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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.

 

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo