Binomial Expression in Mendelian Genetics — NEET Biology
📚 Practice ConceptThe 1/4: 1/2: 1/4 ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax +by)², that has the gametes bearing genes T or t in equal frequency of ½. The expression is expanded as given below:
Which of the following statements regarding dihybrid crosses and Mendel's laws are correct? S1: In a cross between true-breeding parents RRYY and rryy, the F1 generation (RrYy) will produce four types of gametes (RY, Ry, rY, ry) in equal proportions. S2: The Law of Independent Assortment directly explains the 9:3:3:1 phenotypic ratio observed in the F2 generation of a dihybrid cross when genes are unlinked. S3: The segregation of alleles for seed colour is entirely dependent on the segregation of alleles for seed shape during gamete formation in a dihybrid cross. S4: The probability of an offspring having the genotype rryy from a self-pollinated F1 dihybrid (RrYy x RrYy) is 1/8. S5: If two genes are tightly linked on the same chromosome, their F2 generation phenotypic ratio in a dihybrid cross will significantly deviate from 9:3:3:1.
View solution & NCERT explanation
Correct answer: C — S1, S2 and S5 only
The core concept tested here involves the principles of dihybrid cross, gamete formation, Mendelian laws, and the impact of linkage. S1: This statement is correct. In a dihybrid cross, the F1 heterozygote (RrYy) will produce four types of gametes (RY, Ry, rY, ry) in equal proportions (25% each) due to independent assortment of genes. S2: This statement is correct. Mendel's Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair. This independence leads to the characteristic 9:3:3:1 phenotypic ratio in the F2 generation of a dihybrid cross where genes are unlinked. S3: This statement is incorrect. The Law of Independent Assortment explicitly states that the segregation of alleles for one character is independent of the segregation of alleles for another character during gamete formation, not dependent. S4: This statement is incorrect. From a self-pollinated F1 dihybrid (RrYy x RrYy), the probability of producing an offspring with genotype rryy is (1/4 for rr) × (1/4 for yy) = 1/16, not 1/8. S5: This statement is correct. Morgan's work on Drosophila showed that when two genes are located on the same chromosome and are tightly linked, they do not segregate independently, leading to a significant deviation from the expected 9:3:3:1 ratio in the F2 generation, with higher proportions of parental combinations. Therefore, statements S1, S2, and S5 are correct. Option A is incorrect because S3 and S4 are false. Option B is incorrect because S3 and S4 are false. Option C is correct because S1, S2, and S5 are true. Option D is incorrect because S4 is false. The question assesses understanding of dihybrid cross outcomes, gamete formation, Mendel's laws, and the concept of linkage.
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