[long_answer] Mendel's law of independent assortment holds true only when the two genes considered are located on different (non-homologous) chromosomes. Explain why the results of a dihybrid cross would be fundamentally different if both genes were located on the same chromosome. What change would you expect in the F2 phenotypic ratio, and why?
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
Mendel's Law of Independent Assortment and Linked Genes
Mendel's law of independent assortment states that two different traits are inherited independently of each other. This holds true only when the two genes are located on different (non-homologous) chromosomes, so they segregate independently during gamete formation.
If both genes are on the same chromosome (linked genes):
- The two genes tend to be inherited together as a unit, since they are physically attached on the same chromosome.
- During gamete formation, linked genes do not assort independently; they travel together to the same gamete.
- New combinations of traits (recombinants) would appear far less frequently than expected, unlike the free mixing seen in a standard dihybrid cross.
Effect on F2 phenotypic ratio:
- In a normal dihybrid cross (genes on different chromosomes), the F2 ratio is 9:3:3:1.
- If both genes are linked, this ratio would not be obtained. Instead, parental combinations (e.g., tall + round seeds; short + wrinkled seeds) would appear in a much higher proportion, and recombinant types (tall + wrinkled; short + round) would be very rare.
- The ratio would deviate significantly from 9:3:3:1, approximating a 3:1 ratio (like a monohybrid cross) if linkage is complete.
Thus, linkage prevents independent assortment, reducing genetic variation in offspring.
Source: Chapter 8, Section 8.2.2 – Rules for the Inheritance of Traits
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Explanation
- The examiner expects you to first state what independent assortment means and its condition (genes on non-homologous chromosomes).
- Then explain the concept of linkage — genes on the same chromosome travel together.
- Finally, clearly state the change in F2 ratio: 9:3:3:1 breaks down; parental types dominate; recombinants are rare; ratio approaches 3:1 for complete linkage.
- The NCERT text explicitly shows that new combinations (tall+wrinkled, short+round) arise in F2 because the traits are independently inherited — use this as your contrast point.
- Avoid writing a long essay; keep points crisp and logically ordered.