Q.Assertion (A): Linked genes do not show dihybrid F2 ratio 9 : 3 : 3 : 1. Reason (R): Linked genes do not undergo independent assortment. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
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Start your 14-day free trial to unlock the full solution →Linked genes, being inherited together, do not assort independently, which prevents them from producing the classic 9:3:3:1 dihybrid F2 phenotypic ratio.
To understand why linked genes deviate from the expected dihybrid ratio, we must first recall the basis of that ratio: Mendel's Law of Independent Assortment. When Mendel conducted his dihybrid crosses, studying the inheritance of two different traits simultaneously (for example, seed colour and seed shape in peas), he observed a consistent pattern in the F2 generation. If he crossed a pure-breeding parent with dominant traits (e.g., yellow, round seeds) with a pure-breeding parent with recessive traits (e.g., green, wrinkled seeds), the F1 generation would all display the dominant traits. However, when these F1 individuals were self-pollinated, the F2 generation showed a specific phenotypic ratio of 9 : 3 : 3 : 1.
This 9:3:3:1 ratio represents:
- 9 parts showing both dominant traits (e.g., yellow, round)
- 3 parts showing one dominant and one recessive trait (e.g., yellow, wrinkled)
- 3 parts showing the other dominant and one recessive trait (e.g., green, round)
- 1 part showing both recessive traits (e.g., green, wrinkled)
This precise ratio is a direct consequence of Mendel's Law of Independent Assortment, which states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. In simpler terms, the alleles for one gene (like seed colour) separate and distribute into gametes independently of the alleles for another gene (like seed shape). This independent segregation happens because these genes are typically located on different chromosomes, or are very far apart on the same chromosome, allowing for all possible combinations of alleles to form in the gametes with equal probability.
The 9:3:3:1 dihybrid F2 phenotypic ratio is the hallmark outcome when two genes assort independently.
However, not all genes behave this way. The concept of "linked genes" describes genes that are located close together on the same chromosome. Because they are physically close, they tend to be inherited together as a single unit during meiosis, rather than assorting independently. This phenomenon is known as genetic linkage.
When genes are linked, the formation of gametes does not follow the independent assortment pattern. Instead, the parental combinations of alleles are much more likely to be passed on together to the offspring. For instance, if an individual inherited a chromosome carrying alleles for 'yellow' and 'round' from one parent, and 'green' and 'wrinkled' from the other, and these genes are linked, then the gametes produced will predominantly carry either 'yellow' and 'round' together, or 'green' and 'wrinkled' together. The recombinant gametes (e.g., 'yellow' and 'wrinkled', or 'green' and 'round') will be formed much less frequently, primarily through crossing over events, which are less common between closely linked genes. …
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