) in rajma, and noted the
intermediate appearance of F1 and the effect of
environmental factors on flowering time. He
concluded that these characters also were inherited
according to the same laws of segregation and
independent assortment.
xi) Mendel noted the appearance of many new, nonparental flower colours in rajma. He suggested that
colour in rajma may be a complex character, i.e.,
governed by several genes inherited according to the
same laws of inheritance.
xii) He concluded that a larger number of progeny would
increase the precision of observations. He also
realized that the ratios obtained from biological
studies cannot be expected to be as accurate as
those in the case of mathematical calculations.
xiii) Mendel stated that his explanations were based on
two important assumptions:
a) equal proportion of different gametes produced
and
by F1,
b) equal chance for each gamete thus produced to
effect
fertilization. He presented experimental
evidence
through test cross data to show that
the different gametes produced by the F1 were in
equal frequency.
xiv) He studied the F2 from an interspecific hybrid
Phaseolus and obtained deviation from the 3:1 ratio.
He explained this to be mainly due to high sterility of
the F1 hybrid and the small number of progeny in F2.
He suggested that the inheritance in such crosses
also followed the same laws.
xv) He also attempted to explain the permanent hybrids
of some plant species, e.g., Hieraceum, reported by
Gaertner. Unfortunately, Mendel had no way to know
that these species are apomictic, and present a
peculiar pattern of inheritance.
From the above discussion it can be concluded that
without knowing about chromosomes or the details of cell
division, he had found the basic laws of Genetics. The
laws Mendel deduced seem common-sense now, but
were radically new in his days:
1. Law of paired factors (Genes): For the expression
of any trait, a pair of factor is required, later this
factor was termed as allelomorph by Bateson and
Gene by Johnson.
2. Law of dominance: The law states that in a cross
of parents that are pure for contrasting traits, only
one form of the trait will appear in the next
generation. The trait which expresses itself in the
next generation is called as dominant trait, while the
trait which remains unexpressed or hidden is called
recessive.
3. Law of segregation: The law states that for any
particular trait, the pair of alleles separate during
gamete formation and only one allele passes from
each parent on to an offspring.
4. Law of independent assortment: Factors for
different traits behave independently. No two
characters need be always together. Different
combinations of characters could be brought about.
The yellowness is not associated with roundness and
greenness with wrinkledness.
F.
Reasons for failure of Mendel’s predecessors
In his 1865 paper, Mendel presented a brilliant
analysis of the deficiencies in the experimental
approaches of his predecessors. These are summarized
below.
a) The scientists studied the plant as a whole, i.e., its
total
appearance consisting of a large number of
characters. Therefore, the plants could not be classified
into few clear
cut classes. These workers did not
attempt an exhaustive
classification of the different
forms of the characteristics
present in the progeny.
b) The scientists were more concerned with the
description of various forms appearing in the
progeny. An attempt to determine the frequencies of
different characteristic forms in the progeny was not
made.
c) In many cases, the data from different generations
were not
kept accurately and separately.
d) In many cases, a complete control on pollination in
the F1 was lacking.
e) In many studies, the F1 was an interspecific hybrid
exhibiting partial to considerable sterility.
f) The number of plants studied in F2 was relatively
small.
g) In addition, most of the characters studied by the
earlier workers were quantitative in nature.
Mendel’s Principle of Dominance
Mendel’s Law of Dominance - The second law of
Mendel (first law being Law of Unit Characters) states
that “in a cross of parents that are pure for contrasting
traits, only one form of the trait will appear in the next
generation”. Offspring that are hybrid for a trait will have
only the dominant trait in the phenotype.
While Mendel was crossing (reproducing) his pea
plants (over and over again), he noticed something
interesting. When he crossed pure tall plants with pure
short plants, all the new pea plants (referred to as the
F1 generation) were tall (Fig. 16). Similarly, crossing pure
yellow seeded pea plants and pure green seeded pea
plants produced an F1 generation of all yellow seeded pea
plants (Fig.17). The same was true for other pea traits
(Table 2). So, what he noticed was that when the parent
plants had contrasting forms of a trait (tall vs short,
green vs yellow, etc.) the phenotypes of the offspring
resembled only one of the parent plants with respect to
that trait. So, Mendel proposed a law in which he
proposed that there is a factor that makes pea plants tall,
and another factor that makes pea plants short.
Furthermore, when the factors were mixed, the tall factor
seemed to dominate over the short factor.
Table 2: Results of Mendel’s monohybrid cross.
Parental strains
F1
F2 progeny
Ratio
progeny
Tall plants x Short Tall plants
plants
Round
seeds
x Round
wrinkled seeds
seeds
Yellow seeds x green Yellow
seeds
seeds
Purple
flowers
x Purple
white flowers
flowers
Inflated
pods
x Inflated
constricted pods
pods
Green pods x Yellow Green
pods
pods
Axial
flowers
x Axial
terminal flowers
flowers
787
tall,
277
dwarf
5474 round, 1850
wrinkled
6022 yellow, 2001
green
705 purple, 224
white
882 inflated, 299
constricted
428 green, 152
yellow
651 axial, 207
terminal
2.84
:1
2.96
:1
3.01
:1
3.15
:1
2.95
:1
2.82
:1
3.14
:1
Now, from our modern wisdom, we use allele or
gene instead of what Mendel called factors (Fig. 18).
There is a gene in the DNA of pea plants that controls
plant height (makes them either tall or short). One form
of the gene (allele) codes for tall, and the other allele for
plant height codes for short. For abbreviations, we use
the capital “T” for the dominant tall allele, and the
lowercase “t” for the recessive short allele.
A.
Mechanism of monohybrid cross
A cross in which only a single pair of alleles is
considered is called a monohybrid cross. Fig. 19 is a
mathematical representation of the cross between tall
and dwarf peas in terms of Mendel’s interpretation. In
this T is the symbol which stands for the factor or gene
controlling tallness, and t is the symbol used to denote
the factor or gene controlling dwarfness. The factors or
genes, as postulated by Mendel, always occur in pairs.
Both tall and the dwarf plants which are crossed are
homozygous (i.e., both the genes in a pair are identical).
These plants are “pure” for tallness and dwarfness,
respectively, and if self-pollinated will always breed true,
producing only tall and dwarf plants, respectively.
In the present monohybrid cross, the tall parent,
which is homozygous, is shown as TT, and the dwarf
parent shown as tt. During the course of sexual
reproduction both kinds of plants produce gametes;
these gametes contain but one factor of each pair (i.e.,
either T or t). The gametes produced by the tall plant
contain T gene (Fig. 20), while the gametes of dwarf
plant possess t gene (Fig. 21). The fusion of a gamete
from the tall plant with a gamete from the dwarf plant
produces a tall plant in F1 generation, because the gene
for tallness (T) is dominant over the dwarfness (t). The
new plant in the F1 generation is shown in the Fig. 19 as
Tt. It is a heterozygous plant because it possesses a pair
of homologous chromosomes carrying one allele for
tallness and one for shortness (Fig. 22). The
heterozygous plants produce two kinds of gametes or sex
cells, male gamete and female gamete. Half of the male
gametes contain T gene and the other half t gene.
Similarly half of the female gametes possess T gene and
the other half t gene. During the process of fertilization,
these two kinds of gametes i.e., male and female, unite
at random and produce F2 generation. As a result of
these chance combinations, an approximate phenotypic
ratio of 3 tall plants to 1 dwarf plant (i.e., 3:1 ratio) is
normally obtained (Fig. 23). All plants with TT and Tt
genes will be tall, and the plants possessing tt (both
recessive) genes will be dwarf.
Mendel further noted that the results from reciprocal
crosses were identical. Since garden peas are sexual
plants, each cross can be performed in two ways. The
two crosses in which the same two parents are involved
but the strain which serves as the male parent in one
cross is used as the female parent in the other, and viceversa, is called reciprocal cross (Fig. 24).
B. Certain other examples of Law of Dominance
After Mendel several Geneticists tested the validity of
the law of dominance in several plants and animals. They
found its wide application.
a. Law of Dominance in Plants: Besides pea plant, the
law of dominance has also been observed in various
other plants (Table 3).
Table 3: The dominant and recessive characters in
plants.
Name of
Dominant
Recessive
the plant
character state
character state
Nettle
Serrated leaves
Smooth
margined
leaves
Sunflower
Branched habit
Un-branched habit
Cotton
Coloured lint
White lint
Maize
Round
starchy Wrinkle,
sugary
kernel
kernel
Snapdragon Red flower
Non-red flower
Barely
Beardlessness
Beardness
Wheat
Susceptibility
Immunity to rust
Tomato
Two-celled fruit
Many-celled fruit
b. Law of Dominance in animals:
The
law
of
dominance is applicable well to the animals. For instance,
when a homozygous black guinea pig is crossed with a
homozygous brown guinea pig (Fig. 25), all hybrids of F1
are found to be black. The black hybrids of F1 when
mated among themselves they produce black and brown
offsprings in 3:1 ratio. This shows that black coat colour
dominates over brown coat colour.
The dominant and recessive character states of some
other animals are tabulated in Table 4.
Table: 4: The dominant and recessive characters in
animals.
Name of
animal
Cat
Dog
Cattle
Horse
Sheep
Fruit Fly
Salamander
Land snail
Body
character
Skin colour
Length
of
Hair
Skin colour
Tall
Colour
of
face
Horn
Skin colour
Movement
Wool
Eye colour
Wings
Body colour
Shape
of
shell
Dominant
state
Tabby
Short hairs
Recessive
state
Black or blue
Long hairs
Grey
Stumpy
White
Black
Normal tall
Coloured
Polled
Black
Trotting
White
Red
Flat & yellow
Dark
Unbanded
shell
Horned
Red
Pacing
Black
White
Curled & white
Light
Banded
shell
c. Dominant and recessive character states in
humans: The rules of inheritance discovered by Mendel
are applicable to humans also. Consider the inherited
disease of humans, Cystic Fibrosis. This is the most
common lethal genetic disease afflicting Caucasians. It is
caused by a mutant recessive gene carried by one in 20
people of European descent (over 12 million people in the
United States alone). This means that one in 400
Caucasian couples will both be carriers and, as can be
expected by the monohybrid inheritance pattern, one in
four of their children will have the disease.
If two parents who are carriers of the recessive gene
Cfcf, (heterozygous) produce offspring, as can be seen in
the Punnett square (Fig. 26), one in four of their children
will be homozygous and have cystic fibrosis.
Of course, it should be noted that this one in four
probability is just an expectation, and that in fact two
carriers could produce any number of perfectly normal
children. However, the greatest probability is for one in
four children to be affected. Presently one out of every
29 Americans is a symptom-less carrier of the gene.
B.
Phenotype and genotype
The term phenotype refers to the external
appearance of the organism; the term genotype refers to
the internal, genetic make-up of the individual. The
concepts of phenotype and genotype were clearly stated
by Mendel, but the terms themselves were introduced
much later. A tall plant may be either homozygous (TT)
or heterozygous (Tt) genotypically, but in either state it
will have the same phenotype (appearance) i.e., tall (Fig
15). Thus the genotype of individuals with the dominant
character cannot be known from their phenotype; it can
be known only after studying their progeny (either from
selfing or from a test cross). On the otherhand, dwarf
plants can only have the genotype tt. Therefore, the
genotype of individuals possessing a recessive trait can
be inferred from their phenotypes themselves.
The relationship between the genotype and
phenotype is a simple one. The genotype carried by all
living organisms, holds the critical instructions that are
used and interpreted by the cellular machinery of the
cells to produce phenotype of the organism. Thus, all the
physical parts, the molecules, macromolecules, cells and
other structures, are built and maintained by cells
following the instructions give by the genotype. As these
physical structures begin to act and interact with one
another they can produce larger and more complex
phenomena such as metabolism, energy utilization,
tissues, organs, reflexes and behaviors; anything that is
part of the observable structure, function or behavior of a
living organism.
Mendel’s principle of segregation
The 3:1 ratio can be explained with reference to Fig.
27. This is the heart of Mendelian genetics. The figure
illustrates the following key features of single-gene
inheritance:
a. Genes come in pairs, which mean that a cell or
individual has two copies (alleles) of each gene.
b. For each pair of genes, the alleles may be identical
(homozygous TT or homozygous tt), or they may be
different (heterozygous Tt).
c. Each reproductive cell (gamete) produced by an
individual contains only one allele of each gene (that
is, either T or t)
d.
In the formation of gametes, any particular gamete
is equally likely to include either allele (hence, from a
heterozygous Tt genotype, half the gametes contain
T and the other half contains t).
e. The union of male and female reproductive cells is a
random process that reunites the alleles in pairs.
The essential feature of transmission genetics is the
separation, technically called segregation, in unaltered
form, of the two alleles in an individual during the
formation of its reproductive cells. The principle of
segregation is sometimes called Mandel’s first law
(because law of unit characters and law of dominance do
not hold true in all cases).
The principle of segregation is, as a matter of fact,
the later half of the law of dominance. It states that
“alleles will separate from each other during the
production of gametes so that they are equally
transmitted to the progeny”. This principle is often
referred to as the principle of “purity of gametes”,
because gametes have only one gene of each pair.
A.
Assumptions involved in segregation as an
explanation for the 3:1 ratio
The explanation of 3:1 ratio according to the law of
segregation is based on the following assumptions:
a) The production of two types of gametes in equal
frequencies in F1,
b) Equal survival or function of the different gametes,
c) Random union of male and female gametes, and
d) Equal survival of different zygotes thus produced.
Mendel himself was aware of the first and the third
assumptions, and he devised the test cross to test the
equal frequency of the gametes from F1. A failure of the
second and fourth assumptions may occur in some cases,
which may lead to significant deviations from the typical
3:1 ratio. Fortunately, the failure of these two
assumptions is not common, and it didn’t occur in the
cases studied by Mendel.
The evidence for equal frequency of the two types of
gametes produced by segregation of a single gene is both
indirect as well as direct. Indirect evidence is based on
the frequencies of different types of zygotes produced in
a test cross. The direct evidence, on the other hand,
derives from a direct classification of gametes and
scoring the frequencies of the two classes.
B. Certain other examples of law of segregation
The law of segregation is universal in its application,
and it has been found to occur in both plants and
animals. The examples which are cited above in the law
of dominance can also be considered for the law of
segregation. However, to understand the mechanism of
segregation more clearly in animals, it will be helpful to
consider an original experiment of T.H. Morgan on
Drosophila.
Morgan crossed a homozygous long winged (wildtype) Drosophila with a homozygous vestigial winged
Drosophila (Fig. 28). The F1 heterozygote or hybrids were
found to be long winged. When the F1 hybrids were
allowed to mate among themselves, they produced longwinged and vestigial hybrids in 3:1 ratio in the F2
generation.
In this cross, the mechanism of segregation can be
well understood by assuming that homozygous longwinged Drosophila has a pair of alleles LL for longness of
wing and, similarly, homozygous vestigial-winged
Drosophila has the alleles ll for vestigial nature of wings.
The long-winged Drosophila thus produces the gametes
with the single allele L and the vestigial-winged fly
produces the gametes with the single allele l. The
gametes of both parents unite in the process of
fertilization and produce a hybrid with long wings in the
F1 generation. The genotype of the F1 hybrid is Ll, and in
which allele L is dominant over the l, which is recessive.
At the time of gametogenesis, these alleles (L and l) are
separated along with chromosomes to form two types of
gametes, half of the gametes having the allele L and the
other half the allele l. These gametes unite in three
possible combinations, viz. LL, Ll and ll, to produce
genotypically three types of individuals LL, Ll and ll in
1:2:1 ratio in F2 generation. Phenotypically there occur
three long-winged and one vestigial-winged Drosophila.
Thus the dominant and recessive alleles remain together
for long time without contaminating or mixing with one
another, and segregate during gametogenesis.
C.
Physical basis of segregation
The phenomenon of segregation can be easily
explained on the basis of behaviour of homologous
chromosomes during meiosis. As a consequence of
segregation, the two alleles of gene separate and go into
different gametes. Similarly, the two members of the
homologous chromosome pair separate at anaphase-I of
meiosis and move to the opposite poles of a cell.
In F1 hybrid or a heterozygote, one of the two alleles
of a gene is located in one chromosome, while the other
allele is present in the homologue of this chromosome.
The two homologous chromosomes pair during prophaseI and orient at the metaphase plate during metaphase-I.
At anaphase-I, one of the two homologous chromosomes
moves to one pole, while the other chromosome of the
pair moves to the opposite pole. Thus, each pole receives
only one member of a homologous pair of chromosomes.
As a result, one of the two alleles goes to one pole, and
the other allele goes to the opposite pole Fig. 29. At
anaphase-II, the two sister chromatids of each
chromosome separate and move to the opposite poles,
producing four daughter cells, each having a single
chromatid from each homologous pair of chromosomes.
Two of these four cells receive the sister chromatids from
one of the two homologues, while the other two receive
the sister chromatid from the other homologue. As a
result, two of the four cells receive the dominant allele,
while in the remaining two cells the recessive allele of the
allelic pair is present (Fig. 29). Thus, the separation of
homologous chromosomes during meiosis may be
regarded as the reason for the segregation of the two
alleles of a gene, since the alleles are located in identical
positions in the homologous chromosomes.
As far as the time of segregation is concerned, two
situations may arise:
a. When there is no crossing over between a gene and
the centromere of the chromosome carrying this
gene (Fig. 29), the alleles of the gene segregate
during the first meiotic division, which is the
reductional division in such cases.
b.
But, when there is a crossing over between the gene
and centromere (Fig. 30), the two alleles of the gene
segregate during second meiotic division, which is
the reductional division in such cases.
D.
Verification of principle of segregation- Test
cross and back cross
The tall plants in Fig. 15 conceal a genotypic ratio of
1TT : 2Tt. To say the same thing in another way, among
the F2 plants that are tall (or, more generally, among
organisms that show the dominant morphological
phenotype), 1/3 are homozygous (in this example, TT)
and 2/3 are heterozygous (in this example, Tt). Unless
you know something about Genetics it would be a very
bold hypothesis, because it applies that two organisms
with the same morphological phenotype (in this case, tall
plants) might nevertheless differ in molecular phenotype
and in genotype.
Yet this is what Mendel exactly proposed. But how
could this hypothesis be tested experimentally? He
realized that it could be tested via self-fertilization of the
F2 plants. With self fertilization, plants grown from the
homozygous TT genotypes should yield tall and dwarf
plants in the ratio of 3:1. On the other hand, the plants
grown from seeds of dwarf plants should be truebreeding for dwarfness, because these plants are
homozygous tt.
An important feature of the homozygous tall and
dwarf plants produced in the F2 and F3 generations is that
the phenotypes are exactly the same as those observed
in the original parents in the P generation. This makes
sense in terms of DNA, because the DNA of each allele
remains unaltered unless a new mutation happens to
occur. Mendel described this result in a letter by saying
that in the progeny of crosses “the two parental traits
appear, separated and unchanged, and there is nothing
to indicate that one of them has either inherited or taken
over anything from the other”. From this finding, he
concluded that the heredity determinants for the traits in
the parental lines were transmitted as two diferrent
elements that retain their purity in the hybrids. In other
words, the heredity determinants do not “mix” or
“contaminate” each other. In modern terminology, this
means that, with rare but important exceptions, genes
are
transmitted
unchanged
from generation
to
generation.
Another straightforward way of testing the genetic
hypothesis in Fig. 27, is by means of a test cross, a
cross between an organism that is heterozygous for one
or more genes (for example, Tt) and an organism that is
homozygous for the recessive allele (for example tt). The
result of such a testcross is shown in Fig. 31. Because the
heterozygous parent is expected to produce T and t
gametes in equal numbers, and the homozygous
recessive produces only t gametes, the expected progeny
are ½ with the genotype Tt and ½ with the genotype tt.
The former have the dominant phenotype. Mendel carried
out a series of test crosses with various characters. The
results are shown in Table 4. In all cases, the ratio of
phenotypes among the test cross progeny is very close to
1:1, as expected from segregation of the alleles in the
heterozygous parent.
Table 4: Mendel’s test cross results.
Test Cross
Progeny from
Ratio
(F1 heterozygote
homozygous
recessive)
Tall x dwarf plants
x test cross
87 tall, 79 dwarf
1.10
1
Round x wrinkled seeds 193
round,
192 1.01
wrinkled
1
Yellow x green seeds
196 yellow, 189 green 1.04
1
Purple x white flowers
85 purple, 81 white
1.05
1
:
:
:
:
Another valuable type of cross is a back cross, in
which hybrid organisms are crossed with one of the
parental genotypes. Back crosses are commonly used by
genetists and by plant and animal breeders.