5
Chapter
PRINCIPLES OF INHERITANCE
AND VARIATION
NCERT CRUX
Genetics is a branch of biology that deals with the
study of heredity and variations. Mendelism
Heredity is the transfer of characters from parents to
Gregor Johann Mendel began his genetic experiments
their offspring. These hereditary characters are present
on garden pea in 1856.
on the chromosomes in the form of genes.
Selection of pea plant: The main reasons for adopting garden
Variations may be defined as the differences in
pea (Pisum sativum) for experiments were:
characters of offspring. It mainly depends upon
unique process of crossing-over that occurs during Pea has many distinct contrasting characters.
meiosis. This is one of the main reasons of producing Life span of pea plant is short.
recombination. Flowers show self-pollination.
Gene is a unit of inheritance which passes from one It is easy to artificially cross-pollinate the pea flowers.
generation to the next through the gamete and controls The hybrids thus produced were fertile.
the expression of a character. Experimental method: Mendel’s experiment were performed
Allele or Allelomorph represents the two alternative in three stages:
forms of gene. Selection of pure breed as parents.
Character is the feature of the individual e.g., stem Hybridisation and obtaining F1 generation of plants.
height.
Self pollination of hybrid plants and raising of
Trait is an inherited character and its detectable variant subsequent generations.
e.g., tall or dwarf.
Table: Contrasting Traits Studied by Mendel in Pea
Homozygote (Homozygous individuals) is an
individual which contains identical alleles of a gene on S. No. Characters Contrasting Traits
its homologous chromosomes e.g., TT or tt 1. Stem height Tall/dwarf
Heterozygote (Heterozygous individual) is an 2. Flower colour Violet/white
individual which contains two different alleles of a gene 3. Flower position Axial/terminal
on its homologous chromosomes, e.g., Tt 4. Pod shape Inflated/constricted
F1 or Filial 1 Progeny is the generation of hybrids 5. Pod colour Green/yellow
produced from a cross between the genetically different 6. Seed shape Round/wrinkled
individuals called parents. 7. Seed colour Yellow/green
F2 or Filial 2 Progeny is the generation of individuals
which arises as a result of interbreeding amongst
individuals of F1 progeny. Mendel’s Work and Results
Genotype is the genetic constitution of an individual. Mendel studied the obtained results and on their
Phenotype is the observable characteristic of an basis, he proposed certain laws known as “Laws of
individual which is a result of gene products. inheritance”. These laws are discussed below:
, 80 NTA CUET (UG) - Biology PW
Law of Dominance The principle of segregation is also called the principle
This law states that: of purity of gametes because segregation of the two
factors of a trait results in gamete receiving only one
Characters are controlled by discrete units called factors.
factor out of a pair. As a result gametes are always pure
Factors occurs in pairs. for a character.
When two contrasting genes for a character come
together in an organism, only one is expressed externally Law of Independent Assortment
and shows visible effect. It is called dominant gene and It states that inheritance of two or more genes when
the other gene of the pair which does not express and occur at one time, their distribution in the gametes and
remains hidden is called recessive. in the progeny of subsequent generations is independent
Law of Segregation or Purity of Gametes of each other.
This law states that: To prove this, he did a dihybrid cross. He crossed
homozygous dominant round and yellow seeded
Both parental alleles (recessive and dominant) separate
(RRYY) plants with homozygous recessive green and
and are expressed phenotypically in F2 generation.
wrinkled seeded (rryy) plants.
When F2 generation was produced by allowing F1 hybrid
The F1 hybrid was self-pollinated and F2 generation
to self-pollinate, to find out segregation or separation it
was obtained with the phenotypic ratio of 9 : 3 : 3 :
was observed that both dominant and recessive plants
1 and genotypic ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
appeared in 3 : 1 ratio. It can be understood by Mendel’s
monohybrid cross, say between a pure tall pea plant
(TT) and dwarf (tt) pea plant on the reciprocal cross.
The hybrids produced in F1 generations are all tall
though they have also received the factor for dwarfness.
If the hybrids are allowed to self breed, the plants of
the F2 generation appear to be both tall and dwarf in
the phenotypic ratio of 3 : 1 and genotypic ratio of
1 : 2 : 1.
Phenotypic ratio: Round yellow (R * Y*) : Round green (R * yy) :
Wrinkled yellow (rr Y *) : Wrinkled green (rryy)
Fig.: Results of a dihybrid cross where the two parents
differed in two pairs of contrasting traits:
seed colour and seed shape.
Fig.: Diagrammatic representation of monohybrid cross [* – It refers that allele could be either dominant or recessive]
Chapter
PRINCIPLES OF INHERITANCE
AND VARIATION
NCERT CRUX
Genetics is a branch of biology that deals with the
study of heredity and variations. Mendelism
Heredity is the transfer of characters from parents to
Gregor Johann Mendel began his genetic experiments
their offspring. These hereditary characters are present
on garden pea in 1856.
on the chromosomes in the form of genes.
Selection of pea plant: The main reasons for adopting garden
Variations may be defined as the differences in
pea (Pisum sativum) for experiments were:
characters of offspring. It mainly depends upon
unique process of crossing-over that occurs during Pea has many distinct contrasting characters.
meiosis. This is one of the main reasons of producing Life span of pea plant is short.
recombination. Flowers show self-pollination.
Gene is a unit of inheritance which passes from one It is easy to artificially cross-pollinate the pea flowers.
generation to the next through the gamete and controls The hybrids thus produced were fertile.
the expression of a character. Experimental method: Mendel’s experiment were performed
Allele or Allelomorph represents the two alternative in three stages:
forms of gene. Selection of pure breed as parents.
Character is the feature of the individual e.g., stem Hybridisation and obtaining F1 generation of plants.
height.
Self pollination of hybrid plants and raising of
Trait is an inherited character and its detectable variant subsequent generations.
e.g., tall or dwarf.
Table: Contrasting Traits Studied by Mendel in Pea
Homozygote (Homozygous individuals) is an
individual which contains identical alleles of a gene on S. No. Characters Contrasting Traits
its homologous chromosomes e.g., TT or tt 1. Stem height Tall/dwarf
Heterozygote (Heterozygous individual) is an 2. Flower colour Violet/white
individual which contains two different alleles of a gene 3. Flower position Axial/terminal
on its homologous chromosomes, e.g., Tt 4. Pod shape Inflated/constricted
F1 or Filial 1 Progeny is the generation of hybrids 5. Pod colour Green/yellow
produced from a cross between the genetically different 6. Seed shape Round/wrinkled
individuals called parents. 7. Seed colour Yellow/green
F2 or Filial 2 Progeny is the generation of individuals
which arises as a result of interbreeding amongst
individuals of F1 progeny. Mendel’s Work and Results
Genotype is the genetic constitution of an individual. Mendel studied the obtained results and on their
Phenotype is the observable characteristic of an basis, he proposed certain laws known as “Laws of
individual which is a result of gene products. inheritance”. These laws are discussed below:
, 80 NTA CUET (UG) - Biology PW
Law of Dominance The principle of segregation is also called the principle
This law states that: of purity of gametes because segregation of the two
factors of a trait results in gamete receiving only one
Characters are controlled by discrete units called factors.
factor out of a pair. As a result gametes are always pure
Factors occurs in pairs. for a character.
When two contrasting genes for a character come
together in an organism, only one is expressed externally Law of Independent Assortment
and shows visible effect. It is called dominant gene and It states that inheritance of two or more genes when
the other gene of the pair which does not express and occur at one time, their distribution in the gametes and
remains hidden is called recessive. in the progeny of subsequent generations is independent
Law of Segregation or Purity of Gametes of each other.
This law states that: To prove this, he did a dihybrid cross. He crossed
homozygous dominant round and yellow seeded
Both parental alleles (recessive and dominant) separate
(RRYY) plants with homozygous recessive green and
and are expressed phenotypically in F2 generation.
wrinkled seeded (rryy) plants.
When F2 generation was produced by allowing F1 hybrid
The F1 hybrid was self-pollinated and F2 generation
to self-pollinate, to find out segregation or separation it
was obtained with the phenotypic ratio of 9 : 3 : 3 :
was observed that both dominant and recessive plants
1 and genotypic ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
appeared in 3 : 1 ratio. It can be understood by Mendel’s
monohybrid cross, say between a pure tall pea plant
(TT) and dwarf (tt) pea plant on the reciprocal cross.
The hybrids produced in F1 generations are all tall
though they have also received the factor for dwarfness.
If the hybrids are allowed to self breed, the plants of
the F2 generation appear to be both tall and dwarf in
the phenotypic ratio of 3 : 1 and genotypic ratio of
1 : 2 : 1.
Phenotypic ratio: Round yellow (R * Y*) : Round green (R * yy) :
Wrinkled yellow (rr Y *) : Wrinkled green (rryy)
Fig.: Results of a dihybrid cross where the two parents
differed in two pairs of contrasting traits:
seed colour and seed shape.
Fig.: Diagrammatic representation of monohybrid cross [* – It refers that allele could be either dominant or recessive]
