Mendelian Inheritance
Gregor Johann Mendel lived between 1822 and 1884. He was the scientist that gave the first
significant insight into the process involved in the inheritance of biological traits. Mendel carried
out series of experiments that laid foundation for the formal discipline of genetics, which is the
study of heredity and variation. The term heredity refers to the process by which attributes that are
seen in the parents are transferred to the offspring. Variation refers to various changes that are
observed in the expression of attributes in an organism. Variations are of two types: discrete or
discontinuous variation and continuous variation. The attributes that are expressed or found in the
body of an organism is called character or trait. These attributes show discontinuous / discrete
variation or continuous variation and are therefore referred to as discontinuous / discrete characters
/ traits and continuous characters /traits, respectively.
Discrete or discontinuous variation: This is a type of variation that is of kind, distinct and non-
overlapping. Examples include colour of the eyes (brown, blue, green), shape of the nose (pointed,
flat), shape of the fruits, ear attachment (Attached or detached), Blood group (A, B, AB and O),
ability to roll tongue (roller and non-roller), ability to taste some chemicals (Urea,
Phenylthiocarbamide PTC; Taster and non-taster) etc.
Continuous variation: This is a variation that is of degrees i.e. ranges from one extreme to another.
It is non-distinct with several intermediate or overlapping forms. Examples include height of man
or plants, weight of fruits, weight of animals, Intelligent quotient (I.Q.), length of animal tails, size
of leaves etc.
Mendelian experiments
Mendel carried out his experiments using garden pea (Pisum sativum) as his experimental
organism. His choice of garden pea was based on at least three reasons.
i. The garden pea plants exhibit characters or traits that are discrete and non-overlapping
ii. The plants have relatively short life cycle i.e. about 3 to 4 months
iii. The plant is a self-pollinating (inbreeder) and the design of the flowers allow artificial
pollination.
Mendel first observed and identified seven pairs of contrasting characters of the garden pea plants
and studied them one after the other. They are:
i. Seed shape – round seed versus wrinkled seed
ii. Seed colour – yellow versus green
iii. Pod shape – full or inflated versus constricted
iv. Pod colour – green versus yellow
v. Flower colour – violet or pink versus white
vi. Flower position – axial versus terminal
1
, vii. Stem height – tall versus dwarf
He first obtained seeds of the plant that is breeding true (Pure line) for each character. A plant is
said to be breeding true for a character if the character of the plant does not change from generation
to generation. He was able to achieve this by isolating and planting the seed of a plant for several
generations. The seeds of such plants whose character expression does not change from generation
to generation were isolated for each of the pair of characters and preserved.
Monohybrid Cross
Mendel planted the seeds for each pair of contrasting characters separately (i.e., the seeds of the
true breeding tall plant and true breeding dwarf plant). At flowering, he collected the pollen grains
from flower of tall plant to pollinate the stigma of the flower of the short plant whose male parts
have been removed a day to opening of flower, this is what is called crossing. He did this in reverse
order (i.e., forward and backward direction), in what is termed reciprocal crossing. A monohybrid
cross therefore is a cross between two plants with contrasting character. He bagged the crossed
flowers immediately to prevent contamination from unknown pollen grains and allow the fruit to
develop. This is the hybrid fruit because it is the product of a cross between plants with contrasting
character and the seeds within this fruit are the hybrid seeds (First filial i.e., F1 generation seeds).
The plants that develop when the F1 seeds are planted are the F1 plants.
In a cross between a true breeding tall plant and true breeding dwarf plant, a tall plant is produced
in the first filial generation. When this tall F1 plant is allowed to self-fertilize itself, the population
of plants produced at F2 (second filial generation) is observed to comprise both tall and dwarf
plants in which there are more tall plants than the dwarf plants. Irrespective of which of the plant
is used as male or female, the result is the same (i.e., the choice of parent does not influence the
inheritance of the trait). This experiment was performed for all the seven pairs of contrasting
characters in pea plants and the results are found to be the same.
The following observations were made by Mendel:
i. that in a cross between a pair of contrasting characters, only one of the characters is expressed
in the offspring (hybrid).
ii. that there is something that represents each character in the plant and he referred to it as factor.
iii. that the character that is expressed at F1 is the dominant while the one that is not expressed is
the recessive.
iv. that the character that is not expressed at F1 is only hidden but not lost nor destroyed.
v. that both characters are expressed at F2 with frequency of tall character three times the dwarf
character.
vi. that this F2 observation is only possible if each character is represented by a pair of factors
These observations lead to the formulation of first Mendelian law which states that, ‘Of a pair of
factors affecting a trait/ character, only one is represented in the gamete’. This is the law of
segregation.
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Gregor Johann Mendel lived between 1822 and 1884. He was the scientist that gave the first
significant insight into the process involved in the inheritance of biological traits. Mendel carried
out series of experiments that laid foundation for the formal discipline of genetics, which is the
study of heredity and variation. The term heredity refers to the process by which attributes that are
seen in the parents are transferred to the offspring. Variation refers to various changes that are
observed in the expression of attributes in an organism. Variations are of two types: discrete or
discontinuous variation and continuous variation. The attributes that are expressed or found in the
body of an organism is called character or trait. These attributes show discontinuous / discrete
variation or continuous variation and are therefore referred to as discontinuous / discrete characters
/ traits and continuous characters /traits, respectively.
Discrete or discontinuous variation: This is a type of variation that is of kind, distinct and non-
overlapping. Examples include colour of the eyes (brown, blue, green), shape of the nose (pointed,
flat), shape of the fruits, ear attachment (Attached or detached), Blood group (A, B, AB and O),
ability to roll tongue (roller and non-roller), ability to taste some chemicals (Urea,
Phenylthiocarbamide PTC; Taster and non-taster) etc.
Continuous variation: This is a variation that is of degrees i.e. ranges from one extreme to another.
It is non-distinct with several intermediate or overlapping forms. Examples include height of man
or plants, weight of fruits, weight of animals, Intelligent quotient (I.Q.), length of animal tails, size
of leaves etc.
Mendelian experiments
Mendel carried out his experiments using garden pea (Pisum sativum) as his experimental
organism. His choice of garden pea was based on at least three reasons.
i. The garden pea plants exhibit characters or traits that are discrete and non-overlapping
ii. The plants have relatively short life cycle i.e. about 3 to 4 months
iii. The plant is a self-pollinating (inbreeder) and the design of the flowers allow artificial
pollination.
Mendel first observed and identified seven pairs of contrasting characters of the garden pea plants
and studied them one after the other. They are:
i. Seed shape – round seed versus wrinkled seed
ii. Seed colour – yellow versus green
iii. Pod shape – full or inflated versus constricted
iv. Pod colour – green versus yellow
v. Flower colour – violet or pink versus white
vi. Flower position – axial versus terminal
1
, vii. Stem height – tall versus dwarf
He first obtained seeds of the plant that is breeding true (Pure line) for each character. A plant is
said to be breeding true for a character if the character of the plant does not change from generation
to generation. He was able to achieve this by isolating and planting the seed of a plant for several
generations. The seeds of such plants whose character expression does not change from generation
to generation were isolated for each of the pair of characters and preserved.
Monohybrid Cross
Mendel planted the seeds for each pair of contrasting characters separately (i.e., the seeds of the
true breeding tall plant and true breeding dwarf plant). At flowering, he collected the pollen grains
from flower of tall plant to pollinate the stigma of the flower of the short plant whose male parts
have been removed a day to opening of flower, this is what is called crossing. He did this in reverse
order (i.e., forward and backward direction), in what is termed reciprocal crossing. A monohybrid
cross therefore is a cross between two plants with contrasting character. He bagged the crossed
flowers immediately to prevent contamination from unknown pollen grains and allow the fruit to
develop. This is the hybrid fruit because it is the product of a cross between plants with contrasting
character and the seeds within this fruit are the hybrid seeds (First filial i.e., F1 generation seeds).
The plants that develop when the F1 seeds are planted are the F1 plants.
In a cross between a true breeding tall plant and true breeding dwarf plant, a tall plant is produced
in the first filial generation. When this tall F1 plant is allowed to self-fertilize itself, the population
of plants produced at F2 (second filial generation) is observed to comprise both tall and dwarf
plants in which there are more tall plants than the dwarf plants. Irrespective of which of the plant
is used as male or female, the result is the same (i.e., the choice of parent does not influence the
inheritance of the trait). This experiment was performed for all the seven pairs of contrasting
characters in pea plants and the results are found to be the same.
The following observations were made by Mendel:
i. that in a cross between a pair of contrasting characters, only one of the characters is expressed
in the offspring (hybrid).
ii. that there is something that represents each character in the plant and he referred to it as factor.
iii. that the character that is expressed at F1 is the dominant while the one that is not expressed is
the recessive.
iv. that the character that is not expressed at F1 is only hidden but not lost nor destroyed.
v. that both characters are expressed at F2 with frequency of tall character three times the dwarf
character.
vi. that this F2 observation is only possible if each character is represented by a pair of factors
These observations lead to the formulation of first Mendelian law which states that, ‘Of a pair of
factors affecting a trait/ character, only one is represented in the gamete’. This is the law of
segregation.
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