Identify the incorrect statement with reference to the law of dominance.
1. Characters are controlled by discrete units called factors.
2. In a dissimilar pair of factors, one member of the pair is dominant and the other is recessive.
3. Factors always remain together.
4. In a monohybrid cross, this law is used to explain the expression of traits of both parents in the F1 generation.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 3
Note: The given answer is as per the official NTA Answer Key. However, the question can have more than one correct answer, i.e. both Option (3) and Option (4), because statement 4 is also incorrect, as explained below.
The law of dominance says three things. Characters are controlled by discrete units called factors. Factors occur in pairs. In a dissimilar pair of factors, one member (dominant) hides the other (recessive). So in a monohybrid cross only one parental trait shows in the F1, and both show again in the F2.
1. Characters are controlled by discrete units called factors. — A correct statement. Mendel called these units factors; today we call them genes.
2. In a dissimilar pair of factors, one member of the pair is dominant and the other is recessive. — A correct statement. This is the heart of the law.
3. Factors always remain together. — An incorrect statement. The two factors of a pair do not stay together. They separate (segregate) when gametes form, so each gamete gets only one of them. This is the law of segregation. This is the answer given by NTA.
4. In a monohybrid cross, this law is used to explain the expression of traits of both parents in the F1 generation. — Also an incorrect statement. The law of dominance explains why only one of the parental traits is expressed in the F1. Both parental traits are expressed only in the F2.
NCERT Extract:
Law of Dominance – “(i) Characters are controlled by discrete units called factors. (ii) Factors occur in pairs. (iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive). The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2.”