Homozygosity for extremely rare mutations in a humangene called SCN9A cause complete insensitivity topain (congenital pain insensitivity or CPA) and a totallack of the sense of smell (anosmia). The SCN9A geneencodes a sodium channel protein required for transmission of electrical signals from particular nerves inthe body to the brain. The failure to feel pain is a dangerous condition as people cannot sense injuries.The SCN9A gene has 26 exons and encodes a1977-amino acid polypeptide. Consanguineous matings in three different families have resulted in individuals with CPA/anosmia. In Family 1, a G-to-Atransition in exon 15 results in a truncated protein that is898 amino acids long; in Family 2, deletion of a singlebase results in a 766-amino acid polypeptide; and inFamily 3, a C-to-G transversion in exon 10 yields a458-amino acid protein.a. Hypothesize as to how each of the three SCN9Amutations affects gene structure: Why are truncatedproteins made in each case? b. How would you classify the mutant alleles? Dothese cause loss of function or gain of function?Are they amorphs, hypomorphs, hypermorphs,neomorphs, or antimorphs?c. Explain in molecular terms why CPA/anosmia is arecessive condition

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Asked Mar 10, 2020
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Homozygosity for extremely rare mutations in a human
gene called SCN9A cause complete insensitivity to
pain (congenital pain insensitivity or CPA) and a total
lack of the sense of smell (anosmia). The SCN9A gene
encodes a sodium channel protein required for transmission of electrical signals from particular nerves in
the body to the brain. The failure to feel pain is a dangerous condition as people cannot sense injuries.
The SCN9A gene has 26 exons and encodes a
1977-amino acid polypeptide. Consanguineous matings in three different families have resulted in individuals with CPA/anosmia. In Family 1, a G-to-A
transition in exon 15 results in a truncated protein that is898 amino acids long; in Family 2, deletion of a single
base results in a 766-amino acid polypeptide; and in
Family 3, a C-to-G transversion in exon 10 yields a
458-amino acid protein.
a. Hypothesize as to how each of the three SCN9A
mutations affects gene structure: Why are truncated
proteins made in each case?
b. How would you classify the mutant alleles? Do
these cause loss of function or gain of function?
Are they amorphs, hypomorphs, hypermorphs,
neomorphs, or antimorphs?
c. Explain in molecular terms why CPA/anosmia is a
recessive condition

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Expert Answer

Step 1

The SCN9A gene encodes for a sodium ion channel that is dependent on voltage. It is composed of 26 exons and its exact chromosomal location is 2q24.3. These receptors are abundant on the surface of sensory neurons that detect pain and odor. Mutation in this gene can lead to loss of sensation to pain and odor.

Step 2

The different mutations like G to A transition, base deletion, and C to G transversion in the different exons of the SCN9A gene produce truncated proteins. The truncated protein has an amino acid length that is shorter than the wildtype protein. These mutations must be causing the formation of a premature termination codon. The normal codon coding for a specific amino acid must be getting mutated to form a termination codon. In the wildtype mRNA (messenger ribonucleic acid),...

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