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CLASS 10
Monohybrid Inheritance:

Gregor Johann Mendel (Father of Genetics)
Ø Then he selfed the F1 plants to produce the next generation called F2
...

Ø Even though in the F2 generation, both tall and dwarf progenies were there, their
number was NOT equal
...

Ø The F2 dwarf plant on selfing produced all dwarf progenies (F3 generation)
...

Ø The 2/3 of the tall F2 on selfing produce tall and dwarf plants in 3 : 1 ratio (F3
generation)
...
(Instead of
the apparent twotypes)
...


Tall homozygous (pure)

2
...


Dwarf homozygous (pure) :

tt

– 25%

Phenotypic ratio of monohybrid cross: 3 : 1 (3 tall : 1 dwarf)
Genotypic ratio of monohybrid cross: 1 : 2 : 1 (1 homozygous tall : 2
heterozygous tall : 1 homozygous dwarf)
Mendel’s explanations for his monohybrid cross:
Ø The Tall and Dwarf traits in plants are determined by a pair of
contrasting factors (or determinants)
...

Ø If a plant possesses the determinant for tallness (T) the plant will be tall in its
phenotype
...

Ø The determinants for each character will occur in a pair and are received from
their parents
...

Ø The one which expresses in its heterozygous condition is called Dominant
trait (T)
...

Ø In the case of plant height, from monohybrid cross, Mendel concluded that Tall is
dominant and Dwarf is recessive
...

Ø Even though they stay together (such as Tt) they never mix each other (or
contaminate each other)
...

Ø The factor ‘T’ (for tallness) and ‘t’ (for dwarfness) are thus two separate entities
...

Ø When the F1 hybrid (Tt) is selfed, the two entities separate or segregate out and
unite independently produce tall and dwarf parts
...
They were:
(1)
...
Law of Segregation
(3)
...

Ø Among these three laws, the first two (Law of Dominance and Law of
Segregation) were based on his results of monohybrid cross
...
Law of Dominance

This is the first law of Mendelian inheritance
...

Ø In Mendel’s monohybrid cross, tall (represented by ‘T’) is dominant and Dwarf
(represented by ‘t’) is recessive characters
...

Ø In the F1, the genotype of the progeny will be ‘Tt’
...


(2)
...

Ø Law of segregation is also called as Law of Purity of Gametes
...

Ø Pure tall plants (TT) will produce gametes only with ‘T’ allele
...

Ø The hybrid tall (Tt) will produce two types of gametes, 50% will be with ‘T’ and
the rest 50% will be with ‘t’ allele
...
This is called purity
of gametes
...
In one of
his monohybrid cross, he selected plant height as the character
...
The
dominant allele of plant height is Tall and he represented it as ‘T’
...
These two
phenotypes were morphologically very distinct and can be differentiated very easily
...

Ø He generated these true-breeding parent plants by repeated selfing for several
generations
...


Ø The progenies of the first cross were called as the F1 generation (First filial
generation)
...

Ø Then he selfed the F1 plants to produce the next generation called F2
...

Ø Even though in the F2 generation, both tall and dwarf progenies were there, their
number was NOT equal
...

Ø The F2 dwarf plant on selfing produced all dwarf progenies (F3 generation)
...

Ø The 2/3 of the tall F2 on selfing produce tall and dwarf plants in 3 : 1 ratio (F3
generation)
...
(Instead of
the apparent twotypes)
...


Tall homozygous (pure)

2
...


Dwarf homozygous (pure) :

tt

– 25%

Phenotypic ratio of monohybrid cross: 3 : 1 (3 tall : 1 dwarf)
Genotypic ratio of monohybrid cross: 1 : 2 : 1 (1 homozygous tall : 2
heterozygous tall : 1 homozygous dwarf)
Mendel’s explanations for his monohybrid cross:
Ø The Tall and Dwarf traits in plants are determined by a pair of
contrasting factors (or determinants)
...

Ø If a plant possesses the determinant for tallness (T) the plant will be tall in its
phenotype
...

Ø The determinants for each character will occur in a pair and are received from
their parents
...

Ø The one which expresses in its heterozygous condition is called Dominant
trait (T)
...

Ø In the case of plant height, from monohybrid cross, Mendel concluded that Tall is
dominant and Dwarf is recessive
...

Ø Even though they stay together (such as Tt) they never mix each other (or
contaminate each other)
...

Ø The factor ‘T’ (for tallness) and ‘t’ (for dwarfness) are thus two separate entities
...

Ø When the F1 hybrid (Tt) is selfed, the two entities separate or segregate out and
unite independently produce tall and dwarf parts
...
They were:
(1)
...
Law of Segregation
(3)
...

Ø Among these three laws, the first two (Law of Dominance and Law of
Segregation) were based on his results of monohybrid cross
...
Law of Dominance

This is the first law of Mendelian inheritance
...


Ø In Mendel’s monohybrid cross, tall (represented by ‘T’) is dominant and Dwarf
(represented by ‘t’) is recessive characters
...

Ø In the F1, the genotype of the progeny will be ‘Tt’
...


(2)
...

Ø Law of segregation is also called as Law of Purity of Gametes
...

Ø Pure tall plants (TT) will produce gametes only with ‘T’ allele
...

Ø The hybrid tall (Tt) will produce two types of gametes, 50% will be with ‘T’ and
the rest 50% will be with ‘t’ allele
...
This is called purity
of gametes

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