- Gregor Mendel is known as the father of genetics for his work studying inheritance in pea plants. He discovered that traits are inherited through discrete factors (now known as genes) located on chromosomes, and these factors segregate and assort independently during gamete formation and reproduction. - Mendel identified dominant and recessive alleles and established that organisms have two copies of each gene (one from each parent). He showed that hybridization and self-pollination can be used to study inheritance patterns. - Mendel's laws of inheritance include the law of dominance, the law of segregation, and the law of independent assortment. Deviations from simple Mendelian patterns can occur through gene interactions like incomplete

1. INHERITANCE AND VARIATION
Gregor Mendel –
Father of Genetics
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BIOLOGY (XII)
CHAPTER 3
(PART 1)
BY Dr. JANAKI V.
PANDEY

2. Alleles
Cell is 1-100 µm
Nucleus is 6 µm
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3. CHROMOSOMES AND MECHANISM OF
INHERITANCE
Gregor Mendel gave the accurate
explanation for the mechanism of
inheritance by using hybridization
technique.
He stated that traits are not inherited physically but by
something called factor present inside the gametic cell.
The factors are located on separate chromosomes and
factors are transmitted from generation to generation.
The factors (gene) occur in pair in parent and separate
segregate from each other during gamete formation
without blending.
Heredity/ Inheritance: It transmission
of genetic information from generation
to generation.
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4. Genetic Terminology
Factor: It is a unit of heredity. It is responsible for
inheritance and expression of character.
Gene: Gene: It is a particular segment of DNA
which is responsible for heredity.
Character: a specific feature of an
organism. Eg. height of stem, col. of
flower.
Trait: an inherited character and its
detectable variant. Eg. Tall or
dwarf.
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5. Alleles: the two or more alternative forms of a given
gene. Eg. Tall-T and dwarf-t
Dominant: allele that express its trait in
heterozygous condition. eg. Tt.
Recessive: allele that is does not express its trait in
heterozygous condition. Eg. Tt.
Phenotype: external appearance of an individual.
Eg. Tallness or dwarfness.
Genotype: Genetic constitution of organism.
Eg.TT, tt.
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6. 6

7. Homozygous : An individual possessing identical alleles
for a particular trait. Eg. RR
Phenotypic ratio: the ratio of the offsprings produced in
F2 & subsequent generation with
respect to their physical appearance.
Eg. 3:1 for monohybrid.
Genotypic ratio: The ratio of the
offsprings produced in F2 &
subsequent generation with
respect to their genetic makeup.
Eg. 1:2:1
Heterozygous : An individual possessing contrasting
alleles for a particular trait. Eg. Rr
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8. Pureline: an individual or a
group of individuals which is homozygous or true
breeding for one or more traits. Eg. TT or tt.
Monohybrid: a hybrid produced from a cross between
two pure parents differing in single pair of contrasting
character. Eg. Tt.
Dihybrid: a hybrid produced from a cross between two
pure parents differing in two pairs of contrasting
character. Eg. RrYy.
Homologous Chromosome:
morphologically, genetically
and structurally essentially
identical chromosomes
present in a diploid cell.
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9. Mendelian cross
Monohybrid cross: A cross between parents
differing in only one heritable trait.
Phenotypic ratio is 3:1
Genotypic ratio is 1:2:1
There is no blending or mixing of character due to which white colour of flower reappears9

10. Dihybrid cross: A cross between parents differing
in two heritable traits.
Phenotypic ratio is 9:3:3:1
Genotypic ratio is 1:2:2:4:1:2:1:2:1
Alleles present on the separate chromosomes independently separate from each other
during gamete formation.
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11. MENDEL’S LAWS OF INHERITANCE
1. Law of Dominance: When two homozygous
individuals with one or more sets of contrasting
characters are crossed, the
alleles that appear in F1 are
dominant and those which do
not appear in F1 are recessive.
2. Law of segregation/ Law of
purity of gametes: When hybrid
forms gametes, the alleles
segregate from each other and
enter in different gametes.
Alleles do not show blending or mixing.
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12. 3. Law of Independent Assortment: When hybrid
possessing two or more pairs of contrasting factors
form gametes, the factors in each pair segregate
independently of the other pair.
YYRR yyrr
YR yr
Parents
Gametes
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13. RR Rr
Rr rr
Cross between RR (homozygous ) and rr
Red and White (rr) flower.
R r
R
r
F1 ---------- Rr (monohybrid)
Phenotypic ratio is 3:1
Genotypic ratio is 1:2:1
Cross between unknown fls and rr
Selfing of F1
Red flower
Rr Rr
Rr Rr
R R
r
r
All flowers are Red colour.
Unknown flower was homozygous dominant
Cross between unknown fls and rr Rr rr
Rr rr
R r
r
r
½ flowers are Red and ½ are white in colour. The genotype and phenotype is 1:1
(RR homozygous)
(Rr heterozygous)
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14. Back cross: The cross of F1 hybrid with one of the
parents either recessive or dominant.
Test cross: The cross of F1 hybrid with homozygous
recessive parent.
Ratio of test cross is 1:1
Use: to find out genotype of any plant.
If the ratio of any phenotype is 1:1, what does it interpret?
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15. Gene Interaction
• Neo – Mendelism:
Deviation from Mendelian pattern of inheritance.
Gene Interaction
Intragenic Intergenic
Incomplete
dominance
Co-dominance
Multiple alleles Pleiotropy
Polygene
Epistasis
Supplementary
complementary
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16. Incomplete Dominance
• Both the alleles in express
themselves partially.
• There is an intermediate
expression in the F1 hybrid.
• Eg. Flower colour of Mirabilis
jalapa.
• Result: genotypic ratio –
1RR : 2Rr : 1rr
• Phenotypic ratio-
1 Red : 2 Pink : 1 White.
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17. Incomplete Dominance
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18. Co- dominance
• Both the alleles express them self
equally.
• Characters of both the alleles are
seen physically.
• Eg. Coat colour in cattle.
• Result: genotypic ratio –
1RR : 2RW : 1WW
• Phenotypic ratio-
1 Red : 2 Roan : 1 White.
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19. Co-dominance
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20. R r
R RR
RED
Rr
PINK
R Rr
PINK
rr
WHITE
?
INCOMPLETE
DOMINANCE
Phenotypic ratio 1:2:1
Genotypic ratio 1:2:1
CO-DOMINANCE
Phenotypic ratio 3:1
Genotypic ratio 1:2:1
Phenotypic ratio 1:2:1
Genotypic ratio 1:2:1
?
?
COMPLETE
DOMINANCE
R r
R RR
RED
Rr
RED
R Rr
RED
rr
WHITE
R W
R RR
RED
Rr
RED-
WHITE
W Rr
RED-
WHITE
rr
WHITE
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22. Multiple Alleles
• Arises due to mutations of wild
type of gene.
• Different alleles in a series
show dominant-recessive,
co-dominance or incomplete
dominance among themselves.
• Eg. Sizes of wing in
Drosophilia.
Blood groups (A, B, O, AB )in
human being.
• More than two alleles in a population occupying same
locus on the chromosome.
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23. Multiple Alleles
Blood group
Wings in Drosophila
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24. Pleiotropy
• A single gene controls two or more different
traits.
• The phenotypic ratio is 1:2 instead of 3:1 .
• Eg. Sickle cell anaemia.
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26. Thank You All.
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