Grade 12 notes (_Mendel_law_of_Inheritance.pdf)

Mendel law of inheritance
Mendel as a founder of genetics
 Mendel experiments with Pisium sativum open a new chapter in biology. He
published his conclusion “Inheritances of characters” in 1866 in the “Annual
proceeding of the Natural History Society” of Brunn.
 But his work was recognized only in 1900 when three scientists named Hugo de
vries of Holland, Erich von Tschermark of Austria and Karl correns of Germany
independently drew a conclusion of Mendel.
 Since, Mendel's laws were widely accepted throughout the world. This led to the
establishment of genetics as a science. So, he is considered as the pioneer in the
field of genetic and called the father of genetics.

Mendel's, Experiment
1. Selection of materials
Mendel selected the pea plant for his experiment because of following reasons
 It has varieties of well-defined characters among them he selected 7 different
characters „unit characters‟ to follow inheritance.
 It has bisexual flowers in which the cross-pollination as well as self-pollination
can be done easily.
 It can be easily cultivated that requires small space to grow each plant.
 It has very short life span and thus many generations can be formed in only a
single growing season.
 The hybrids of pea plant are perfectly fertile.
 It can be easily grow in different geo-climatic condition.
 A large number of progeny (offspring) can be obtained from a single plant.

Mendel studied seven characteristics of pea plant, each of which occurred in two
contrasting traits such as
 Seed texture or shape - Round and Wrinkled
 Seed cotyledon colour -Yellow and green
 Colour of flower - Purple and white
 Pod shape - inflated and constricted
 Pod colour -Green and Yellow
 Position of flower - Axial and terminal
 Height of plants - Tall and dwarf

Selection of pure breeding plant
 Mendel selected pure breeding varieties for his experiments. He conform the
plant as pure breeding by selfing the plant for several generations.
 For example - Seeds collected from red flowered plant always produced having
red flowers.
Hybridization
 Hybridization is the process of obtaining hybrids by crossing two genetically
different parents.
 Mendel studied the inheritance of -
 Single pair of contrasting traits (Monohybrid cross) and
 Two pairs of contrasting traits (Dihybrid cross).

1. Monohybrid cross
 Monohybrid cross is a cross between two individual that differ only in one
characters while other remain same. For this cross, Mendel used only one of the
seven pairs of contrasting characters at a time.
 For example - Mendel took pure breeding tall plant (TT) and pure breeding dwarf
plant (tt).
 He named them parental generation (P generation).
 He removes the anther of the tall variety before bursting a pollen sac and
pollinated them with pollen from dwarf plant.
 He collected the seed produced by this cross and raised them into plants. He
named them first filial generation (F1 generation).
 All the plant of F1 generation will tall, he also performed reciprocal crosses.

Phenotypic ratio = 3 tall: 1 dwarf
Genotypic ratio = 1 tall: 2 hybrid tall: 1 dwarf
Fig: Result of monohybrid cross

2. Dihybrid cross
 A cross made between two parents which differs only in two contrasting,
characters is called dihybrid cross.
 The cross is made to study the inheritance of two characters at a time.
 For dihybrid cross, Mendel took pure breeding tall plant with red flower (TTRR)
and pure breeding dwarf plant with white flower (ttrr). In F1 generation, all plants
were tall and red.

Phenotypic ratio = 9 tall and red: 3 tall and white: 3 dwarf and tall: 1 dwarf and white
Genotypic ratio = 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 ( Which is the multiplication of two
monohybrid genotypic ratio i.e. (1:2:1) (1:2:1).
Fig: Result of dihybrid cross

Result of Mendel’s experiment
1. In all the seven traits, he found that the F1 generation resembled only one of the
parents. He obtained same result in reciprocal cross. The trait expressed in F1
generation was called dominant and the one that remained hidden was known as
recessive.
2. In F2 generation both the parental forms reappeared.
3. The ratio of the dominant form to recessive was always 3:1 i.e. the dominant form
was three times the number the recessive form.
4. In F3 and F4 generation, out of three tall plants produce tall plants, Similarly two
out of three tall plant produced both talls and dwarf in 3:1. One dwarf plant always
produces dwarf plant in subsequent generation. Mendel concluded that the
observed ratio (Phenotype) was 3:1 (tall: dwarf) and genetic ratio (genotype) 1
pure tall: 2 hybrid tall; 1 pure dwarf.

5. In dihybrid cross, he found two recombinant forms in F2 generation which were
not present in parents. In dihybrid experiment, tall plants with white flowers and
dwarf plant with red flowers were recombinant forms.
Mendel's principles (laws) of inheritance
On the basis of the result obtained Mendel‟s experiment, he proposed the following
three laws
1. Law of Dominance
Law of dominance states that “ When a cross is made between a pair of pure
contrasting characters, only one of them is able to express itself phenotypically
while the other remain hidden in F1 generation”. The character which is expressed
is dominant and the other is recessive.

For example - When tall plant (TT) are crossed with dwarf plants (tt), only tall plant
(Tt) appear in F1 generation, although they have factors for both tallness and a
dwarfness. Thus the tallness as it express called dominant and dwarfness remain
unexpressed called recessive.
Law of segregation (Purity of gametes)
This law states that the factors for each character segregate at the time of gametes
formation. The characters never mix or contaminate with each other. They remain
together for a long time and segregate at the time of gametogenesis. As a result,
each gamete receives only one factor from a pair of each factor. Therefore, the
gametes are always pure for a particular character and law of segregation is also
called the law of purity of gametes.

For example – Tallness (T) and dwarfness (t) are two contrasting factors present in
hybrid (Tt). It develops tall character but possesses factor for having both tall and
dwarf, while in further generation factor T and t separate i.e. 50% of the gametes
contain T and 50% contain t.
3. Law of independent Assortment:
Mendel performed dihybrid cross and formulated law of independent assortment. It
states that “when two pairs of contrasting alleles present in parents, inherit to
offspring, one pairs of alleles is independent to another pair, they do not remain linked
and assort randomly”.

For eg - When a pea plant of two different characters tall plant with red flows (TTRR)
is crossed with a dwarf plant with white flower (ttrr). The height characters segregates
and is inherited independent of the flower colour. When two alleles of a pair (T, t and
R, r) segregate, both pairs assort randomly. It means in a gamete T may assort with
R or r, similarly t may assort with R or r. So four types of gametes (TR,Tr, tR and tr)
are formed in F1 generation that on selfing result all combination of the characteristics
in F2 generation. The ratio of 9 tall red: 3 tall white: 3 dwarf red: 1 dwarf white.

Reason for Mendel’s success.
1. Pre- mendelian experiments on heredity failed because they concentrated on
inheritance of many characters at a time. Mendel focused on inheritance of a
single character.
2. He used pure breeding plants which have two contrasting characters. There was
no intermediate.
3. Mendel kept accurate records in crosses and even maintained exact number of
seeds as plants produced.
4. He used self-pollinated plant in which undesirable cross pollination was totally
prevented.
5. He made reciprocal and test cross to conform his result.

1. Importance of Mendelism.
2. Dominant and recessive characters can be found out.
3. Crosses can be made to produce a hybrid with desired characters.
4. Agriculturally useful plants can be improved.
5. Pure but recessive characters can be used whatever required.
6. The genotype and phenotypes likely to be produced in the next generation can be
predicted even before the cross can be made.

Grade 12 notes (_Mendel_law_of_Inheritance.pdf)

Grade 12 notes (_Mendel_law_of_Inheritance.pdf)

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Grade 12 notes (_Mendel_law_of_Inheritance.pdf)