Law of Segregation is the second Law of Inheritance given by Mendel which was
formulated based on the observations noted down while performing Monohybrid
cross.
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) whereas
genotypic ratio is obtained as 1 : 2 : 1 (Pure tall : Hybrid tall : Pure
recessive).
Thus, we could observe 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
which is Law of Segregation or Law of Purity of Gametes.
Topics Covered:
What is Law of Segregation?
Mendel's Law of Segregation or Law of Purity of Gametes can be defined
as:
"When a pair of allelomorphs (contrasting characters) are brought together
in the hybrid (F1 progeny), they remain together in the hybrid without
blending but separate complete and pure during gamete formation".
Thus, the
traits of recessive allele which disappeared in F1 generation reappears
unchanged in F2 generation. Thus, showing gametes segregate pure during gamete formation and do not
blend or mix with one another.
Experiment performed by Mendel to study Law of Segregation:
1) Mendel carried out monohybrid cross between pure tall (TT) and
pure dwarf (tt) pea plants and a hybrid was formed (Tt).
2) In F1 generation, one of the two characters appeared as dominant (T)
while other remained recessive (t) or unexpressed.
3)
During gamete formation,
segregation takes place and only one factor of the pair of alleles alone enter one gamete and the
other enters other gamete.
Thus, F1 hybrid (Tt) produced two types of gametes: Dominant (T) and
recessive (t).
4) On self fertilization of hybrid (Tt) formed in F1 generation, the
individuals formed in F2 generation showed Phenotype in ratio 3:1 (3
dominant : 1 dwarf) while Genotype in ratio 1:2:1 (TT: Tt : tt).
5) Each parent thus produces two types of gametes which have equal chances
of combining together.
Hence, Mendel's experiment shows segregation of two alleles in F1 and F2
generations. The two contrasting characters segregated or separated in F2
generation.
Why Law of Segregation is called Law of Purity of Gametes?
Law of Segregation is also called as Law of Purity of Gametes because the
two factors of alleles (representing a character) do not mix each other and
gets separated during gamete formation, expressing themselves separately in
F2 generation in a ratio of 3:1.
Importance of Law of Segregation
The Law of Segregation is important because it introduced the
concept of HEREDITARY FACTORS which do not become altered or blended
when present together in the same individual.
Although traits of recessive allele disappeared in F1 generation, the
recessive characters were re-appeared unchanged in F2 generation. Thus, disapproving the blending theory of traits.
Where does Law of Segregation holds true?
1) Law of Segregation applies only to diploid organisms that form
haploid gametes to reproduce sexually.
2) It applies only to traits controlled exclusively by a
single gene pair in which of the two alleles, one is dominant over
the other.
Limitations of Law of Segregation (Exceptions)
1) It does not apply to alleles which are incompletely dominant or
co-dominant.
2) It is not applicable to genes that collaborate or vary in their
expressivity or penetrance.
3) It is not applicable to genes that are pleiotropic, complementary or
influenced by epistasis
4) It does not apply to traits caused by many gene pairs.
Suggested Readings:
- Law of Dominance
- Difference between Monohybrid and Dihybrid cross
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