Principles of Inheritance and Variation - Notes | Class 12

Inheritance of Two Genes

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

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

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 the original phenotype.
- Modified allele: It is generally recessive.
E.g., consider a gene that contains information for producing an enzyme. The normal allele of that gene produces a normal enzyme. The modified allele is responsible for the production of:
- Normal/less efficient enzyme, or
- A non-functional enzyme, or
- No enzyme at all.
In the first case: The modified allele will produce the same phenotype as the unmodified allele. Thus, the modified allele is equivalent to the unmodified allele.
In the 2^nd and 3^rd cases: The phenotype will depend only on the functioning of the unmodified allele. Thus, the modified allele becomes recessive.