This document provides an overview of genetics and summarizes Gregor Mendel's experiments with pea plants. It discusses key genetics concepts like heredity, variation, factors/genes, phenotypes, genotypes, dominance, and segregation. It describes Mendel's monohybrid and dihybrid crosses and how they led to his laws of inheritance - the law of dominance and the law of independent assortment. Mendel's work established genetics as a field and provided the foundation for modern understanding of heredity.

1. Genetic Basis of Inheritance
Deparment Of Botany
Preapared by
Dr. P. B.Cholke
(Assistant Professor in Botany)
Pune District Education Association’s
Anantrao Pawar College ,Pirangut,
Tal-Mulshi, Dist-Pune- 412115

2. What is genetics?

3. What is genetics?
• Genetics (Greek word ‘genesis’ = to grow into)
• i. The branch of biology which deals with the study of heredity
and variation is known as genetics.
• ii. The term genetics were coined by Bateson (1906).
• 2. Heredity :
• The transmission of character from one generation to the others
is called as heredity.
• e.g.
• i. Height of plant
• ii. Colour of flower
• iii. Colour of eyes
• iv. Shape of nose
• 3. Variations :
• The differences observed between parents and offsprings as well
as among offsprings of the same parents are called variations.
•

4. • Like begets Like”.
• “The tendency of living organisms to produce
the individuals of their own kind is called like
begets like.”
• e.g.
• i. Human produces human baby.
• ii. Mango seed produces mango plants.
• iii. Lions give birth to lions.

5. The father of genetics
Gregor Johann Mendel

6. • Mendel was born in July 22, 1822 in Heinzendorf, a small village in
Austria.
• ii. He completed his graduation in 1840 and Joined a monastery
in Brunn.
• iii. In 1847 he become a priest and worked as a teacher at higher
secondary school of Brunn during 1854 to 1868.
• iv. Mendel carried out his experiment on garden pea (Pisum
sativum) plant in monastery garden from 1857 to 1865.
• v. He published his research papers entitled “Experiment in
plant hybridization” in the annual proceedings of the “Natural
history society of Brunn” in 1866.
• vi. Unfortunately his outstanding work remains unnoticed for 35
years.
• vii. Mendel died on 6th January 1884.
• viii. In 1950, three scientist Hugo De vries of Holland, Karl correns
of Germany and Erich.V. Tschermak of Austria discover the original
work of Mendel and arrived at similar conclusions as those of
mendel.
• ix. Mendel’s laws form the basis of modern genetics so mendel is
called “The father of genetics”.

8. Mendels Experiments
• Selection of Material
• Mendel selected garden pea
• ( Pisum sativum L) because
• i. Garden pea plant shows many contrasting
characters.
• ii. Plant is annual herb i.e. having short life span.
• iii. Many plants can be easily cultivated in small plot.
• iv. Flowers are naturally self pollinated.
• v. Garden pea plants produce fertile offsprings.
• vi. No intermediate characters are obtained.

9. Selection of characters

10. 10

11. 11

12. • iii) Procedure of Experiments :
• a) Mendel was very methodical in carrying out
his experiments.
• b) First, he studied only one trait at a time,
unlike others who had considered the organism as a
whole.
• c) Then, he studied two traits and three traits
at a time by performing monohybrid, dihybrid and
trihybrid crosses.
• d) He started his experiments with true
breeding (pure line) plants and maintained a complete
record of the actual number of each type of offsprings.
• e) He conducted several crosses to eliminate
chance factor.

13. • Mendel conducted experiments in the following
three steps :
• Step 1 - Selection of parents and obtaining
pure lines.
• Mendel started with pure lines that were
available. He also ensured that the selected male
and female parent plants are breeding true for
the selected trait/traits by selfing them for three
generations. (Breeding true or 'true breeding'
means they produce offsprings with the same
selected trait/traits only).

15. • Step 2 - Artificial cross of the selected parents to
raise F1 generation.
• Mendel first emasculated the flowers of the
plant which he had selected as a female parent.
Then, pollens from the flower of selected male
parent were dusted on the stigma of the
emasculated flower, i.e. artificial- cross. Mendel
crossed many flowers, collected seeds and raised
the hybrids that represent first filial generation or
F1 generation.
•

16. • Step 3 - Selfing of F1 hybrids to raise F2
generation.
• Mendel allowed the natural self-
pollination in each F1 hybrid; collected seeds
separately and raised F2 generation, i.e.
second filial generation. (F2 generation was
obtained by selfing of F1 hybrids.)

17. Classical Method of symbolism
Parental generation P or P1
First filial generation F1
Second filial generation F2
Third filial generation F3
Dominant Capital ‘T’ for Tall
,
Capital ‘Y’ for Yellow.
Recessive small ‘t’ for dwarf
(Not D or d), small ‘y’ for green (not G
or g) for green
(not G or g.)
Female
♀
Male
♂

18. Factors :
• Unit in the organism which is responsible for
the inheritance and expression of a particular
character is called as factor.
• The term factor was coined by Mendel

19. • ii. Gene :
• Mendel’s factor has came to be known as
gene.
• Gene is a particular segment of DNA
which is responsible for the inheritance and
expression of particular character.
• The term gene was coined by the
Johansson (1909)

20. • iii. Allel or Allelomorph :
• Two or more alternative forms of a gene
controlling the same character and located at
the same loci in the homologous
chromosomes are known as alleles.
• Term coined by Bateson.

21. • iv. Homozygous :
• When a pair of alleles present on
homologous chromosomes are similar, it is
known as homozygous condition.
• e.g.
• ‘TT’ or ‘tt’

22. • v. Heterozygous :
• When the pair of alleles present on
homologous chromosomes are not similar it is
known as heterozygous condition.
• e.g.
• Tt is heterozygous for Tallness.

23. • vi. Phenotype :
• The external appearance of an individual is called
phenotype.
• e.g.
• a. Tallness and dwarfness of plant.
• b. Red or white flowers of pea plant.
• vii. Genotype :
• It is the genetic make up or genetic constitution
of an individual.
• e.g.
• a. Genotype of pure tall is TT
• b. Genotype of hybrid is Tt
• c. Genotype of dwarf is tt

24. • viii. Dominant :
• The character which express itself in F1-
generation is called Dominant character.
• ix. Recessive :
• The character which suppress itself in F1
generation is called recessive character.

25. offsprings.
• The individual obtained at each generation are
called as offsprings.

26. • xi. F1 - generation (first filial generation) :
• The first generation obtained by parental
cross is called F1 generation.
• xii. F2 - generation (second filial generation) :
• The generation of offsprings produced by
selfing of F1 generation is called F2-generation.
• xiii.Pure line (True breeds) :
• The generation of homozygous individuals
which produce offspring of similar types on
selfing is called pure line.
• e.g.
• a. TT - pure tall
• b. tt - pure dwarf.
•

27. • Monohybrid cross :
• A cross between two homogygous
individuals differing in only one pair of
contrasting character is called monohybrid
cross.
• Monohybrid ratio :
• The ratio of offsprings obtained in F2
generation of the monohybrid cross is called
as monohybrid ratio.

28. law of Dominance
First law of inheritance.
When two homozygous parents are crossed
differ in either single or more pairs of
contrasting characters then the character
which appears in F1 generation is said to be
dominant and which do not express called
recessive character.
• This law is not universally applicable.

29. • i. Mendel selected height of pea plant as one
character.
• ii. He cross pure tall (TT) with pure dwarf (tt) and
obtained F1 generation.
• iii. All the individuals of F1 gen were heterozygous
(hybrid) tall (Tt) as tallness is dominant over dwarfness.
• iv. The F1 plant is allowed to self pollination to raise
F2 generation.
• v. In F2 generation both tall and dwarf plants are
produced in 3 : 1 ratio. This is monohybrid phenotype
ratio.

30. • There are many examples of characters in different
organisms which do not follow law of dominance.
• ii. Law of dominance is not applicable in case of
incomplete dominance and co-dominance.
• iii. In case incomplete dominance the F1 trait is
intermediate.
• e.g.
• a. Snapdragon (Antirrhinum majus)
• b. Four ‘o’ clock plant (Mirabilis jalapa)
• iv. In case of co-dominance F1 generation individual both
traits get express.
• e.g.
• a. Coat colour in cattle.
• b. ABO blood group in human.
• v. Thus the law of dominance is not universally applicable.

31. Mendel’s second law of inheritance.
• Second law of inheritance.
– OR
• Law of purity of gamets.
• Statement :
• Member of allelic pair in a hybrid remains together
without mixing with each other and separate or
segregate during gamete formation.
• Thus, gametes receive only one of the two factors are
pure for a given character therefore it is known as law of
segregation.
• Reappearing of recessive characters (dwarfness) in F2
generation proves the law of segregation, every gamete
receive one factor or one gene.
• This is universally true or applicable law.

32. • Explanation :
• Every character is controlled by a pair of alleles e.g.
Tallness in pea plant is controlled by (T) and dwarfness
in pea plant is controlled by (t). These two alleles (T & t)
remain together without mixing with each other.
• At the time of gamete formation these alleles separate
from one another and as a result, each gamete has only
one allele or gene.
• The gametes produced in F1 hybrid are pure for
particular character. So the law of segregation is also
known as "the law of purity of gametes."
• Reappearing of recessive characters (dwarfness) in F2
generation proves the law of segregation, every gamete
receive one factor or one gene.
• This is universally true or applicable law

33. Example
• When a homozygous tall pea plant (TT) is crossed with
dwarf (tt) the offsprings of F1 generation will be hybrid
tall (Tt).
• The hybrid has two factors (Tt) which remain together
without mixing.
• When these F1 hybrids form gametes , the two factors
T and t segregate and each gamete contains only one
factor or gene.
• The dwarf plant reappears in F2 generation because of
segregation of factors.
• In F2 generation both tall and dwarf plants are
produced in 3 : 1 ratio.

34. • Dihybrid cross :
• When homozygous parents differ in two
sets of contrasting character (traits) are
crossed called dihybrid cross

35. • Law of independent assortment :
• ‘‘When the two homozygous parents
differing in two or more pair of characters are
crossed, then the inheritance of one character
is independent of other character.’’

36. • Example:When yellow round seeded pea plant is crossed with green
wrinkled seeded plant. Then all F1 generation off springs producedare
yellow round (YyRr). When F1 dihybrid were selfed the F2 generation
obtained are as
• Yellow round ------- 9
• Yellow wrinkled ------- 3
• Green round ------- 3
• Green wrinkled ------- 1
• The offsprings of F2 generation were produced in the ratio
• 9 : 3 : 3 : 1.
• Explanation:
• In pea plant, In seed colour yellow( Y ) is dominant over green (y). In seed
shape, Round (R) is dominant over wrinkled ( r).
• Hence, the pure yellow round pea plant parent is represented by genotype
YYRR and
• the pure green wrinkled parent plant is represented by yyrr.
• During gamete formation, the paired genes of a character assort
independently of the other pair.
• Thus dominant parentYYRR produces only one type of gamate i.e YR
• The recessive parent yyrr produces only one type of gamate i.e yr

37. • The f1 plants are formed by union of these two types
of gametes is YyRr. As the F1 plants contain dominant Y
and dominant R gene the phenotype of F1 plant is
Yellow round.
• The F1 dihybrid plants are self fertilized among
themselves.
• During F1 gamete formation, the two factors or genes
of a character assort independently of the other pair.
• For example , gene Y may combine with the dominant
gene R or with recessive gene r and enter a gamate.

38. • In the same way, gene y may combine with the dominant gene
R or with recessive gene r and enter a gamete. This is due to
independent assortment of factors or genes.
• So F1 female plant produces four types of gametes . They are
YR, Yr, yR and yr.
• similarly F1 male plants also produces same four types of
gametes.
• When F1 dihybrid were selfed he obtained four types of
off springs in F2 generation.
• Yellow round ------- 9
• Yellow wrinkled ------- 3
• Green round ------- 3
• Green wrinkled ------- 1
• The offsprings of F2 generation were produced in the ratio
• 9 : 3 : 3 : 1.
• This phenotypic ratio is called dihybrid ratio.