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1. PREPARED BY:
SHIVEN R. TRAMBADIA
17BMB053
TY MICROBIOLOGY DEPT.
SHREE M & N VIRANI SCIENCE
COLLEGE1/13/2020 1-TS

2. 1. PRE-MENDELIAN ERA
• Blending Theory of Inheritance
2. MENDEL’S EXPERIMENTS
• Introduction
• Reason for choosing Garden Pea
• Seven visible characters
• Mendel’s experimental methodology
• Monohybrid Cross
• Principle of Inheritance
• Law of Segregation
• Dihybrid cross
3. SUMMARY OF MENDEL’S LAWS
1/13/2020 2-TS
CONTENTS

3. PRE-MENDELIAN ERA
• Various theories about heredity had
been proposed before Mendel
began his experiments, which
included ‘Blending Theory of
Inheritance’ was believed by most
at that time.
• Blending Theory of Inheritance:
o Both sexes contribute equally to a
new individual.
o Parents of contrasting appearance
always produce offspring of
intermediate appearance, which was
not the case always.
o Blending theories ignore
characteristics skipping a generation.1/13/2020 -TS 3
•Blending of two colours of
same species of flower
giving intermediate
appearance

4. PRE-MENDELIAN ERA
•Blending theory of inheritance also helped Charles Darwin
in his hypothesis of ‘pangenesis’, which suggested that every
part of the body produce a representative ‘gemmule’, which by
the blood circulation get collected in the semen.
•Weismann, discounted the concept of gemmules and
proposed that living organisms consist of two kinds of
materials – somatoplasm & germplasm.
•But all of them were not so successful in explaining
inheritance, which was possible when Gregor Johann
Mendel (1822-1884), an Austrian monk presented his results
in his paper entitled “Experiments on Plant Hybrids” given to
the Natural Science Society at Brno.
1/13/2020 -TS 4

5. MENDEL’S EXPERIMENTS
•In 1856. Mendel started his experiments
on controlled hybridisation in ‘Garden Pea’,
in the monastery garden at Brno,
Czechoslovakia.
• Published his results in 1866, but was
poorly recognised and ignored.
•His experimental organism was common
garden pea ‘Pisum sativum’.
1/13/2020 -TS 5

6. MENDEL’S EXPERIMENTS
Why Garden Pea?
Small and easy to grow
Large number of offsprings
Completes 1 life cycle in a season
Large number of true breeding varieties available
Naturally self-pollinating, but can easily be cross-
pollinated
Various number of contrasting characters; like red
vs. white flowers, round vs. wrinkled seeds, etc.
1/13/2020 -TS 6

7. MENDEL’S EXPERIMENTS
1/13/2020 -TS 7
Mendel selected seven visible characters, each with two
contrasting traits, which are as below:

8. MENDEL’S EXPERIMENTS
Mendel’s experimental methodology:
i. Obtaining seeds, which were true to their types, i.e.
Plants with red flowers always produced generations of
red flower plants.
ii. Counting all seeds and grouping them on the basis of
characters that they represent. Thus his method was
quantitative and he kept record of characters of plants of
several generations.
iii. Drawing generalisations on the basis of numerical
relationships between the hybrids having contrasting
characters.
1/13/2020 -TS 8

9. MENDEL’S EXPERIMENTS
MONOHYBRID CROSS:
 In initial experiments, the offsprings are
hybrid – which are products of different
strains that differ in regard to only one
trait, i.e. Monohybrid Cross.
 Mendel called the original parents the P
generation and the first generation
offspring the F1 (for filial) generation.
 He performed Reciprocal crosses: first,
he dusted pollen of tall plants on the
stigmas of dwarf plants, and then, vice
versa.
 In both cases, all F1 offsprings
resembled the tall plant.
1/13/2020 -TS 9

10. MENDEL’S EXPERIMENTS
 These results were contrary to those
predicted by blending theory of inheritance.
 Rather than being intermediate in height, the
offsprings were tall and similar to one
parent.
 When F1 generations were allowed to self-
pollinate, ¾ of F2 generation were tall and
¼ were dwarf, a 3:1 ratio.
 Observed Ratio: 3:1 of Dominant to
Recessive.
 Hidden Ratio: 1:2:1 of ¼ Pure Dominant,
½ Impure Dominant, ¼ Pure Recessive.
 This helped Mendel propose his postulates,
i.e. Principles of Inheritance.
1/13/2020 -TS 10

11. MENDEL’S EXPERIMENTS
Principles of Inheritance:
 Law of Dominance:
i. Characters are controlled by discrete units called
factors.
ii. Factors occur in pairs.
iii. In a dissimilar pair of factors one member of the
pair dominates (dominant) the other (recessive).
1/13/2020 -TS 11

12. MENDEL’S EXPERIMENTS
Law of Segregation / Law of Purity of Gametes:
 Hybrids or heterozygotes of F1 generation have two
contrasting characters or allelomorphs of dominant
and recessive nature.
 These allels though remain together for long time
but do not contaminate or mix with each other and
separate or segregate at the time of gametogenesis,
so that each gamete receives only one allele of a
character either dominant or recessive.
1/13/2020 -TS 12

13. MENDEL’S EXPERIMENTS
DIHYBRID CROSS:
 After studying the single pair of contrasting
characters, Mendel worked on Pea plant with two or
more pairs of contrasting characters.
 A cross between two sets of plants having two pairs
of true breeding characters is referred to as a
Dihybrid Cross.
 For this he selected a set of Pea Plants, which were
true breeding for Yellow & Round seeds, and
another with Green & wrinkled seeds.
1/13/2020 -TS 13

14. MENDEL’S EXPERIMENTS
1/13/2020 -TS 14

15. MENDEL’S EXPERIMENTS
• In F1 generation of the cross, he observed that plants
produced only Round & Yellow seeds, which
suggests that Round & Yellow are dominant over
Wrinkled & Green, respectively.
• When F1 generation were selfed, Mendel postulated
that segregation of one pair of unit factors (genes)
would occur independently of the other pair, i.e.
They assort independently.
• Here, four different combinations are possible:
i) Round-Yellow, ii) Round-Green, iii) Wrinkled-
Yellow, iv) Wrinkled-Green.1/13/2020 -TS 15

16. MENDEL’S EXPERIMENTS
During fertilization, each
Zygote will have equal
probability of receiving
two of these combinations,
one from each parent,
resulting in progeny.
1/13/2020 -TS 16
Ratio
Obtained
Combination
9/16 Round-Yellow
3/16 Wrinkled-Yellow
3/16 Round-Green
1/16 Wrinkled-Green

17. MENDEL’S EXPERIMENTS
• Mendel suggested that when two pairs of contrasting
characters come from the same parent they may not
necessarily stay together but behave as if each
character is an independent unit.
• This laid the foundation of fourth postulate, also
referred as “Principle of Independent Assortment
of Factors.”
• ‘It states that when two individuals differ from each
other in two or more pairs of factors, the inheritance
of one pair is quite independent to that of others.’
1/13/2020 -TS 17

18. MENDEL’S EXPERIMENTS
• In other words, it can also be stated, as when two
independent event occurs simultaneously, the
combined probability of two outcomes is equal to the
product of their individual probabilities of
occurrence.
1/13/2020 -TS 18
For Example:

19. 1/13/2020 -TS 19

20. REFERENCES
1. Cell biology, Genetics, Molecular Biology,
Evolution, and Ecology, by- Dr. P. S. Verma & Dr.
V. K. Agrawal. (S. Chand Publication).
2. Complete Biology for NEET, by- S.S. Bhatia, Mc
Graw Hill Publications.
3. Molecular biology of the Gene, by- James D.
Watson, Tania A. Baker, Stephen P. Bell, Alexander
Gann, Michael Levine and Richard Losick.
4. NCERT XI-XII, Text Book.
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