Lecture Power Point Presentation

1. (T.Y.B.Sc. BOTANY PAPER –III BO.333: GENETICS AND EVOLUTION GSD )
Sir Parashurambhaucollege, Pune-30
Department of Botany
Chapter. 2. MENDELISM
Dr. Dhulgande G. S.
Asst. prof.

2. Genetical Terminology:
1. Haploid - the condition of having only one set of chromosomes per
cell (n)
2. Diploid - the condition of having two sets of chromosomes per cell
(2n)
3. Gamete - a haploid (n) sex cell in plants and animals (egg or
sperm)
4. Zygote - diploid (2n) cell resulting from the union of two gametes
in sexual reproduction
5. Chromatin - the complex of DNA, RNA and proteins that makes
up uncondensed eukaryotic chromosomes.
6. Chromosome - structures within the nucleus of eukaryotic cells
composed of chromatin and visible at cell division (condensed
chromatin).
7. Homologous chromosomes - chromosomes that are similar in
morphology (shape and form) and genetic constitution. In living
organisms one set comes from the father and the other from the
mother.

3. 8. Chromatids - One of the two half of a duplicated chromosome.
9. Centromeres- specialized constricted region of a chromatid, that
contains the kinetochore; sister chromatids are joined at the
centromere during cell division.
10. Recombination - exchange of genetic material between
chromosomes
11. Crossover- the breaking and rejoining of homologous (sister)
chromatids during
prophase-I (pachytene) of meiosis, resulting in recombination.
LZPDD
12. Synapsis- the pairing of homologous chromosomes during
prophase-I (zygotene) of meiosis.
13. Disjunction- separation of homologous chromosomes (or sister
chromatids) during anaphase.
14. Genotype- the genetic make-up (the assemblage of alleles) of an
individual.

4. 15. Phenotype- the physical or chemical expression of an organism’s
gene.
16. Gene- a discrete unit of hereditary information that usually
specifies a protein; a region of DNA (locus) located on a chromosome
that specifies a trait (characteristic).
17. Alleles- genes governing variations of the same characteristic
(trait) that occupy corresponding positions (loci) on homologous
chromosomes; alternative forms of a gene.
18. Dominant allele- an allele that is always expressed when present,
regardless of whether the organism is homozygous or heterozygous
for that gene.
19. Recessive allele- an allele that is only expressed when the
organism is homozygous for that allele and not expressed when
heterozygous (when paired with a dominant allele).
20. Homozygous- possessing a pair of identical alleles for a
particular locus (gene).
21. Heterozygous- possessing a pair of different alleles for a
particular locus (gene).

5. 22. Carrier- a heterozygous individual not expressing a
recessive trait but capable of passing it on to its offspring.
23. Parent generation (P)- the generation that supplies
gametes to the filial generation.
24. Filial generation (F1)- the generation that receives
gametes from the parental generation.
25. Hybrid- an offspring resulting from the mating
between individuals of two different genetic constitutions
(parents).
26. Dihybrid cross- a genetic cross that takes into account
the effect of alleles at two separate loci (two different
genes).
27. Monohybrid cross- a genetic cross that takes into
account the effect of alleles at a single locus (single gene).

6. Selection of the experimental plant material:
Mendel selected garden sweet pea (Pisum sativum L.)
for his hybridization experiments for the following reasons:
1. The plants are annual and easy to cultivate.
2. The plants have very short life cycle.
3. The plants have many distinct, well defined and easy
morphological characters.
4. The flowers are bisexual.
5. The flowers are naturally self-pollinating, but they can also
be easily cross pollinated.
6. The offspring of cross pollinated plants are fertile.
7. The flowers are sufficiently large which helps in easy
emasculation and artificial cross pollination.
8. The plant produce a large number of progenies (offspring’s)
after each successive generations, it will help in correct
conclusions.

7. The seven variable characteristics Mendel investigated in
pea plants were
•Seed texture: (Round vs wrinkled)
•Seed cotyledon color :(Yellow vs green)
•Flower color :(White vs purple)
•Growth habit (Height): (Tall vs dwarf)
•Pod shape: (Inflated vs constricted)
•Pod color :(Green vs yellow)
•Flower position :(Axial vs terminal)

9. Monohybrid Cross:
Definition: “A cross between two parents possessing a
pair of contrasting characters is known as monohybrid
cross”.
A monohybrid cross is a mating between two
organisms with different variations at one genetic
chromosome of interest.
The character(s) being studied in a monohybrid cross
are governed by two or multiple variations for a single locus.
This cross is based on the first law of genetics i.e Law
of Segregation.
e.g. When the pure tall variety of pea plant is crossed with the
pure dwarf variety of pea plant. ‘T’ is used for tall plant
while‘t’ is used for dwarf.

10. Parents: Tall × Dwarf
TT tt
Gametes: T t
F1 Generation: Tt --------------- Hybrid tall
On selfing Tt is crossed with Tt
Male
Female
T t
T TT
Tall
Tt
Tall
t Tt
Tall
tt
Dwarf
The monohybrid phenotypic ratio is 3:1 and the genotypic is 1:2:1.

11. Dihybrid Cross:
Definition: “A cross between two pure or homozygous parents
having two pairs of contrasting characters is called dihybrid cross”.
The F1 hybrids (offsprings) obtained from dihybrid cross are
known as dihybrids.
The phenotypic ratio is called dihybrid ratio; it will be 9:3:3:1.
When pure homozygous pea plant having yellow cotyledons and round seeds with
another homozygous pea plant having green cotyledons and wrinkled seeds.
In this cross there are two pairs of contrasting characters are present i.e.
one pair is yellow cotyledon with rounded seeds while other pair is green cotyledon
with wrinkled seeds.
Here,
Yellow cotyledon color represented by -------------------‘Y’
Rounded seeds represented by ------------------------------‘R’
Green cotyledon color represented by ----------------------‘y’ and
Wrinkled seeds represented by ------------------------------‘r’.
Therefore , the parents will be as Yellow round (YYRR) and green wrinkled (yyrr).

12. Parents: Yellow round × Green wrinkled
YYRR yyrr
Gametes: YR yr
F1 generation: YyRr ------------------------ Yellow round Hybrid
On selfing,
Yellow round × Yellow round
YyRr YyRr
Gametes for F2 generation: YR, Yr, yR, yr
YR Yr yR yr
YR YYRR
Yellow round
YYRr
Yellow round
YyRR
Yellow round
YyRr
Yellow round
Yr YYRr
Yellow round
YYrr
Yellow
wrinkled
YyRr
Yellow round
Yyrr
Yellow
wrinkled
yR YyRR
Yellow round
YyRr
Yellow round
yyRR
Green round
yyRr
Green round
yr YyRr
Yellow round
Yyrr
Yellow
wrinkled
yyRr
Green round
yyrr
Green
wrinkled

13. YR Yr yR yr
YR YYRR
YR
YR YyRR
YR
Assignment

14. Mendel’s Laws of Heredity (Inheritance):
Based on the hybridization experiments, John Gregor
Mendel formulated the laws of inheritance. There are
three laws:
1. Law of Dominance
2. Law of Segregation
3. Law of Independent assortment.

15. 1. LAW OF DOMINANCE:
It is based on monohybrid cross.
This law states that, “In crossing of pure homozygous
organism having only one pair of contrasting character, one
factor in a pair of allele may mask or prevent the expression
of other one”. T t
e.g. When Mendel made a monohybrid cross between tall
and dwarf, then all tall plants were appeared in the F1
generation.
But on selfing of F1 generation, both tall and dwarf
characters appeared in F2 generation. It shows that in F1
hybrid the tall character is dominant while dwarf is
recessive.

16. 2. LAW OF SEGREGATION or LAW OF PURITY OF
GAMETES or LAW OF INCOMPLETE DOMINANCE:
It is also known as law of purity of gametes.
It is based on monohybrid cross.
This law states that, “during gamete formation the two factors
(genes) of each character segregate i.e. one factor of one character in
each gamete will remains. Thus here each gamete is pure from each
other”.
OR
It also states that, “during the formation of gametes, the two traits
carried by each parent will segregate easily”. Thus, the law of
segregation is based on the fact that each gamete contains only one
allele.
e.g. When a red flowered pea plant (RR) is crossed with white
flowered pea plant (rr) then hybrid will have pink flowers (Rr). On
selfing of F1, they produce three colored (Red, pink and white)
flowers in F2 generation. The reappearance of pink color indicates
the process of segregation.

17. 3. LAW OF INDEPENDENT ASSORTMENT:
It is based on dihybrid cross.
It states that, “when the parents differ from each other in two
or more pairs of contrasting characters then the inheritance
of one pair of factor is independent to that of other pair of
factor”.
e.g. When pure homozygous pea plant having yellow
cotyledons and round seeds crossed with another
homozygous pea plant having green cotyledons and
wrinkled seeds.
In this cross there are two pairs of contrasting
characters are present i.e. one pair is yellow cotyledon with
rounded seeds while other pair is green cotyledon with
wrinkled seeds.
Therefore , the parents will be as Yellow round
(YYRR) and green wrinkled (yyrr).

18. BACK CROSS:
“When F1 individuals are crossed with any one of
the parents, then such a cross is called back cross”.
In such cases there are two possibilities:
(A) When F1 (Tt) is crossed to the parent with
dominant phenotype i.e., homozygous tall (TT). In
such a cross plants will be 100% tall.
(B) When F1 plant (Tt) is crossed to the parent
with pure recessive (tt) dwarf plant. In such a cross
50% plants will be tall and 50% plants will be dwarf.

19. Parents: Tall × Dwarf
TT tt
Gametes: T t
F1 Generation: Tt --------------- Hybrid tall
T t
T TT
Tall
Tt
Tall
T TT
Tall
Tt
Tall
A) F2----- Tt × TT B) F2-----Tt × tt
100% Tall 50% Tall and 50% dwarf
T t
t Tt
Tall
tt
Dwarf
t Tt
Tall
tt
Dwarf

20. TEST CROSS:
“When F1 individuals are crossed with one
of its homozygous recessive parent, then such a
cross is called test cross”.
e.g. When F1 plant (Tt) is crossed to the
parent with pure (homozygous) recessive (tt)
dwarf plant.
In such a cross 50% plants will be tall and
50% plants will be dwarf.

21. T t
t Tt
Tall
tt
Dwarf
t Tt
Tall
tt
Dwarf
Parents: Tall × Dwarf
Gametes: TT tt
F1------ T t
F2-----Tt × tt
50% Tall and 50% dwarf

22. Assignments:
Define:
1. Gamete -
2. Diploid-
3. Centromeres-
4. Gene-
Mendel selected garden sweet pea (Pisum sativum L.) for his hybridization
experiments for the following reasons:
1. The plants are --------- and easy to cultivate.
2. The plants have very --------- life cycle.
3. The plants have many distinct, ---------------- and easy morphological characters.
4. The flowers are --------------.
5. The flowers are naturally ----------pollinating, but they can also be easily cross
pollinated.
6. The offspring of ------------- pollinated plants are fertile.
7. The flowers are sufficiently ---------- which helps in easy emasculation and
artificial cross pollination.
8. The plant produce a -------- number of progenies (offspring’s) after each
successive generations, it will help in correct conclusions.

23. 1. Crossing over occurs during----------------
2. Synopsis occurs during----------------

24. T t
T
t
Monohybrid cross

25. Back cross
T t
T
T
T t
t
t