Dihybrid Cross

Now, we will consider 2 different characteristics of pea plant and analyse offsprings produced.

Let us consider Seed Shape and Seed Colour .

• Here - Round and Yellow are dominant traits.

            Wrinkled and Green are recessive traits.

• As observed in the previous example, the F ₁ generation shows dominant traits only - Yellow and Round.
• But the F ₂ generation shows different traits :
• Yellow and Round seeds 
• Green and Round seeds
• Yellow and Wrinkled seeds
• Green and Wrinkled seeds

The Phenotypic ratio of this cross is:

Yellow Round : Green Round : Yellow Wrinkled :Green Wrinkled 

9 : 3 : 3 : 1

Note - 

• A monohybrid cross is the cross of F ₁ generation offspring of parents having two different forms of the same trait .

For Example - Homozygous tall X Homozygous short pea plant.

• A dihybrid cross is the cross of F ₁ generation offspring which have different forms of two different traits .

For Example - Yellow and Round seeds X Green and Wrinkled seeds

What do we observe from these crosses?

1. Characters are controlled by separate units called “ factors ” which are now known as genes .
2. Genes occur in pairs .
3. An individual inherits one gene for a particular trait from each parent.
4. These genes are passed on from one generation to another without getting blended. 
5. If the pair of genes are not similar (heterozygous - Example - Tt) one member of the pair dominates ( dominant ) the other ( recessive ).
6. Certain genes even when passed are not observed in the offsprings but they appear in the subsequent generations .
   For Example - In the cross between homozygous tall (TT) and homozygous short (tt) plants, though the gene from short plants did not appear in the F ₁ generation, it appeared in the F ₂ generation.
7. In the case of Dihybrid cross the traits are inherited independently .
   This is why we notice combinations in offspring that were not seen in the parents.