Question

Question 9 [10 marks / 10 minutes]
Suppose there are two different mutant alleles of the same gene. Sequence analysis shows that both alleles are single nucleotide substitutions, and that different nucleotide positions within the gene are affected in each allele. Phenotypic analysis shows that one allele is a null allele, and the other is dominant negative (antimorphic).
Explain what this information reveals about the two mutations. Give a hypothesis for how these two mutant alleles could show such markedly different effects. [10 marks]

Answer 1

According to the information, the first mutation gives rise to a null-allele. A null-allele is a non-functional gene which does not encode for any protein. A mutation can give rise to such a null-allele from an otherwise normal allele if it disrupts the starting codon of the reading frame. In this manner, translation of the allele would not take place and no protein would be formed.

Secondarily, the second mutation is an antimorphic mutation which means that this mutation is dominant negative and hence the phenotype so obtained after it would be recessive in nature. This can take place only if the mutation takes place in such a region of the DNA which encodes for the functional part of the protein.

In order to hypothesize the possible reasons behind these observations, it must be carefully noted that different amino acids are synthesized by different codons, having a difference of only single nitrogenous base. Thus, a random base pair substitution can undoubtedly give rise to such a non-functional mutation.