Prathmesh Notes
3. Inheritance And Variation (Part-I) (4/6M)
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Q. Define – 1.Genetics 2.Heredity 3. Variation [1M]
A.1. Genetics- It is the branch of biology which deals with the study of heredity and variation.
2. Heredity-[a] Transmission of characters from one generation to next generation, is called
heredity.
[b] Transmission of characters from parents to their offsprings, is called heredity
Hint: Hereditary characters- The characters which are transmitted from one generation
to next generation are called hereditary characters.
e.g. Height, skin colour, hair pattern, eye colour, flower colour, etc.
3. Inheritance : The process of transmission of characters from parents to offsprings, is called
inheritance.
Hint: Inheritance is the basis of heredity.
4. Variation- Differences observed between parents and offsprings as well as among the
offsprings (siblings) of same parent, is called variation.
Hint: Variation plays an important role in evolution.
[A]Mendelism (Mendelian inheritance):
1. Gregor Johann Mendel (1822-1884).
2. Father of genetics (Laid foundation of genetics).
3. Experimental material (Pisum sativum / Garden pea plant) (2n= 14)
4. Hybridization work- 7 years (1856 - 1863)
Q. Mention the names of scientists who rediscovered Mendelian work. [1M]
A. 1. Hugo de Vries (Holland)
2. Karl Correns (Germany)
3. Erich Tschermak (Austria)
Q. Enlist 7- pairs of contrasting characters (traits) selected by Mendel for his experiment. [2/3M]
A.
Sr. No. Characters Dominant Recessive
1. Stem height Tall (T) Dwarf (t)
2. Flower colour Violet (V) White (v)
(Red/ Purple)
3. Flower position Axial (A) Terminal (a)
4. Seed colour Yellow (Y) Green (y)
5. Seed shape Round (R) Wrinkled (r)
6. Pod colour Green (G) Yellow (g)
7. Pod shape Inflated (I) Constricted (i)
1
, Prathmesh Notes
Q. Why did Mendel select pea plant for his experiment? [2/3M]
A. Selection of pea plant-
1. Pea plant is annual herb; complete their life-cycle within 3-4 months.
2. Life-cycle is of short duration. Hence, many generations can be studied within short period of time.
3. Easy to cultivate and easy to handle.
4. Plant shows many contrasting characters.
5. Plant shows complete dominance.
6. Plants are naturally self-pollinated. Hence, plants are pure (homozygous- true breeding) for all
characters.
7. Plant produces fertile offspring by cross-pollination (which produces seeds).
8. Flowers are large, bisexual and easy for emasculation.
Hints: i. Annual plant- Life cycle completes within one year.
ii. Herb (Herbaceous plant)- Plant grows up to 2-4feet.
iii. Emasculation- Removal of stamens, before anthesis, from bisexual flower, without damaging the carpel,
is called emasculation.
iv. Anthesis: Opening of floral bud (flower).
Genetic terms and symbols:
1. Character: Any feature of an organism, is called character.
e.g. Height, skin colour, flower colour, etc.
2. Trait: It is an expression of a character. e.g. tallness or dwarfness.
OR
An inherited character and its detectable variant.
Hint: A character may have two or more traits.
3. Factor (unit of heredity by Mendel): It is a particle present in an organism which is responsible for
inheritance and expression of a character.
4. Gene (unit of heredity by Johannsen): Gene is a specific segment of DNA which is responsible for
inheritance and expression of character.
Hint: Johannsen coined the term gene for Mendelian factor.
5. Alleles/ Allelomorphs: Alleles are the alternative forms of a gene.
OR
Alleles are the (two or more) alternative forms of a gene present on homologous chromosome,
situated at same gene locus (position) and controls a character.
6. Homologous chromosome (Homologue):
Morphologically and genetically identical chromosomes present in diploid cell of an organism, is
called homologous chromosomes.
7. Homozygous (Pure): Organism having similar (identical) alleles for a particular trait, is called
homozygous for that trait. e.g. Tall-TT, Dwarf- tt, Axillary position- AA
8. Heterozygous (Hybrid): Organism having dissimilar (non- identical) alleles for a particular trait, is called
heterozygous for that trait. e.g. Tall-Tt, Axillary position- Aa
9. Dominant trait : The trait of a parent which express (appear) in F1 generation, is called dominant trait.
e.g. Tallness.
10. Recessive trait: The trait of a parent which does not express (appear) in F1 generation, is called
recessive trait. e.g. Dwarfness.
11. Phenotype: External appearance of an organism, is called phenotype.
2
, Prathmesh Notes
12. Genotype: Genetic make-up (constitution) of an organism, is called genotype.
e.g. Phenotype Genotype
Tall TT, Tt
Dwarf tt
Hint: Phenotype = Genotype + Environment i.e. P = G + E
13. True-breeding(Pure lines):Plants which are always homozygous and produce offsprings with same trait
after self-pollination generation after generation, are called true-breeding plants.
Hint: True breeding plants show stable trait inheritance and expression for several generations.
14. Hybrid: It is an heterozygous individual (plant) obtained by crossing 2- pure parents differing in 1 or
more pairs of contrasting characters.
(a) Monohybrid: Hybrid obtained by crossing 2- pure parents differing in one pair of contrasting
character, is called monohybrid. e.g. Tt
(b) Dihybrid: Hybrid obtained by crossing 2- pure parents differing in two pairs of contrasting
characters, is called dihybrid. e.g. TtAa
15. Monohybrid cross (and ratio): A cross between 2- pure parents differing in one pair of contrasting
character, is called monohybrid cross (and ratio obtained in F2 generation, is called monohybrid ratio).
Hint: Monohybrid ratio-3:1
16. Dihybrid cross (and ratio): A cross between2- pure parents differing in two pairs of contrasting
characters, is called dihybrid cross (and ratio obtained in F2 generation, is called dihybrid ratio).
Hint: Dihybrid ratio-9:3:3:1
17. Phenotypic ratio: The ratio of individuals obtained in F2 generation on the basis of phenotype,is called
phenotypic ratio.
Hint: [a] Monohybrid phenotypic ratio- 3:1
[b] Dihybrid phenotypic ratio- 9:3:3:1
18. Genotypic ratio: The ratio of individuals obtained in F2 generation on the basis of genotype, is called
genotypic ratio.
Hint: [a] Monohybrid genotypic ratio- 1:2:1
[b] Dihybrid genotypic ratio- 1:2:2:4:1:2:1:2:1
19. Parents: Plants (Individuals) used in initial cross, are called parents.
Hint: P1…. Dominant (Generally female)
P2….Recessive (Generally male)
20. F1 Generation (First filial generation)- [Filial- son / offspring]:
Offspring obtained by crossing initial parents, is called F1 generation.
21. F2 generation (Second filial generation):
Offspring obtained by selfing of F1 generation, is called F2 generation.
22. Inbreeding: Cross between closely related organisms, is called inbreeding.
e.g. Tt x Tt, TT x TT, tt x tt, RR x RR , etc.
Hint:[a] Here, 2-plants having same genotype are crossed.
[b] Results of selfing and inbreeding are same (identical).
23. Reciprocal cross: Cross with reverse sexes, is called reciprocal cross.
OR
A cross in which one parent used as female parent is used as male parent in another cross and the male
parent in one cross is used as female parent in other cross, is called reciprocal cross.
i.e. A (female) x B (male) : A (male) x B (female)
3
, Prathmesh Notes
Checker board / Punnet square:
Q. Write a note on Punnet square (Checker board). [2M]
A. Def: “Representation of results of various crosses by using squares (boxes), is called Punnet square.”
Explanation:-
1. It is simple mathematical method.
2. This method is introduced by R.C. Punnet. Hence, the name ‘Punnet’s square.’
3. This method is used for calculation of offsprings in F2 generations.
4. Helps in easy and systematic representation of phenotype and genotype.
Significance:- Phenotypic and genotypic ratios of F2 generation is easily obtained.
Reasons of Mendel’s success:
Q. What are the reasons of Mendel’s success? [2M]
A.1. Selection of pea plant-
Pea plant has many advantages viz.,short life-cycle, natural self-pollination, homozygous condition,
easy emasculation and cross-pollination.
2. Characters-
Selection of 7-pairs of contrasting characters. Of which, one is completely dominant over other.
3. Statistical method-
He used statistical method to represent phenotype and genotype in systematic way.
4. Accurate record-
He kept accurate record of the experiment from beginning up to the end of experiment.
5. Mendel’s luck-
Luckily, 7-characters selected by Mendel are present on different chromosomes. Hence, do not show
linkage.
Hint: Linkage- Association of two or more genes present on same chromosome.
Types- Two types- (a) Complete linkage (strong).
(b) Incomplete linkage (weak).
Inheritance of one gene:
[A] Monohybrid cross and ratio-
Q. State and explain monohybrid ratio with suitable example. (3/4M)
A.a) Statement-
“A cross between 2- pure parents differing in one pair of contrasting character, is called monohybrid
cross and ratio obtained in F2 generation, is called monohybrid ratio.”
b) Explanation-
1. Mendel selected 2-pure pea plants differing in one pair of contrasting character.
2. He selected height as a character for his experiment.
3. He crossed pure tall (TT) plant with pure dwarf (tt) plant to obtain F1 generation i.e. F1 hybrid (Tt).
4. After selfing of F1, F2 generation is obtained. As a result, 2-types of gametes are formed which on
random mating (fusion) produces 4-offsprings in 3:1 ratio. This is called monohybrid ratio.
4
3. Inheritance And Variation (Part-I) (4/6M)
--------------------------------------------------------------------------------------------------------------------------------------------------
Q. Define – 1.Genetics 2.Heredity 3. Variation [1M]
A.1. Genetics- It is the branch of biology which deals with the study of heredity and variation.
2. Heredity-[a] Transmission of characters from one generation to next generation, is called
heredity.
[b] Transmission of characters from parents to their offsprings, is called heredity
Hint: Hereditary characters- The characters which are transmitted from one generation
to next generation are called hereditary characters.
e.g. Height, skin colour, hair pattern, eye colour, flower colour, etc.
3. Inheritance : The process of transmission of characters from parents to offsprings, is called
inheritance.
Hint: Inheritance is the basis of heredity.
4. Variation- Differences observed between parents and offsprings as well as among the
offsprings (siblings) of same parent, is called variation.
Hint: Variation plays an important role in evolution.
[A]Mendelism (Mendelian inheritance):
1. Gregor Johann Mendel (1822-1884).
2. Father of genetics (Laid foundation of genetics).
3. Experimental material (Pisum sativum / Garden pea plant) (2n= 14)
4. Hybridization work- 7 years (1856 - 1863)
Q. Mention the names of scientists who rediscovered Mendelian work. [1M]
A. 1. Hugo de Vries (Holland)
2. Karl Correns (Germany)
3. Erich Tschermak (Austria)
Q. Enlist 7- pairs of contrasting characters (traits) selected by Mendel for his experiment. [2/3M]
A.
Sr. No. Characters Dominant Recessive
1. Stem height Tall (T) Dwarf (t)
2. Flower colour Violet (V) White (v)
(Red/ Purple)
3. Flower position Axial (A) Terminal (a)
4. Seed colour Yellow (Y) Green (y)
5. Seed shape Round (R) Wrinkled (r)
6. Pod colour Green (G) Yellow (g)
7. Pod shape Inflated (I) Constricted (i)
1
, Prathmesh Notes
Q. Why did Mendel select pea plant for his experiment? [2/3M]
A. Selection of pea plant-
1. Pea plant is annual herb; complete their life-cycle within 3-4 months.
2. Life-cycle is of short duration. Hence, many generations can be studied within short period of time.
3. Easy to cultivate and easy to handle.
4. Plant shows many contrasting characters.
5. Plant shows complete dominance.
6. Plants are naturally self-pollinated. Hence, plants are pure (homozygous- true breeding) for all
characters.
7. Plant produces fertile offspring by cross-pollination (which produces seeds).
8. Flowers are large, bisexual and easy for emasculation.
Hints: i. Annual plant- Life cycle completes within one year.
ii. Herb (Herbaceous plant)- Plant grows up to 2-4feet.
iii. Emasculation- Removal of stamens, before anthesis, from bisexual flower, without damaging the carpel,
is called emasculation.
iv. Anthesis: Opening of floral bud (flower).
Genetic terms and symbols:
1. Character: Any feature of an organism, is called character.
e.g. Height, skin colour, flower colour, etc.
2. Trait: It is an expression of a character. e.g. tallness or dwarfness.
OR
An inherited character and its detectable variant.
Hint: A character may have two or more traits.
3. Factor (unit of heredity by Mendel): It is a particle present in an organism which is responsible for
inheritance and expression of a character.
4. Gene (unit of heredity by Johannsen): Gene is a specific segment of DNA which is responsible for
inheritance and expression of character.
Hint: Johannsen coined the term gene for Mendelian factor.
5. Alleles/ Allelomorphs: Alleles are the alternative forms of a gene.
OR
Alleles are the (two or more) alternative forms of a gene present on homologous chromosome,
situated at same gene locus (position) and controls a character.
6. Homologous chromosome (Homologue):
Morphologically and genetically identical chromosomes present in diploid cell of an organism, is
called homologous chromosomes.
7. Homozygous (Pure): Organism having similar (identical) alleles for a particular trait, is called
homozygous for that trait. e.g. Tall-TT, Dwarf- tt, Axillary position- AA
8. Heterozygous (Hybrid): Organism having dissimilar (non- identical) alleles for a particular trait, is called
heterozygous for that trait. e.g. Tall-Tt, Axillary position- Aa
9. Dominant trait : The trait of a parent which express (appear) in F1 generation, is called dominant trait.
e.g. Tallness.
10. Recessive trait: The trait of a parent which does not express (appear) in F1 generation, is called
recessive trait. e.g. Dwarfness.
11. Phenotype: External appearance of an organism, is called phenotype.
2
, Prathmesh Notes
12. Genotype: Genetic make-up (constitution) of an organism, is called genotype.
e.g. Phenotype Genotype
Tall TT, Tt
Dwarf tt
Hint: Phenotype = Genotype + Environment i.e. P = G + E
13. True-breeding(Pure lines):Plants which are always homozygous and produce offsprings with same trait
after self-pollination generation after generation, are called true-breeding plants.
Hint: True breeding plants show stable trait inheritance and expression for several generations.
14. Hybrid: It is an heterozygous individual (plant) obtained by crossing 2- pure parents differing in 1 or
more pairs of contrasting characters.
(a) Monohybrid: Hybrid obtained by crossing 2- pure parents differing in one pair of contrasting
character, is called monohybrid. e.g. Tt
(b) Dihybrid: Hybrid obtained by crossing 2- pure parents differing in two pairs of contrasting
characters, is called dihybrid. e.g. TtAa
15. Monohybrid cross (and ratio): A cross between 2- pure parents differing in one pair of contrasting
character, is called monohybrid cross (and ratio obtained in F2 generation, is called monohybrid ratio).
Hint: Monohybrid ratio-3:1
16. Dihybrid cross (and ratio): A cross between2- pure parents differing in two pairs of contrasting
characters, is called dihybrid cross (and ratio obtained in F2 generation, is called dihybrid ratio).
Hint: Dihybrid ratio-9:3:3:1
17. Phenotypic ratio: The ratio of individuals obtained in F2 generation on the basis of phenotype,is called
phenotypic ratio.
Hint: [a] Monohybrid phenotypic ratio- 3:1
[b] Dihybrid phenotypic ratio- 9:3:3:1
18. Genotypic ratio: The ratio of individuals obtained in F2 generation on the basis of genotype, is called
genotypic ratio.
Hint: [a] Monohybrid genotypic ratio- 1:2:1
[b] Dihybrid genotypic ratio- 1:2:2:4:1:2:1:2:1
19. Parents: Plants (Individuals) used in initial cross, are called parents.
Hint: P1…. Dominant (Generally female)
P2….Recessive (Generally male)
20. F1 Generation (First filial generation)- [Filial- son / offspring]:
Offspring obtained by crossing initial parents, is called F1 generation.
21. F2 generation (Second filial generation):
Offspring obtained by selfing of F1 generation, is called F2 generation.
22. Inbreeding: Cross between closely related organisms, is called inbreeding.
e.g. Tt x Tt, TT x TT, tt x tt, RR x RR , etc.
Hint:[a] Here, 2-plants having same genotype are crossed.
[b] Results of selfing and inbreeding are same (identical).
23. Reciprocal cross: Cross with reverse sexes, is called reciprocal cross.
OR
A cross in which one parent used as female parent is used as male parent in another cross and the male
parent in one cross is used as female parent in other cross, is called reciprocal cross.
i.e. A (female) x B (male) : A (male) x B (female)
3
, Prathmesh Notes
Checker board / Punnet square:
Q. Write a note on Punnet square (Checker board). [2M]
A. Def: “Representation of results of various crosses by using squares (boxes), is called Punnet square.”
Explanation:-
1. It is simple mathematical method.
2. This method is introduced by R.C. Punnet. Hence, the name ‘Punnet’s square.’
3. This method is used for calculation of offsprings in F2 generations.
4. Helps in easy and systematic representation of phenotype and genotype.
Significance:- Phenotypic and genotypic ratios of F2 generation is easily obtained.
Reasons of Mendel’s success:
Q. What are the reasons of Mendel’s success? [2M]
A.1. Selection of pea plant-
Pea plant has many advantages viz.,short life-cycle, natural self-pollination, homozygous condition,
easy emasculation and cross-pollination.
2. Characters-
Selection of 7-pairs of contrasting characters. Of which, one is completely dominant over other.
3. Statistical method-
He used statistical method to represent phenotype and genotype in systematic way.
4. Accurate record-
He kept accurate record of the experiment from beginning up to the end of experiment.
5. Mendel’s luck-
Luckily, 7-characters selected by Mendel are present on different chromosomes. Hence, do not show
linkage.
Hint: Linkage- Association of two or more genes present on same chromosome.
Types- Two types- (a) Complete linkage (strong).
(b) Incomplete linkage (weak).
Inheritance of one gene:
[A] Monohybrid cross and ratio-
Q. State and explain monohybrid ratio with suitable example. (3/4M)
A.a) Statement-
“A cross between 2- pure parents differing in one pair of contrasting character, is called monohybrid
cross and ratio obtained in F2 generation, is called monohybrid ratio.”
b) Explanation-
1. Mendel selected 2-pure pea plants differing in one pair of contrasting character.
2. He selected height as a character for his experiment.
3. He crossed pure tall (TT) plant with pure dwarf (tt) plant to obtain F1 generation i.e. F1 hybrid (Tt).
4. After selfing of F1, F2 generation is obtained. As a result, 2-types of gametes are formed which on
random mating (fusion) produces 4-offsprings in 3:1 ratio. This is called monohybrid ratio.
4
