Biology · Principles of Inheritance and Variation · NEET
It is about which allele makes a working enzyme. NCERT gives the example of a gene that codes for an enzyme. The normal (unmodified) allele makes the normal enzyme that changes a substrate S into a product. If the other allele is modified and makes a non-functional enzyme or no enzyme, then only the normal allele does the job. That normal, working allele is the dominant allele, and the faulty one is the recessive allele. So dominance is about function, not strength.
No. NCERT lists three possibilities for a modified allele: it may make (i) a normal or less-efficient enzyme, (ii) a non-functional enzyme, or (iii) no enzyme at all. In case (i) the modified allele acts the same as the normal one, so both look alike and this equivalent pair is very common. The trait becomes recessive only in cases (ii) and (iii), where the enzyme is non-functional or absent, so the phenotype depends only on the working allele.
In a heterozygote (Aa), the dominant allele A still makes the working enzyme. Usually one working copy makes enough enzyme to complete the job, for example changing substrate S into its product. The recessive allele a makes a faulty or no enzyme, so it adds nothing. Because the phenotype depends only on the functioning allele, the heterozygote looks the same as the homozygous dominant. That is why A hides a.
No. This is a common wrong idea. Dominant does not mean stronger, better, or more common. It only means that this allele makes a working product (enzyme), so its effect is seen. Recessive means the product is faulty or missing, so its effect is hidden when a working allele is present. Some harmful traits are dominant and some helpful traits are recessive, so dominance says nothing about which is better.
The recessive trait needs both alleles to be faulty. In aa, there is no working allele left, so no functional enzyme is made and substrate S is not changed. This gives the recessive phenotype. If even one dominant allele (A) is present, it supplies the working enzyme and the dominant trait shows. So a recessive trait appears only when the individual is homozygous recessive (aa).
Mendel saw the pattern, and the enzyme idea explains why. Mendel's Law of Dominance says that in a dissimilar pair of factors, one factor dominates and the other stays hidden, so only one parental character shows in the F1 monohybrid cross. The enzyme basis explains the mechanism: the dominant factor is the allele that makes a working enzyme, and the recessive factor makes a faulty or no enzyme. NCERT places this explanation right after the Law of Dominance for this reason.
Which one of the following can be explained on the basis of Mendel's law of Dominance? A. Out of one pair of factors one is dominant and the other is recessive. B. Alleles do not show any expression and both the character appear as such in F2 generation. C. Factors occur in pairs in normal diploid plants. D. The discrete unit controlling a particular character is called factor. E. The expression of only one of the parental characters is found in a monohybrid cross.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The dominant allele makes a working enzyme, so its trait shows; the recessive allele makes a faulty or no enzyme, so its trait stays hidden when a working allele is present.
NCERT Class 12 Biology, Chapter 4 (Principles of Inheritance and Variation), right after the Law of Dominance, using the example of a gene that codes for an enzyme acting on substrate S.
Yes. If the modified allele still makes a normal or nearly normal enzyme, it acts the same as the original allele. Such equivalent allele pairs are very common, and neither is hidden.
NEET often tests the Law of Dominance and its reason. Knowing that dominant means a functional enzyme (not a stronger allele) helps you avoid trap options and solve monohybrid and disorder questions correctly.
Learn incomplete dominance, where the heterozygote looks in-between the two parents (like the pink flower in Snapdragon), because one working allele does not make enough product to fully show the trait.
