Mendel’s Laws of Inheritance
Gregor
Johann Mendel (1822 – 1884) is appropriately called father of genetics. With
the help of his experiments on garden pea (Pisum sativum) was able to formulate
laws, which explain the manner of inheritance of characters. The results of his
work were published in Germany in 1866 paper entitled “Experiments in Plant
Hybridization”.
Mendel’s
contemporaries did not understand his finding, probably partly because
i. Used mathematical principles of
probability for explanations he gave in his results. This was something new and
unacceptable to biologists of that time. They believed that biological
phenomena were too complex to be reduced to a mathematical treatment.
ii. Mendel studies the inheritance
of contrasting pairs of characters exhibiting discontinuous variation. Many of
his contemporaries i.e., Darwin, Galton etc, were pre occupied with characters
exhibiting continuous variation. They regarded discontinuous variation as being
unimportant in evolution.
iii. Mendel failed to demonstrate
the validity of his conclusions in other species. He was unlucky in selecting
Hieraceum (hawkweed) which showed apomixes (embryos arised directly from
diploid tissue in the ovary without fertilization) and honey bees (having
haploid males) as the experimental material.
His
work was not appreciated by the rest of the scientific community until 1900,
when three botanists, Carl Correns (Germany) working on Oenothera, Hugo de
Vries(Netherlands) working on Xenia, and Erich von Tschermak(Austria)
working on various flowering plants, rediscovered his work after each had
apparently independently reached similar conclusions. Mendel’s original paper
was republished in Flora, 89, 364 (1901).
Mendel
chose the garden pea as his experimental organism because
i. It is an annual plant with well-defined characteristics, and it can
be grown and crossed easily.
ii. Its short life cycle makes it possible to study several
generations within a short period.
iii. Moreover, garden peas, have perfect or bisexual flowers,
flowers that contain both female and male sex organs, and
iv. They are ordinarily self-fertilized, i.e., the ovule (female gamete)
is fertilized by pollen (male gamete) from the same plants.
v. Because of self-fertilization,
plants are homozygous. It is therefore, easy to get pure lines for several
generations.
vi. Pollen from another plant can
be experimentally introduced to the stigma of flower, but cross-pollination is
rare without human intervention.
vii.Mendel was fortunate in choosing a diploid plant because
diploid organisms contain only two sets of chromosomes. If he had chosen a
polyploidy organism, an organism with more than two sets of chromosomes, he
would not have obtained simple, understandable results.
In
seven pairs of contrasting traits or characters Mendel chose to study, two
parental forms exhibited well-defined, contrasting alternative or visible
morphologies on the plants. The seven characters that Mendel used were:
i. Stem tall (6-7 feet) or dwarf
(1/4 to 1 1/2 feet)
ii. Flowers may be axillary or
terminal
iii.Unripe pods may be green or yellow
iv. Ripe pods inflated or
constricted
v. Seeds round or wrinkled
vi Seed coat white or brown
vii.Cotyledons green or yellow.
Much
of Mendel’s success in his first experiments may be attributed to his good
judgment in making crosses, as far as possible, between parents that differed
in only one trait.
For
each of the seven pairs of characters, plants with one alternative trait was
treated as female, and those with other alternative as male. Reciprocal crosses
were also made i.e., each of these crosses was made in two ways.
Example:
Tall (♂)X Dwarf (♀)
Dwarf(♂)
X Tall(♀)
The
parental generation is called P (or P1 and P2, when the two parents need to
be distinguished). The progeny obtained as a result of crossing parents is
called as first filial or F1 generation. The progeny obtained by self-fertilization of F1 plants is called as
second filial or F2 generation. Similarly F3, F4 etc. generations can also
be obtained.
To
prevent self-fertilisation, the anthers were removed from the female plant.
This is known as emasculation. He collected pollen from male parent and dusted
on the feathery stigmas of emasculated female flowers. This is called crossing
or hybridization. These cross pollinated flowers were enclosed in separate bags
to avoid further deposition of pollen from other sources.
A
cross between two parents differing in a single pair of contrasting characters
is known as monohybrid cross.
A
cross between two parents differing in two pair of contrasting characters is
known as dihybrid cross.
Phenotype (Gr: form that is shown)
: A class of individuals recognised based on outward appearance of a trait in
an individual is the phenotype, e.g. Smooth-seeded shape or wrinkled shape of
seeds represent two different phenotypes.
Genotype : A class of individuals
recognised based on its genetic constitution and breeding behaviour is called
the genotype, e.g., the genotype of pure smooth seeded parent pea plant is SS
and it will always breed true for smooth-seeded character, but plants having Ss
on selfing would give rise to a population represented by 3 : 1 ratio for
smooth seeded plants and wrinkled seeded plants.
Trait : is the morphologically or
physiologically visible character, e.g. colour of flower, and shape of seed.
Factor : The unit of inheritance
and expression of a particular character is controlled by inheritable units
called factor (gene) which are present in pairs in parental cells and singly in
the gametes. Z
Gene : A segment of DNA
molecule which determines the unit of inheritance and expression of a
particular character.
Alleles
or Allelomorphs :
Two or more alternative forms of a gene are called alleles. For example in pea
plant, the gene for producing seed shape may occur in two alternative forms:
smooth (S) and wrinkled (s). Genes for smooth wrinkled seeds are alleles of
each other, and occupy same locus on homologous chromosomes.
Homozygous : An individual
possessing identical alleles for a trait is termed homozygous e.g. SS is
homozygous condition for smooth seeded character in garden-pea.
Heterozygous : An individual with dissimilar
alleles for a trait is termed heterozygous for e.g. Ss represents the
heterozygous condition for smooth seeded character in garden pea.
Dominant
trait :
Out of the two alleles or allelorrorphs of a trait, the one which expresses
itself in a heterogygous organism in the F1 hybrid is called the dominant trait
(dominant allele). Thus, if the allelic
combination in an organism is Tt, and T (tallness) expresses itself but t
(dwarfness) cannot, so T is the dominant allele, and tallness is dominant on
dwarfness represented by “t’.
Recessive
trait :
Out of the two alleles for a trait, the one which is suppressed (does not
express) in the F1 hybrid is called the recessive trait (recessive allele). But
the Recessive allele does express itself only in the homozygous state (e.g.
tt).
Parent
generations :
The parents used for the first cross represent the parent (or P1) generation.
F1
generation :
The progeny produced from a cross between two parents (P1) is called First
filial or F1 generation.
and
the one that remains masked in F1 individual but gets expressed in the next
generation (F2), is called recessive.
F2
generation :
The progeny resulting from self pollination or inbreeding of F1 individuals is
called Second Filial or F2 generation.
Monohybrid
cross :
The cross between two parents differing in a single pair of contrasting
characters is called monohybrid cross and the F1 offspring is Monohybrid. The
phenotypic ratio of 3 dominants : 1 recessive obtained in F2 generation from
the monohybrid crosses by Mendel was mentioned as 3:1 monohybrid ratio.
Dihybridcross: The cross in which two
parents differing in two pairs of contrasting characters are considered
simultaneously for the inheritance pattern is called dihybrid cross. The
phenotypic ratio obtained in the F2 generation from a dihybrid cross is called
Mendelian dihybrid ratio (9 : 3 : 3 : 1), and the F1-individual is called
dihybrid (SsTt).
Hybridisation: Crossing organisms belonging
to different species for getting desirable qualities in the offspring. z
Test cross : is the Crossing of the F1
progeny with the homozygous recessive parent. If F1 progeny is heterozygous,
then test cross always yields the ratio of 1 : 1 between its different
genotypes and phenotypes.
Reciprocal
cross :
Is the cross in which the sex of the parents is reversed. That is if in the first
cross male was dwarf and female tall, then in the reciprocal cross, dwarf
parent will be female and tall parent male.
Principle
of Segregation:
In one
experiment, Mendel crossed a tall plant with a dwarf plant of garden
peas. All the offspring in the first (F1) generation well tall. The
dwarf trait had disappeared in the F1 progeny.
When F1 plants
were self-fertilised and progeny or second filial (F2) generation
were classified, three-fourths were tall and one-fourth were dwarfs. To be
exact, an F2 of 1064 plants consisted of 787 tall pl ants and
277 dwarf plants, a nearly perfect 3:1 ratio. This ratio is known as monohybrid
phenotypic ratio.
On basis
of his hypothesis, he predicted that about one – third of F2 tall
plants would produce only tall F3 progeny, whereas, two thirds
would produce both tall and dwarf progeny. The F2 short plants
were expected to produce all short F3 progeny.
Mendel
self-fertilised F2 plants, and predicted results were obtained.
Indeed, the tall F2 plants were of two types; about one-third
of the tall plants produce only tall progeny and about two-thirds of them
produce both tall and short progeny. In contrast, all the short F2 plants
produced only short progeny.
It means, F2 generation
consisted of three types of plants (instead of apparent two types):
i. Tall homozygous (pure) – 25% (TT)
ii. Tall heterozygous (hybrid) – 50% (Tt)
iii. Dwarf homozygous (pure) – 25% (tt)
From his
observation, Mendel concluded that F2 ratio is more accurately
considered as 1:2:1. This ratio is known as monohybrid genotypic ratio.
An important
feature of Mendel’s results was that the F1progeny plants all
exhibited the trait expressed in one of the parent plants. The trait expressed
in the other plant did not appear among F1 progeny. The
factor or particles specifying one of the traits has an over ridding or
dominant controlling effect over the factor specifying the other trait. This
trait that is expressed in F1 is termed as dominant trait or
character.
The other trait
that disappeared or was masked in the F1 but reappeared in
one-fourth of the F2 progeny is termed as recessive trait or
character.
Similarly, the
factor (now called a gene) that specifies the trait expressed in the F1 generation
is called the dominant factor, and the factor that controls the trait that is
masked in the F1 generation is termed as recessive factor. In
other words, a trait or character which appear only in homozygous individual is
called a recessive character (e.g., dwarfness). A character which
phenotypically expresses itself in the homozygous as well as heterozygous
individual is called dominant character (e.g. tallness).
Mendel
concluded that, particles or factors were transmitted intact through the
gametes from parents to progeny. He suggested that these factors existed in
several alternative forms (now called alleles), determining the different
phenotypes observed.
During sexual
reproduction, the members of each pair of alleles separated into different
reproductive cells or gametes of the male and female parents,that fuse and give
rise to progeny. Mendel called this separating or segregation process as
“splitting of hybrids”. Thus, paired factors or genes (allelic pairs) separate
from one another and are distributed to different sex cells or gametes. This is
called as law of segregation.
According to
this law, in a heterozygote a dominant and a recessive allele remain together
throughout the life (from zygote to the gametogenesis stage). Without
contaminating or mixing with each other they finally separate or segregate from
each other during gametogenesis. So that each gamete receives only one allele.
As the gametes are pure for a given character, this law is also known as Law of
purity of gametes.
Mendel
concluded that, particles or factors were transmitted intact through the
gametes from parents to progeny
During sexual
reproduction, the members of each pair of alleles separated into different
reproductive cells or gametes of the male and female parents,that fuse and give
rise to progeny. Mendel called this separating or segregation process as
“splitting of hybrids”. Thus, paired factors or genes (allelic pairs) separate
from one another and are distributed to different sex cells or gametes. This is
called as law of segregation.
Principle of Independent Assortment :
Mendel
crossed plants that differed in two pairs of alleles. In this cross, designed
to clarify the relation of different pairs of alleles.
Mendel
crossed a homozygous pea plant having round and yellow seeds (YYRR) with the
homozygous pea plant having wrinkled and green seeds (yyrr). The F1 plants were round and yellow
seeded (YyRr) just like the homozygous
parent, displaying complete dominance
This
type of cross in which the parents have different parental contributions for
two traits is called a dihybrid cross. This shows that round shape and yellow
color were dominant and wrinkled and green condition recessive.
When
these F1 plants
were self fertilized, they produced four types of gametes with two parental and
two new combinations i.e., YR, Yr, yR and yr are formed in approximately equal
number. Thus, recombination of genes takes place at the time of gamete
formation in F1 plants.
From
a total of 556 seeds, the following distribution was observed: 315 – round and
yellow, 108 – round and green, 101 – wrinkled and yellow, and 32 – wrinkled and
green. These results closely fit a ration of 9:3:3:1.
At
the top of the checker board or Punnett square the four kinds of gamestes from
the female plant are shown. The four gametes from the pollen parent or male parent
are represented in a vertical row at the left.
Thus,
any allele of one gene is equally likely to combine with any allele of the
other gene and pass into the same gamete. A random union among these gametes
gives rise to 16 possible zygotes. These zygotes yield a 9:3:3:1 phenotypic
ratio, which is known as the typical dihybrid ratio.
Two
of these traits were similar to parental combination (round and yellow seeds,
wrinkled and green seeds), while the other two were new combinations (round and
green seeds and wrinkle and yellow seeds).
The
nine different types of genotypes expected in F2 generation occur
in the ratio of 1:2:2:4:1:2:1:2:1. This is known as Dihybrid genotypic ratio.
The
result showed the assortment of two independent pairs of alleles, each showing
dominance of one member. Not only did the members of each pairs of alleles
segregate, but the allelic pairs of different genes behaved independently with
respect to each other i.e., members of different pairs of alleles assort
independently into gametes.
According
to law of independent assortment “ the factors or genes for different pairs of
contrasting characters present in a parent assort (separate) independently from
one another during gamete formation”.
Back
Cross and Test Cross
Mendel used two
types of tests to distinguish homozygous one from heterozygous having the same
phenotype ( e.g. TT and Tt for tall phenotype). If a homozygous tall plant (TT)
is selfed, it will breed true, producing only tall plantsl. But when
heterozygous tall plant (Tt) are selfed, tall and dwarf appear in 3:1 ratio.
Back
Cross:
When F1 individuals
are crossed with one of the two parents from which they were derived, then such
a cross is called back cross.
When F1 is
back crossed to the parent with dominant characters, no recessive individuals
are obtained in the progeny.
Test
Cross:
When F1 individuals
are crossed with recessive parent, both phenotypes appear in the progeny. This
is known as test cross, because it is used to test whether an individual is
homozygous (pure) or heterozygous (hybrid). For a monohybrid the test cross
ratio is 1:1, but, for a dihybrid the test cross ratio is 1:1:1:1.
Dihybrid
Test Cross
Yellow Yellow Green Green
Round Wrinkle Round Wrinkle
Mendelian genetics is based on the transmission of
chemical units or genes from parents to progeny and thus from generation to
generation. The mechanism of transmission includes 1. Segregation, the
sepeartion of pairs of alleles into different gametes, and 2. Independent
assortment, the independent segregation of members of different pairs of
alleles, as demonstrated in dihybrid crosses.
Hereditary mechanism operate in all plants and
animals. Probability is involved in genetic mechanisms and must be recognised
in predicting the transmission and expression of both dominant and recessive
alleless.
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