Law of Dominance is the first law of inheritance, given by Gregor Mendel
which states that "out of a pair of allelomorphic characters, one is dominant over the
other."
When monohybrid cross was performed by Mendel, the F1 generation showed the
ratio of 3:1 between dominant and recessive character.
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Breeding Experiment performed by Mendel:
When Mendel performed monohybrid cross between pure dominant tall plant
(TT) and pure recessive dwarf plant (tt), he found out that
F1 generation showed dominant phenotype characters i.e., the
F1 progeny (Tt) was tall pea plant while recessive character (dwarf) was not
shown in F1 generation.
This established Mendel's first law inheritance called as
Law of Dominance.
Below is given a diagrammatic representation of monohybrid cross between homozygous tall plant and homozygous dwarf plant which shows Law of Dominance in F1 progeny.
What is Law of Dominance?
Law of Dominance can be defined as
"out of a pair of allelomorphic character (or contrasting characters),
one is dominant over the other".
Mendel found out that the characters present as a pair of alleles in one
plant dominate over that of the other plants.
When a pair of allelomorphs (contrasting characters) or alleles are brought
together in F1 generation (heterozygous hybrid), one character expresses
itself ( called as dominant trait) while other remains
unexpressed ( called as recessive trait).
Monohybrid Cross (Monohybrid hybridization)
a) In a Monohybrid cross, inheritance of one pair of contrasting
character is studied.
For example in this case, we will study one pair of character of pea
plant in a cross:
- Height of plant [Tall (TT) or Dwarf (tt) ]
b) On the basis of Monohybrid Cross, Mendel gave two Laws of
Inheritance:
- Law of Dominance
In F1 progeny, we can observe that only dominant trait is expressed while recessive trait remains unexpressed. Thus, it concludes that out of a pair of allelomorphic characters, one is dominant over the other.
- Law of Segregation
When self-hybridization of F1 progeny is performed, we could see that in F2 generation, the phenotypic ratio is obtained as 3 : 1 (Tall : Dwarf), while genotypic ratio is obtained as 1 : 2 : 1 (Pure tall : Hybrid tall : Pure recessive).
Thus, we could see that the recessive character which disappeared in F1 generation, reappears in F2 generation and shows that alleles or genes do not mix with one another and separated pure during gamete formation.
Dihybrid Cross
In a Dihybrid cross, inheritance of two pairs of contrasting characters
is studied.
In a breeding experiment, when cross between two parents is performed
considering alternative forms of two different characters, the cross is
called dihybrid cross.
Example:
Cross bewteen pure homozygous Yellow cotyledon, Round seed pea variety
(YYRR) with Green cotyledon, wrinkled seed pea variety (yyrr).
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