Principles of Inheritance and Variation - Notes | Class 12 | Part 3: Inheritance of Two Genes


Dihybrid cross: 

It is a cross between two parents differing in 2 pairs of contrasting characters.

E.g. Cross b/w pea plant with homozygous round shaped & yellow coloured seeds (RRYY) and wrinkled shaped & green coloured seeds (rryy).

On observing the F2, Mendel found that yellow and green colour segregated in a 3:1 ratio.

Round & wrinkled seed shape also segregated in a 3:1 ratio.

Dihybrid Phenotypic ratio:

9 Round yellow: 3 Round green: 3 Wrinkled yellow: 1 Wrinkled green = 9:3:3:1

The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round: 1 wrinkled.

i.e. (3: 1) (3: 1) = 9: 3: 3: 1

Dihybrid genotypic ratio: 


RRYY =1 
RRYy =2 
RrYY =2
RrYy =4 
RRyy =1 
Rryy =2
rrYY =1 
rrYy =2 
rryy =1

Mendel’s 3rd Law: Law of Independent Assortment 

It is based on the results of dihybrid crosses.

It states that “When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.

The concept of dominance 

Every gene contains information to express a particular trait.

In heterozygotes, there are 2 types of alleles:
  • Unmodified (normal or functioning) allele: It is generally dominant and represents original phenotype.
  • Modified allele: It is generally recessive.
E.g. Consider a gene that contains information for producing an enzyme. Normal allele of that gene produces a normal enzyme. Modified allele is responsible for production of
  1. Normal/less efficient enzyme or
  2. A non-functional enzyme or
  3. No enzyme at all
In the first case: The modified allele will produce the same phenotype like unmodified allele. Thus, modified allele is equivalent to unmodified allele.

In 2nd and 3rd cases: The phenotype will dependent only on the functioning of the unmodified allele. Thus the modified allele becomes recessive.

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