A gene contains the information for producing an enzyme that is needed for the transformation of a substrate S. In a heterozygote, the unmodified allele produces the normal enzyme, while the modified allele produces no enzyme at all. Which of the following is expected in this heterozygote?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The phenotype depends on the functioning of the unmodified allele, which behaves as the dominant allele.
A diploid organism has two copies of each gene, a pair of alleles. One allele may be modified. Here the normal (unmodified) allele makes the enzyme that changes substrate S, and the modified allele makes no enzyme. The heterozygote still has one working allele, so it still makes the enzyme. Dominance depends on the gene product: the allele whose product decides the phenotype behaves as the dominant allele.
Option (1) The phenotype depends on the functioning of the unmodified allele, which behaves as the dominant allele. — Correct. The unmodified allele makes the normal enzyme, so S is still transformed and the original phenotype appears. The phenotype depends only on this functioning allele, so it is the dominant allele.
Option (2) The phenotype depends on the modified allele, which behaves as the dominant allele. — Incorrect. The modified allele makes no enzyme, so it adds nothing to the phenotype. Such an allele is generally the recessive allele. Its effect shows only when no working allele is present.
Option (3) Both alleles express their own products together, which is an example of co-dominance. — Incorrect. In co-dominance both alleles express their own products together, as IA and IB make two different sugars in the AB blood group. Here only one allele makes a product, so this is not co-dominance.
Option (4) The substrate S is not transformed, so the heterozygote shows the recessive trait. — Incorrect. The recessive trait is seen when the enzyme is non-functional or not made at all, that is, when both alleles are modified. The heterozygote has one normal allele that still makes the enzyme, so S is transformed.
NCERT Extract:
Explanation of the Concept of Dominance – “… Let’s take an example of a gene that contains the information for producing an enzyme. Now there are two copies of this gene, the two allelic forms. Let us assume (as is more common) that the normal allele produces the normal enzyme that is needed for the transformation of a substrate S. … But, if the allele produces a non-functional enzyme or no enzyme, the phenotype may be effected. The phenotype/trait will only be dependent on the functioning of the unmodified allele. The unmodified (functioning) allele, which represents the original phenotype is the dominant allele and the modified allele is generally the recessive allele. Hence, in the example above the recessive trait is seen due to non-functional enzyme or because no enzyme is produced.”