Question

In Mendelian inheritance, the phenotypic ratio of a dihybrid cross is:

• A. 1 : 2 : 1
• B. 3 : 1
• C. 9 : 3 : 3 : 1
• D. 15 : 1

Hint:

A 9 : 3 : 3 : 1 ratio is always associated with a Mendelian dihybrid cross showing independent assortment.

Answer 1

The Correct Option is C

Step 1: Understanding a dihybrid cross.
A dihybrid cross involves the inheritance of two different traits simultaneously. Mendel studied such crosses to understand how traits are passed independently from parents to offspring.
Step 2: Applying Mendel’s law of independent assortment.
According to the law of independent assortment, alleles of different genes assort independently during gamete formation, provided the genes are located on different chromosomes.
Step 3: Formation of gametes.
In a typical dihybrid cross such as RrYy × RrYy, each parent produces four types of gametes: RY, Ry, rY, and ry.
Step 4: Phenotypic outcomes.
When these gametes combine, they produce four phenotypic classes in the ratio:
9 showing both dominant traits,
3 showing the first dominant and second recessive trait,
3 showing the first recessive and second dominant trait,
1 showing both recessive traits.
Step 5: Conclusion.
Thus, the phenotypic ratio of a dihybrid cross is 9 : 3 : 3 : 1.