Question

State and explain Mendel’s Law of Independent Assortment with reference to a dihybrid cross involving pea plant. Consider the traits of seed shape (round vs. wrinkled) and seed colour (yellow vs. green).

Hint:

Independent assortment only occurs for genes located on different chromosomes or very far apart on the same chromosome. If genes are close together, they show "Linkage" instead.

Answer 1

Step 1: Understanding the Concept:
The Law of Independent Assortment is based on dihybrid crosses, where two pairs of contrasting characters are studied simultaneously.
It explains how different genes separate from one another when reproductive cells develop.
Step 2: Key Formula or Approach:
Consider a cross between a pea plant with Round Yellow seeds (RRYY) and a plant with Wrinkled Green seeds (rryy).
1. Parental Generation (P): \( RRYY \times rryy \)
2. Gametes: \( RY \) and \( ry \)
3. F1 Generation: \( RrYy \) (All are Round and Yellow).
4. F2 Generation (Selfing F1): \( RrYy \times RrYy \)
Step 3: Detailed Explanation:
During gamete formation in the F1 hybrid (\( RrYy \)), the segregation of the 'R' and 'r' alleles is independent of the 'Y' and 'y' alleles.
This results in four types of gametes produced in equal frequency (25% each): \( RY, Ry, rY, \text{ and } ry \).
When these gametes fuse randomly during fertilization, they produce 16 possible combinations in the F2 generation.
The phenotypic ratio obtained is:
- Round Yellow: 9
- Round Green: 3
- Wrinkled Yellow: 3
- Wrinkled Green: 1
The appearance of new combinations like Round Green and Wrinkled Yellow proves that the traits for seed shape and seed color assorted independently.
Step 4: Final Answer:
The dihybrid phenotypic ratio of 9:3:3:1 confirms that the inheritance of one trait does not influence the inheritance of another.