Mendel performed two types of crosses to study Laws of Inheritance:
- Monohybrid cross and
- Dihybrid cross
Let's study the differences between Monohybrid and Dihybrid Cross.
A. MONOHYBRID CROSS
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) ]
Experiment:
- Cross pure or homozygous tall pea plant (TT) with dwarf pea plant (tt) [TT x tt].
- Gametes formed : T and t
- F1 generation progeny : Hetrozygous hybrid formed (Tt) showing tall dominant character (Law of Dominance)
- Perform self hybridization or selfing of hybrid formed in F1 generation [Tt x Tt]
Observations of Selfing [F2 generation]:
1) Phenotype of F2 generation:
- 3:1 (Tall : Dwarf or Dominant : Recessive)
- 1:2:1 (TT : Tt : tt) [Pure tall : Hybrid tall : Pure recessive]
3) Recessive characters reappear in F2 generation [Law of Segregation or Law of Purity of Gametes]
Conclusions from Monohybrid Cross:
On the basis of Monohybrid Cross, Mendel gave two Laws of Inheritance:
a) Law of Dominance
When a pair of allelomorphs (contrasting characters) or alleles are brought together in F1 generation (heterozygous hybrid), one character expresses itself (dominant trait) while other remains unexpressed (recessive trait).
Example:
In the monohybrid cross shown above, in F1 generation, a heterozygous hybrid (Tt) is formed which expresses dominant parent character (TALL).
- Phenotypic ratio of F1 progeny = 3 : 1 (Tall : Dwarf)
b) Law of Segregation or Law of Purity of Gametes
When a pair of allelomorphs (contrasting characters) or alleles are brought together in F1 generation (heterozygous hybrid), they remain together without mixing or blending and separate complete and pure during gamete formation.
Example:
In the monohybrid cross shown above, when selfing of F1 generation hybrid (Tt) is performed, it results in genotype of 1 : 2 : 1 (TT : Tt : tt) [Pure tall : Hybrid tall : Pure dwarf] in F2 generation.
Thus, each F1 hybrid parent (Tt) produced two types of gametes: T (tall) and t (dwarf), which had equal chances of combining together.
The recessive character (dwarf) which disappeared in F1 generation, reappeared unchanged in F2 generation. Thus, proving that pair of allelomorphs remained together without mixing or blending with each other.
B. DIHYBRID CROSS
In a Dihybrid cross, inheritance of two pairs of contrasting characters is studied.
For example in this case, we will study following two pairs of characters of pea variety simultaneously in one cross :
- color of cotyledons [yellow (YY) or Green (yy)]
- shape of seed [Round (RR) or Wrinkled (rr)]
Experiment:
- Cross pure homozygous Yellow cotyledon, Round seed pea variety (YYRR) with Green cotyledon, wrinkled seed variety (yyrr) [YYRR x yyrr]
- Gametes formed: YR, yr
- F1 generation progeny: Heterozygous hybrid YyRr (Yellow cotyledon, Round seed)
- Perform selfing of F1 generation hybrid (YyRr x YyRr)
Observations of Selfing [F2 generation]:
1) Phenotype of F2 generation:
- 9:3:3:1 (Yellow Round : Yellow Wrinkled : Yellow : Green Round : Green Wrinkled)
- 1:2:2:4:1:2:1:2:1
3) Recessive characters reappear in F2 generation [Law of Segregation or Law of Purity of Gametes]
Conclusions of Dihybrid Cross:
On the basis of Dihybrid cross Mendel gave Law of Independent Assortment
a) Law of Independent Assortment
Law of Independent Assortment can be studied with the help of dihybrid cross. The Law of Independent Assortment states that "when two pairs of contrasting characters are crossed together, the alleles of two (or more) different genes get sorted into gametes independently of one another which means that an allele received by the gamete for one gene does not influence or affect the allele received for another gene."
Real life examples of Dihybrid cross includes:
- Blood Type
- Eye color
- Skin color of cow
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