Question

Which of the following reasons may be attributed to the recessive nature of one of the alleles in a heterozygous individual?

I) The allele is responsible for the production of a non-functional enzyme.
II) The allele is responsible for the production of no enzyme at all.
III) The allele is responsible for the production of a normal/less efficient enzyme.

• A. All of the above
• B. I and II only
• C. II and III only
• D. I and III only

Answer 1

The correct option is B I and II only

Consider a gene that contains the information for the production of an enzyme. This gene has two alleles.

Out of the two alleles, one is a normal allele (unmodified allele) and produces the normal enzyme that causes the transformation of the substrate.

Another allele is a modified allele due to certain changes that it has undergone.

If a modified allele produces a normal or less efficient enzyme, the modified allele is equivalent to the unmodified allele (normal allele) and results in the transformation of the substrate, i.e., it will produce the same phenotype/trait.

Hence the statement III cannot be the reason for the recessive nature of one of the alleles in a heterozygous individual.

The phenotype is affected when the modified allele produces a non-functional enzyme or no enzyme at all. In such a case the phenotype will be expressed due to the functioning of the unmodified allele.

The unmodified allele is the dominant allele and the modified allele is generally the recessive allele.

Hence, the recessive trait is seen due to a non-functional enzyme or because of no enzyme produced.